« A DICTIONARY OF PHOTOGRAPHY, EDITED BY THOMAS SUTTON, B.A., Editor of "Photographic Notes," AND GEORGE DAWSON, M. A., Lecturer on Photography, King's College, London. fllustrutctr tottlj mtnurnus giagnras. LONDON: SAMPSON LOW, SON, & MARSTON, MILTON HOUSE, 59, LUDGATE HILL. 1867. [A 11 rights reserved. ] LONDON : WILLIAM 3TEVEKS, PBIHTBK, 37, BELL YA TEMPLE BAB. PREFACE. The first edition of the Dictionary of Photography, pub- lished in 1858, has been out of print for several years. In consequence of this, and the continued demand for the work, the publishers have entrusted the present editors with its entire supervision, in the belief that a new edition would be a desirable boon to photographers, by supplying them with a book of easy reference, in an alphabetical form, whereby they will be saved the trouble of wading through elaborate and often badly arranged treatises to find information on the subject to which they wish to refer. It has been the desire of the editors to condense rather than to expand the matter contained in this volume, and to make it as thoroughly practical as possible. With this object in view, they have eliminated the debateable theo- ries and speculative articles which occupied a prominent position, especially in the first part of the previous edition, and they have abridged other articles which, at the pre- sent time, are of less importance to photographers than they were in the year 1858. In lieu thereof, a great deal of practical matter has been added, including all the new useful processes ; many of the iv PREFACE. original articles have been re-written in a spirit more con- sistent with the present advanced state of the art ; the preparation and properties of the necessary chemicals and the theory and construction of the different optical instru- ments required have been succinctly described ; but a detailed account of the different kinds of cameras, printing frames, stands, and some other photographic items has not been given, because a complete description of what can be seen at any photographic warehouse would be entirely out of place in a work of this kind, and would have occupied too much space. The aim of the editors has been to make this work a photographic multum in parvo. How far they have suc- ceeded photographers themselves must determine. The editors would take this opportunity of expressing their obligations to an American gentleman, who desires in the meantime to be anonymous, for some valuable hints and articles kindly volunteered by him for this edition of the Dictionary. THOMAS SUTTON. GEORGE DAWSON. January St/i, 1867. PHOTOGRAPHIC DICTIONARY. Aberration. (Lat., ab, from, errare, to wander). This is a term much used in Optics. When a pencil of light suffers either refraction or reflection at the surface of any medium, it generally happens that the directions of the refracted or reflected rays do not all pass accurately through a point, or focus. This error is called " aber- ration," when the pencil is a direct one. The term, however, is not applied to those cases of confusion which occur in refraction through a lens, when both the incident and refracted pencils are oblique. It will be understood that we speak now of " spherical aberration;" that is to say, of the aberration which is produced entirely by the spherical form of the surface of the medium ; the term spherical including the case of refraction at a plane surface, since a plane may be considered as a sphere of infinite radius. Pig. 1 will serve to explain a common case of spherical aberra- tion in a large pencil refracted through a lens, the incidence at the front surface being oblique, and the emergence at the posterior surface being direct. Fig. 1. A Let A B be a single convex lens, and QABa pencil of light incident upon it, proceeding from a luminous point Q. The pencil, B 2 ABE after refraction through the lens, will not form a cone of light in which all the rays come to a common focus, but an effect will take place which it is important clearly to understand. In the first place, the emergent pencil is symmetrical with respect to an axis F S, which axis produced passes through the centre 0 of the pos- terior spherical surface A S B. The refracted rays which emerge from the immediate neighbourhood of the point S form a small pencil, which may be considered as having a focus F, called the " geometrical focus." The outer rays of the emergent pencil will cut the line S F at points c, d, b, as shown in the figure ; the dis- tances F c, F d, F b, increasing as the distance from S of the point of emergence of a ray increases. F b is therefore called the aberra- tion of the ray Q, B, F d of the ray Q D, F c of the ray Q C, and so on. Moreover, these distances F b, F c, &c, are affected with the sign ■+■ (plus), in order to indicate that they are measured from the geometrical focus F towards the origin Q, of the incident pencil. Had they fallen on the other side of F, they would have been affected with the sign — (minus). Aberration is therefore called " positive " or " negative," according as it is affected with the sign plus or minus. When F S is large compared with S A, the aberration F d is pro- portional to the square of S A. It is impossible to construct a single lens with spherical surfaces, so that the pencils shall be entirely free from aberration ; but, by combining two or more lenses, made either of different kinds of glass, or of the same kind of glass, spherical aberration may be to a great extent, although not entirely, corrected. Such compound lenses, or combinations, are said to be " aplanatic." There are certain forms of reflecting and refracting surfaces and lenses in which a particular pencil is reflected or refracted without aberration. The cases with which the photographer is chiefly con- cerned are those of the parabolic and spheroidal mirrors. All rays incident on the surface of a parabolic reflector, in directions parallel to its axis, are reflected in directions which pass through its focus ; and all rays incident on the surface of a prolate spheroid, in direc- tions which pass through one of its foci, are reflected in directions which pass through the other focus. It is a common error to confound spherical aberration with cur- vature of the image. Curvature of the image may exist where there is no spherical aberration, and vice versa. See u Curvature of the Image." The nearest approach to a correct focus which can be obtained ABE 3 with a lens, when a large aperture is employed, is called the " least circle of aberration." It will be seen in Fig. 2 that, if the various rays of a refracted pencil are produced through the axis, there will be a certain posi- tion, m, of a circular area through which they all pass, in which the diameter of that circle will be the least possible. If F a be the aberration of the pencil, the distance of this least circle of aberration from F is three-fourths of F a ; and if the aperture of the lens A B be small compared with its focal length, C F, then the diameter of the least circle of aberration is proportional to the cube of the diameter of the part of the lens employed. Fig. 2. A When we speak of the focus of a direct pencil, we mean the least circle of aberration of that pencil. It is evident that the smaller the diaphragm of the lens, the smaller will be the diameter of this circle. In view-lenses, in which correction for spherical aberration would be inconsistent with other important conditions, a small diaphragm is the only means which can be employed for obtaining sharp definition. Aberration, Chromatic. The light which proceeds from the sun, and most luminous bodies, is found to be heterogeneous; that is, composed of different kinds of light, of different degrees of refran- gibility. If a ray of such light be refracted through a prism or lens, it will be decomposed into its constituent rays ; and if a direct pencil passes through a lens, there will be formed a system of emergent pencils, corresponding to the different kinds of light of which the incident pencil is composed. Sunshine is found to be composed of light of seven different colours; viz., red, orange, yellow, green, blue, indigo, violet ; arranged in the order of their refran- gibility, red being the least and violet the most refrangible. When, therefore, a pencil of sunshine is refracted through a convex lens, the foci of the coloured pencils are arranged along the axis of the lens in the order of refrangibility. If we designate these foci by the .4 ACC ACE letters r, o, y, g, b, i, v (r being farthest from, and v nearest to the lens), the distance between r and v is called the " chromatic aberra- tion " of the pencil. It is found that by combining n lenses made of n different kinds of glass, according to a certain formula, n different coloured foci may be united in the same point upon the axis. When two or more foci are thus united, the lens, or combination of lenses, is said to be " achromatic," or corrected for chromatic aberration. If it were not for the property called " irrationality of dispersion," the same arrangement by which two of the coloured foci were combined would suffice to combine all the colours. Accelerator. This name is often given to any substance which shortens the time of exposure, either in the camera or in the print- ing-frame. The term is, however, so indefinite in its application that it ought to be discontinued. Accessories. By this term is commonly meant the various ornamental articles of furniture, vases, columns, balustrades, curtains, &c, which photographers employ in connection with portraiture. With respect to taste in the choice and arrangement of such objects, little can with propriety be said in a work of this kind. The portraitist will find it a highly advantageous plan to have all such objects coloured of some shade of grey— lighter or darker, as the case may be — and not of a variety of colours which have different actinic properties, and which therefore might mislead the eye in arranging the light and shade of the picture. Varnished surfaces should also, as a rule, be avoided, because they produce by reflec- tions disagreeable spots of high light in the photograph. It is hardly necessary to remark, also, that solid accessories should always be preferred to flat pictures of objects in questionable perspective. Care should always be taken not to overload a picture with acces- sories ; the aim should rather be to make them so entirely subser- vient to the principal figure as that the eye should not be inclined to dwell upon them too much, or the mind become too conscious of their presence. Acetates. Compounds of acetic acid with bases. Some of the acetates are used extensively in Photography for displacing free nitric or sulphuric acids from solutions, in which such acids are injurious. They are also used in conjunction with chloride of gold in the toning bath. They are formed by dissolving the oxides in the acid, or by adding acetic acid to the proper carbonates, when carbonic acid is expelled with effervescence. The salts of acetic acid are almost, without exception, soluble in water. The general ACE 5 formula for the acetates is MO, C 4 H 3 0 3 . The acetates used in Photography will be described under their respective bases. Acetic Acid. C 4 H 3 0 3 , HO or Ac 0 3 HO = 60. Acetic acid is obtained either by the oxidation of alcohol or by the destructive distillation of wood. When wine, beer, and other fermented liquors are exposed to the air under certain conditions, an oxidising action, sometimes erroneously called acetous fermentation, is set up, and the alcohol which they contain is gradually converted into acetic acid, or vinegar, by the removal, first, of two equivalents of hydrogen, whereby aldehyde is formed. The aldehyde then combines with two equivalents of oxygen, and is converted into acetic acid. Another method of preparing acetic acid consists in subjecting, for several hours, any of the hard woods, such as beech, oak, &c, which must be quite dry, to a red heat, in an iron retort or closed cylinder, to which is attached a condenser. Charcoal remains in the retort, and in the condenser is found a liquid, consisting of acetic acid, tar, water, &c. This liquid is redistilled, and afterwards converted into an impure acetate of soda, which is purified from tarry matter by cautious fusion and recrvstallization. It is then decomposed by strong sulphuric acid, diluted with about half its weight of water. The acetic acid thus formed is purified by dis- tillation. Other methods have been devised and are practised for obtaining acetic acid from wood, but the above process yields the purest acid. Acetic acid, prepared by either of the above methods, contains a large proportion of water. By repeated distillation it becomes stronger ; but the acetic acid commonly called " glacial," which crystallizes at 40°Fahr., is most conveniently obtained by distilling 82 parts of anhydrous acetate of soda with 49 parts of monohy- drated sulphuric acid. The strength of pure acetic acid is very variable, and cannot be determined by its specific gravity. The most constant is the " glacial," which below 40° of temperature becomes solid. When it does not solidify, the only means of knowing its strength is by observing the quantity of crystallized carbonate of soda which is necessary to neutralize it. The dilute acetic acid of the London Pharmacopoeia, sold by the druggists, should be such that one fluid ounce saturates 57 grains : the acetic acid fortior, of the Phar- macopoeia, or the acid called Beaufoy's, should take 390 grains to the fluid ounce ; and the glacial requires 1036 to the ounce, or about 129 to the fluid drachm. When pure, any of these may be used by 6 ACH ACI calculating from these numbers the equivalent measures. The numbers 1, 7, and 18 are near enough ; but it would be better if a standard acid were used by photographers, easily verified by a Standard solution of carbonate of soda. The acetic acid of commerce is often contaminated with impurities, which are very injurious in some photographic processes in which it is employed. Nitrate of silver is a very convenient method of detect- ing those that are hurtful. Proceed in this way : — To a small portion of the suspected acid placed in a test-tube or wine-glass, add a few drops of solution of nitrate of silver. If a milkiness or precipitate of any kind is produced, the acid probably contains hydrochloric or sulphurous acid. If no precipitate is immediately formed, place the test-tube in a strong light for some time, when, if any organic im- purity is present, the liquid will become discoloured. In several of the photographic processes acetic acid is exten- sively used, and it is therefore important to be able to obtain it pure, and of a definite strength. By the above tests any injurious impurity can immediately be detected, and the strength can be estimated by the method already pointed out. Achromatic. A lens, or combination of lenses, which is em- ployed to give a real image of an object placed before it, is said to be achromatic when the image is seen free from any coloured fringe round its edges. Eye-pieces of microscopes and telescopes are said to be achromatic when all the heterogeneous rays of which an emergent pencil is composed are parallel, and therefore enter the eye in such a manner as to produce a focus of white light upon the retina. The conditions of achromatism are therefore different for object-glasses and eye-pieces. Huygens' eye-piece is achro- matic; Eamsden's not. It is customary now to use the term " actinic," instead of achromatic, when speaking of photographic lenticular combinations, in which the visual and chemical rays are united, in preference to the yellow and the blue. Acids. Acids have been defined by Gerhardt to be salts of hydrogen. According to this view, the most common way in which they react or form combinations with other substances is by double decomposition, whereby one equivalent of the metal takes the place of one of the hydrogen. This is, however, still a dis- puted scientific question, which need not be discussed here. The properties common to most acids are — 1. They have a sour taste. 2. They redden blue colours. 3. They are soluble in water. 4. They destroy the characteristic properties of alkalis. 5. They decompose most of the carbonates. But it must not be ACT 7 supposed that all acids possess these properties, nor that there are not other substances to which they are equally applicable. Alum (sulphate of potash and alumina), for instance, which is not an acid, possesses them all. Acids may, for convenience, be divided into two classes — 1. Oxacids, or oxygen united to a metalloid, such as sulphuric acid (sulphur and oxygen). 2. Hydracids, or hydrogen united to a metalloid, as hydrochloric acid (chlorine and hydrogen). When several acids are formed from the same elements, they are distinguished by their termination, or by a prefix. Thus, to take a familiar example, sulplmrow* acid contains the smallest proportion of oxygen of the sulphur acids, and sulphuric acid the highest ; but when an acid, containing the same elements, but less oxygen, inter- venes between the two, the prefix hypo is used, as /^osulphurous acid. Or, again, if an acid is discovered containing more oxygen than the terminal ic would denote, the prefix hyper, or, more shortly, per, is adopted ; as permanganic acid, which contains more Oxygen than manganic acid. It may be observed, that when the terminal syllable of the name of an acid is ic, the salts formed from it end in ate, as nitrate of silver (nitric acid and silver). Salts formed from acids ending in' ous have the terminal ite, as mtvite of silver (nitrous acid and silver). Also the distinguishing prefix, when there is any, must be used ; as hyposulphite of soda (hyposulphurous acid and soda). These rules hold good in every case without exception. The different acids employed in Photography will be treated of in this Dictionary under their respective headings. Actinic. A compound lens is said to be " actinic " when the real image which it gives upon a screen is such that a large num- ber of those coloured rays which exert a chemical action upon the substances composing the sensitive photographic tablet are com- bined with sufficient of the luminous rays to render the image visible. According to this definition, an achromatic lens is not necessarily actinic. Some lenses, which give an image devoid of colour, have not their chemical and visual foci coincident. Actinism. (Greek, ciictiv, a ray). A ray of light, whether pro- ceeding from a heavenly or terrestrial body, is found, in general, to possess three properties ; viz., the luminous, the calorific, and the actinic. The actinic is that property of the ray which produces important chemical changes in certain substances submitted to its action, and on the use of which the whole art of Photography is founded. When a ray of light is decomposed by refraction through 8 ACT ALB a prism into its constituent colours, it is found that actinism exists chiefly among the violet, and scarcely at all among the yellow and red rays. Hence the importance of combining the violet or actinic focus of a photographic lens with the yellow or luminous focus : for the actinic rays produce the photographic picture, while the luminous rays render the image visible upon the focussing screen. It must be remembered, when speaking of actinism, that both light and heat produce chemical changes in some bodies. It is not, therefore, sufficiently exact to say that actinism is the che- mical property of rays. Actinograph. An instrument for measuring the chemical action of the sun's rays. Hunt and others have described several forms of this instrument, all of them depending on the same principle ; viz., the depth of the blackening effect of the chemical rays allowed to fall on a sensitive sheet of paper during a given time. Affinity, Chemical. That power of attraction by which dis- similar substances combine with each other to form compounds. Bodies invariably combine with each other in definite proportions, and the compound formed possesses properties different from either of its elements. Agate Burnisher. Positive prints on plain paper are sometimes "mealy " and deficient in vigour. In such cases it is thought by some persons an improvement to impart a smoothness and glaze to the surface of the paper by rubbing it all over with a polished piece of agate. Tn this operation the paper should be laid face upwards on a slab of plate-glass, marble, or other hard polished substance. Alabastrine Positives. This is a term applied to collodion positives, in which the film, after being coloured with dry pigments, is rendered permeable to varnish, and thus shows the colour in the collodion itself. There have been many methods described by which this can be effected, and some of the results are very beauti- ful, but the process is now little practised, chiefly, perhaps, because it entails a great deal of trouble. Album. This word means, in its literal sense, anything white. The term is now generally applied to books with handsome bindings, in which photographs, &c, are preserved in a fit state to be examined without fingering or injuring the articles exhibited. Albumen. This compound is found dissolved in flesh and in animal fluids, but in Photography it is derived from the white of an egg, which is its purest natural form. In this state it is soluble in water, but a heat of from 140° to 160° coagidates it or changes it into ALB 9 an isomeric compound, no longer soluble in water. Albumen is also coagulated or precipitated by strong acids, by many of the metallic salts, and by several organic reagents. When carefully dried at a temperature under 140°Pahr., it shrinks up into an amber-coloured gummy mass, which cannot now be coagulated by heat, although it is still soluble in water, thus showing that the presence of water is necessaiy for its coagulation. . A characteristic of albumen, which is of importance in Photo- graphy, is its property of forming with nitrate of silver a definite compound, from which the whole of the silver cannot afterwards be removed by the usual fixing agents. See " Silver, Albuminate of." When albumen has been kept for some time in aqueous solution, it decomposes and, among other products, yields sulphuretted hydrogen, easily recognized by the smell. In this state it is quite unfitted for all photographic purposes, and should never be used. Two or three drops of strong ammonia added to half a pint of albumen solution tends to preserve it from putrefaction for a longer time. A little acetic acid is said to have the same effect. The chemical composition of albumen is not positively known. Its properties vary slightly with the source from which it has been obtained, the differences sometimes consisting in the mineral con- stituents, and sometimes indicating various modifications of the same substance. Albumen-Negative Process on Glass. To prepare the Albu- men. — Collect in a basin the whites of a number of eggs, carefully separating the germ and all portions of yelk. To each ounce of this albumen add one drachm of distilled water, in which is dissolved 6 grains of iodide of potassium; also to every 5 ounces of the mixture add one drop of ammonia. Beat the whole to a stiff froth with a bunch of quills, and allow the liquid to settle till the following day. To albumenize the Plate. — The glass plate must first be cleaned very thoroughly, and polished with a cambric handkerchief just before use. Attach to the under-side of it a gutta-percha plate- holder, having a wooden handle a foot long. Then breathe on the plate, and, holding it horizontally in the left hand, pour upon the centre of it a sufficient quantity of the albumen from the basin to cover it, allowing the albumen to filter through an opening in the t dry froth or crust. Make the albumen flow backwards and forwards over the plate three or four times, and then let it all run off into a separate basin, from which it must be carefully filtered before being used a second time. In coating the plate, be particularly careful 10 ALB to prevent air-bubbles from forming upon it. Next, take the handle of the plate-holder between your hands, and, with the plate in a vertical position, spin it round quickly for a minute or so, in order to drive the albumen to the edges by centrifugal force. This done, remove the excess of albumen from the edges by means of a pipette (see " Pipette "), and dry the plate before a clear fire, keeping it rotating all the time by means of the handle, as before directed. When dry it is ready for the next operation. Albumenized plates may be put away in a plate-box, and kept for a considerable time without deterioration in a dry place. Care must be taken, in the operation of albumenizing the plate, that no particles of dust adhere to it. To excite the Plate. — Place it on a dipper, and immerse it quickly' and without hesitation in a vertical bath of aceto-nitrate of silver, made thus: Distilled water, 1 ounce; nitrate of silver, 50 grains; glacial acetic acid, 1 drachm. Leave it in the bath for a couple of minutes, then wash it well in clean water, and lastly in distilled water, and set it up to dry. When dry put it away in the plate-box until ready for use in the camera. It may be preserved in a sensitive state for several days. Some persons add a few drops of a solution of iodide of potassium to a new nitrate bath, and filter it on the fol- lowing day, in order to saturate it with iodide of silver. When this is done a new bath is not so liable to attack the iodide of silver in the film. The Exposure. — Albumenized plates, from which the excess of free nitrate of silver has been removed by washing, are, whether used in a dry or wet state, extremely insensitive to light ; but, when only slightly washed, exposed at once, and developed with a strong developer, a much shorter exposure is sufficient. This should be timed solely with reference to the shadows, the lights being left to take care of themselves. When the camera is properly constructed, so as to prevent stray light from falling on the plate, it is hardly possible to over-expose a dry, washed, albumenized plate. To develope the Image. — First immerse the plate in distilled water ; then place it on a levelling-stand, and pour over it a saturated solution of gallic acid, to which a few drops of aceto-nitrate of silver have been added. The development occupies about twenty minutes. To fix the Picture.— -Wash the plate in rain water, and pour over it a nearly saturated solution of hyposulphite of soda. This will quickly remove the yellow iodide of silver from the film. Then wash % the plate well under a tap, and dry it before the fire. The negative may be varnished with any good varnish (see " Varnish "), but this is not always done. ALB 11 Albumen— Dia-positive Process on Glass. Positives obtained by this process are intended to be viewed by transmitted light. The manipulation is so nearly identical with that of the albumen-negative process, described in the foregoing article, that it is only necessary to point out the difference between them. The negative to be copied is placed either in a copying camera (see " Copying Camera "), or in direct contact with the sensitive plate in a pressure-frame. In the latter case the plate must be used dry, and the exposure to diffused daylight, or artificial light, only occupies a few seconds ; in the former case, the plate may be either dry or wet, and the exposure is considerably longer. The wet process is the least troublesome, and yields the best results, because the operations of exciting, exposing, and developing may then succeed each other at once, and less time is allowed for a combination to take place between the silver and albumen, which causes the lights of the picture to assume a yellow tint. The development is also a much quicker operation in the wet process, more nitrate of silver being allowed to remain on the plate, and for this reason also the lights are less likely to assume a yellow tint. The difference between this and the negative process consists chiefly in the employment of a gold toning-bath, in order to vary and improve the tint of the finished picture. This may be done in the ordinary gold-toning bath (see " Toning Bath"), and the picture afterwards fixed with hyposulphite of soda. Dia-positives on glass should be viewed with the plain side of the glass next the eye ; and against the film the rough side of a finely- ground glass should be placed, the two glasses being bound together at the edges with a strip of tape or paper pasted over them. In this way the print is protected from injury, and has a proper semi- transparent background. The chief use of this process is for printing transparent slides for the stereoscope. In this operation it must be remembered that the picture taken from the right station must be viewed by the right eye, and vice versa ; and also that the objects in the view must not be reversed as regards right and left. It may therefore be necessary to place the negative in the copying frame with its back to the lens. Matters of this kind must be carefully considered by the operator ; and his ingenuity will suggest the proper way of proceeding in every case. No general rules need be laid down in this place. Albumenized Paper. Paper which has been covered with a coating of salted albumen. This paper is used in the positive printing processes in combination with nitrate of silver. The object of 12 ALB the albumen is to give to the surface of the paper a finer finish, and to ensure a greater variety and brilliancy of tints in the prints obtained. It also gives the power of rendering the details of the negative with extreme sharpness and great transparency in the shadows. But, on the other hand, the glazed surface is considered by some objectionable, as being vulgar, and inartistic. To prepare the Albumen. — Take a sufficient number of fresh- laid fowls' eggs, which, on an average, contain one ounce of albumen each. Break each egg on the edge of a cup, and collect the white, carefully rejecting the germ and yelk. Put all these whites together into a large basin, and for each egg add from 5 to 15 grs. of common salt, the weight being regulated by the following considerations : — If the subsequent exciting solution of nitrate of silver be a weak one (not exceeding 50 grs. to the ounce), the proportion of salt should be low ; but if it be strong (90 grs. of silver), from 10 to 15 grs. of salt may safely be used. The higher salted albumen, with a strong exciting bath, is the most sensitive to light, and prints with the deepest tone. Should it be required to give less glaze to the paper, add to the albumen 1 ounce of water for each egg, and rather more than a corresponding quantity of salt, because less of the solution will now adhere to the paper. When the materials are mixed in the proportions which may be thought best for the object required, beat up the whole into a stiff froth with a bunch of quills. This cannot be done too perfectly ; for the more completely the membraneous cells containing the albumen are destroyed, the less liability is there to streaks on the paper* The addition of a few drops of ammonia to the albumen before it is whipped up assists to break the cells, confers greater limpidity, and is generally supposed to preserve the solution from putrefaction for a longer time. It is also not injurious to the paper, because it evaporates while the paper is drying. Let the frothed albumen be transferred to a tall jar (covered over to prevent the ingress of dust), and allowed to repose for at least twenty-four hours. By that time the greater part of the vesicles will have condensed, and the membraneous shreds subsided to the bottom. Then decant gently into a flat-bottomed and shallow dish, the upper portions of clear liquid, avoiding, if possible, the passing over of particles of froth which may be still swimming on the surface, and also the membranes and other impurities which, have settled. A. good way of avoiding these is to filter the solution through two folds of muslin, taking care at the same time that the nozzle of the. ALB 13 funnel is very close to the bottom of the flat dish, otherwise another crop of vesicles of froth will be formed on the surface of the filtered liquid, which will have to be removed before a sheet of paper can be albumenized smoothly and regularly. To albumenize the Paper. — The paper, having been selected according to taste, or the requirements of the process to which it is to be applied, is marked on the right side; i.e., the smooth or upper side of the sheet, which does not show the wire-cloth markings. It is sometimes difficult to detect these wire-marks at a glance, when the paper has been rolled under great pressure, as it usually is ; but it is important that the opposite or smoothest side of the paper should be covered with the albumen, otherwise the marks will be painfully conspicuous in the finished prints. A practised eye can detect them instantly, by holding up the sheet to a strong light, and looking on its surface at an acute angle with the incidence of the light. The safest way of detecting them is to float a small portion of the paper in water for two or three minutes, when the effects of the rolling disappears, and the small right-angled net-work made on the paper by the wires is easily discernible at a glance by any one. At the paper-mills the sheets are all laid in one direction, so that when the right side of one sheet is ascertained, the proper face of all the rest in the ream lies in the same direction ; but in purchasing a quire from a retail house, it may happen that it is made up of odds and ends placed anyhow, in which case each sheet must be examined before being albumenized. The albumenizing should be conducted in a warm and dry room. If not, the albumen sinks into the paper by absorption, before it has had time to dry, and the surface of the paper is left as dead as it was originally. This is the first precaution to adopt. Secondly, arrangements must be made to continue the operation of albumenizing without intermission ; otherwise stringy fibres, the effects of evaporation, are found on the surface of the liquid, and the next sheet laid down is either covered with air-bubbles, or the superfluous albumen drains away in streaky channels. Thirdly, the room should be as free as possible from particles of floating dust, but they should not be laid in the usual way, by watering the floor, because that creates a damp atmosphere, w T hich retards the drying. Lastly, the paper should be perfectly dry. When the above precautions are attended to, and the plain paper piled in order, with the smooth face downwards, and the narrow- end towards the operator, all is ready for work. Pick up the sheet by the upper right and the diagonal left lower corners, with the ALB respective hands. Lay down the lower right-hand corner of the paper on the surface of the solution, close to the edge of the dish nearest the person ; allow that end of the sheet to drop gently towards the left, taking care that no other part of the paper shall touch the albumen ; then, the instant that the left hand is disen- gaged, it should seize the upper left corner of the sheet, and roll it over with slow and gentle pressure. One minute, as a general rule, is quite sufficient for the paper to lie in contact with the albumen. Strongly-sized papers may remain longer with advantage, but those of an absorbent nature should be removed very quickly, to prevent the albumen from penetrating too deeply into its texture, the effect of which is always flat mealy pictures. Whilst the paper is lying on the albumen solution, the operator should watch for air-bubbles. They are easily discernible by a wrinkling-up of the paper at the part where they occur. In such cases have a camel' s-hair brush near at hand, dip it in the albumen, and, after gently raising the paper by the corner nearest to the bubble, moisten the dry part with the brush, and lay down the paper again, when no bad consequences will ensue. The next sheet laid down will probably have an air-bubble at the same spot, from the dried external fibrous outline of the last, unless special care be used. The best way to avoid their recurrence is to rake over, to the edge of the dish, with the brush, the superficial deposit formed at that point before laying down another sheet. In removing the paper from the albumen, it should be seized by the opposite corners at one end, and raised very slowly and steadily, avoiding jerks ; the object being to take up as little surplus albumen as possible, which, if in excess, will run into lines instead of drying into an even surface. Allow to drain for about a minute, over the dish, keeping the sheet fully extended ; then hang it up in the same position by two clips at opposite corners, or over a thick roller, to dry. The drippings may be collected in a convenient vessel, and used again, when slightly diluted. The first few sheets floated on an albumen solution are seldom perfect, no matter what precautions are adopted. There are always little surface-bubbles, and floating stringy fibres, which cannot be avoided ; but they adhere to the paper and are soon removed. The fibrous matter forms again on the surface, after a time, unless the albumenizing of the paper is continuous, and some sheets may be wasted before they are entirely removed. The best way to get rid of them, when a cessation of work is necessary, is to float on the solution a sheet of rough blotting-paper before using the other. In the course of drying, thin paper is apt to curl up before the ALB ALC 15 excess of albumen has reached the draining corners. The solution in consequence may flow back and form streaks. This only occurs when the room is very warm ; but it may be avoided by suspending two clips from the lower corners, or by a plan which the profes- sionals sometimes adopt of moving the papers from place to place. Albumenized paper should not be stowed away, nor piled together, until thoroughly dry. Professional albumenizers almost invariably roll their paper under great pressure before sorting it into quires. This is not only unnecessary, but it is deceptive, because it gives a fictitious glaze which disappears as soon as the paper is laid down on the sensitizing solution ; it may also be injurious by impregnating the paper with particles of iron or copper from the rolling machinery. Albumenized paper will keep without deterioration for many months, if stored in a dry place, but moisture or damp air soon destroys it. See also the article " Paper." Albumenized Paper— Sutton's Patent. This paper is prepared with a solution of india-rubber in benzole, before being albumenized, in order that the albumen may not sink so deeply into it, and that the resulting picture may be as much as possible upon the surface of the paper, and therefore more brilliant, as well as probably more permanent. It is supposed by the inventor of the process that the benzole also removes from the paper some deleterious colouring matter, and renders it whiter. Necessarily this extra expenditure of time and materials adds to the expense of the paper, although it undoubtedly improves its quality. Albumenized Paper, Printing on. See " Printing." Alcohol. Formula, C. t H fi 0 2 = 46. This organic compound is a hydrate or hydrated oxide of ethyle, and is formed when ether and water meet in the nascent state. Practically, however, it is obtained from solutions of sugar, malt, or other saccharine juice, by fermen- tation. The fermented liquor contains alcohol diluted with other substances, from which the alcohol, being more volatile, can be separated by distillation. After the first distillation, it will still contain from 30 to 40 per cent, of water, besides a volatile oil. By a second distillation it is still more highly rectified, and the product is called spirits of wine. A third distillation brings down the specific gravity to '825 from -836, and the product goes under the name of rectified spirits of wine. It still contains from 10 to 20 per cent, of water. But it is impossible, by mere distillation, to reduce alcohol to a much lower specific gravity or to a more absolute state without having recourse to other methods of. dehydrating it. This is done 16 ALC by mixing with the re-distilled liquid some substances which have a powerful affinity for water. The one most usually employed is quick- lime, which is reduced to a fine powder, and mixed with the spirits of wine in a retort. The neck of the retort is closed, and the whole allowed to digest for several hours, with occasional shaking up. The highly-rectified alcohol is then distilled over by means of a water bath, at a lower temperature than boiling water, care being taken that the distillation is not carried too far, otherwise the alcohol would be contaminated with lime. Absolutely dehydrated alcohol has probably never been prepared, and therefore its specific gravity has been differently given by various investigators. The specific gravity of that kind which should be used in the preparation of collodion may vary from *805 to '820. The less rectified varieties are apt to make the collodion glutinous and difficult of manipulation. An inferior quality may be used in other photographic preparations, such as for dissolving caoutchouc and gutta-percha, for making plate-cleaning solutions, for adding to the developer to make it flow well, and for many other purposes where its strength and freedom from impurities are of minor importance. The highly-rectified alcohol for collodion should not contain essential oils, because they quickly contaminate the nitrate of silver bath with organic matter. Fusel oil is the one most likely to be met with ; but it is easy to avoid its passing over in the final recti- fication by distilling at a low temperature — fusel oil being less volatile than alcohol. The tests for an alcohol fit for making good photographic collodion are : — 1st. It should not redden litmus paper, nor restore the blue colour to litmus paper already reddened. 2. It should give no precipitate with nitrate of silver. 3. When mixed with a few drops- of solution of nitrate of silver, and subjected for some hours to sun- shine, there should be no discoloration. 4. The specific gravity should not exceed '820. Alcohol, Methylated. The high excise duties levied on the manufacture of alcohol from sugar, malt, &c, for potable purposes, interfered much with its extensive use in the industrial arts. It was i therefore decreed by the Government that 90 parts of spirits of wine might be mixed with 10 parts of purified wood spirit, and sold, free of duty, under the name of "Methylated Spirit," for manufac- turing purposes. The addition of the " wood spirit," it was then supposed, rendered the mixture so unpalatable and nauseous that it could not be drunk nor employed for any other object than dissolv- ing gums, making varnishes, &c. ALK 17 The ordinary rectified methylated alcohol, which is very cheap, will answer well for all photographic purposes, save one — the manu- facture of collodion. But, by some recent contrivances of filtering it through, or re-distilling it over, charcoal, not only may the spirit be rendered potable, but quite as well suited for making collodion as the purest alcohol. The process to which it is subjected deprives the methylated spirit both of its disagreeable smell and of its nauseous taste, and also fits it for the manufacture of a collodion quite as good as from pure spirit, while the price is only about one-third of the latter. The great objection to the use of the unpurified methylated spirit in collodion is that it always contains fusel or other essential oils, which, combining with the silver in the nitrate bath, and being, partly soluble therein, soon contaminate it with organic impurities, which cause fogging in the negatives. Alkaline Development. A term applied to the development of collodionized sensitive plates under certain conditions. The develop- ment is effected by an alkali, or an alkaline salt, combined with pyro- gallic acid. It is necessary, however, in order to ensure success, that all salts of silver soluble in water should be absent, otherwise a general fogging of the image ensues. See " Tannin Process" and " Developer, Alkaline." Alkalis. "Without being too precise, an alkali may be defined to be a substance which destroys the characteristic properties of acids ; but, strictly speaking, the term can be applied to only four sub- stances — viz., potash, soda, lithia, and ammonia. Photographically and generally speaking, any substance which changes vegetable blues to reds is an alkali. The protoxides of the metals, as a rule, possess this property, and specially do the so-called alkaline earths, baryta, strontia, and lime. In Photography, the actinic effects of alkalis are opposed to those of the acids. Alkalis have a quickening influence, and acids the opposite; but it is only by a careful balancing of the two to a certain standard, ascertained by experiment, that the best effects can be obtained. In a nitrate of silver bath used in the developing pro- cesses, free alkali is objectionable, because, when combined with the developer, its predisposing action is to precipitate silver evenly over the whole plate or paper, instead of on those parts which have been actinically affected. In the nitrate bath for sensitizing positive chlorized paper, which has not to undergo development, alkalinity is sometimes an advantage. Moderate alkalinity is also useful in the toning c 18 ALM AMM processes with chloride of gold, and in the hyposulphite fixing- bath. The most delicate test for alkalinity of any solution is moist litmus paper, previously reddened with the fumes of acetic acid, washed in distilled water, then, while still moist, immersed in the solution to be tested. The paper will turn blue instantly if much alkali be present, but more slowly according to the lessened quan- tity that may be in the solution. Almond Oil. This is a drying oil, as colourless as water, and very useful for rendering paper, or paper negatives, semi-transparent. The paper should be left to steep in it for several hours ; then be wiped and hung up to dry. If the oil should be found too thick for the purpose required, it may be thinned by the addition of benzole. Allotropic. (aXAog, another, rpo7roe, form). When the same substance exists in two or more forms, having different properties, but still chemically the same, as albumen in the liquid and coagu- lated state, the unusual form is said to be allotropic. Light pro- duces allotropic forms in some bodies, and certain temperatures do the same with others. Those who suppose the action of light to consist in a molecular disturbance of the sensitive surface, imagine it to produce by actinism these allotropic conditions of bodies. Amber. A fossil resin found on some sea-coasts, and also in seams of coal ; it is used in making amber varnish. See " Varnish." Ambrotype. Collodion positives are sometimes called " Ambro- types," in America. Ammonia. N H 3 = 17. This substance is a compound of the hypothetical metal ammonium. Its elements are according to the chemical formula given above; but whether the metal ammonium exists or not has never been shown. Ammonia is really a colourless gas, feebly combustible, and of a pungent smell ; but it may be compressed by cold or pressure. Water also dissolves about seven hundred times its bulk of this gaseous substance, and forms a solution which goes under the name of liquor ammonia fortissimus, or, when more largely diluted, of liquor ammonia. But the real strength of the liquid does not depend on these arbitrary distinctions. It can only be estimated from the specific gravity, which diminishes with concentration, or more safely, by ascertaining how much acid of a known strength a given quantity of the ammoniacal liquid will neutralize. The latter is the method which is usually employed. AMM 19 Ammonia is a powerful alkali, and is employed in several photo- graphic processes for neutralizing acidity in solutions, for removing greasy matter from glass plates, and for many other purposes. Its strength is not a matter of much importance, but its freedom from impurities is, in some cases, to be attended to and avoided. An aqueous solution of ammonia may contain carbonate of ammonia chlorine, lime, copper or lead, and empyreumatic oil. None of these can do any harm in photographic solutions, except in those of nitrate of silver. And nitrate of silver may be made the means of detecting any one of them, by adding a single drop of a twenty-grain solution to a drachm of the suspected liquor. If, after the whole has been shaken up and subjected to a strong light for an hour, no precipitate or blackening effect takes place, the ammoniacal liquor may be used for the most delicate photographic purposes. Tolerably pure ammonia can be so readily obtained at any retail shop that there is no necessity for describing here its mode of pre- paration. Ammonia-fuming Process. It has been asserted that albumen- ized and plain salted paper, excited by an ordinary solution of nitrate of silver, is rendered much more sensitive to the action of light after being subjected to the fumes of ammonia. The process has been much practised in America, but in England the advan- tages claimed for it have not been appreciated, probably for good reasons. The paper, after having been sensitized in the usual way and dried, is placed in & fumigating box, exposed to the vapour of liquid am* monia for some minutes. The ammonia converts part of the free nitrate of silver on the surface of the paper into oxide of silver, which is supposed to be the accelerating agent. For the sake of experiment, any wooden box will answer, by pinning down a sheet of sensitized paper (sensitive surface downwards, on the under side of the lid), sprinkling the bottom of the box with strong ammonia, and then shutting down the lid for five minutes. The sheet of paper is afterwards hung up in the dark room to allow the free ammonia to escape. The advantages of this process are very doubtful. Ammonia, Carbonate of. There are several carbonates of ammonia. The one used in Photography for alkaline development, and sometimes for neutralizing acid solutions, is the sesquicarbonate 2 N H 4 0, 3 C 0 2 , but when exposed to the air a white bicarbonate is formed. Ammonia, Hydrosulphate of. NIL S, HS = 51. This c 2 20 AMM substance is obtained by passing sulphuretted hydrogen gas through a solution of ammonia. It is used in Photography for precipi- tating silver from its solutions, and for intensifying negatives. It should be kept in a well -stoppered bottle away from the ope- rating room, because even in minute quantity the fumes from it have a powerful effect in causing fogging of the image. Ammonia, Nitrate of. NH 4 0, NO5 = 80. This substance is formed in the nitrate bath when the iodide or bromide of ammo- nium has been added to the collodion, or when chloride of ammo- nium has been used for salting positive paper. Ammonium. A metal which by analogy is supposed to exist, but it has not yet been isolated. Ammonium, Bromide of. NH 4 Br = 98. This salt is now largely used in the manufacture of collodion. It is more soluble in alcohol than the corresponding salt of potassium, and although not so much so as the cadmium salt, yet it has no tendency to render the collodion glutinous. It is also very stable. It can readily be prepared in a pure form by precipitating bromide of calcium with carbonate of ammonia. Ammonium, Chloride of. NH 4 CI = 53-5. Is much used in salting paper for positive printing, for which it is excellently adapted. It is prepared from the ammoniacal liquid of the gas- works, by a process which yields the salt in a very pure form. Ammonium, Iodide of. NH 4 I = 145. Is a salt very valuable in negative and positive collodion, because it has the property of giving limpidity, sensitiveness, and adherency to that compound, which, with other iodides, it often does not possess. It is, how- ever, an unstable substance, prone to liberate iodine, and to decom- pose the collodion with which it is mixed, unless its bad effects are neutralised by other iodides, such as that of cadmium, which has an opposite tendency. Iodide of ammonium is usually prepared by adding iodine to a strong solution of hydrosulphate of ammonia, until it begins to be coloured with iodine. On first adding the iodine a dense deposit of sulphur occurs. The solution is now acid. Neutralize with am- monia, filter, and crystallize. Dry in vacuo over sulphuric acid, and keep in hermetically-sealed bottles, away from light, if it be required to preserve it for a long time from decomposition. The iodide of ammonium of commerce is often contaminated with many impurities. Sulphate and carbonate of ammonia are the most frequent. To detect either, a few drops of a concentrated solution AMM ANI 21 of chloride of barium will give a white precipitate if one or the other is present. A few drops of strong acetic acid added will distinguish between the two impurities. The precipitate is quickly redissolved after this addition, if it is caused by the carbonate. Not so, if caused by the other impurity. Sulphate of ammonia, unless in quantity sufficient to materially weaken the strength of the iodide, does no other injury to the col- lodion. It is different with the carbonate. When that impurity is present in any iodide dissolved in collodion, it renders the nitrate of silver bath too neutral for good work, by a decomposition which it would be superfluous here to explain. Ammonium, Sulphocyanide of. NH 4 , S 2 Cy = 76. Has been proposed as a substitute for hyposulphite of soda in fixing positive prints. In a concentrated form it acts energetically, and may be considered a safer fixing agent than the hyposulphites, but its greater expense will probably prevent its being extensively used for this purpose. Amorphous. This term is applied to those substances which do not crystallize in a definite form. Amphitype. (Greek, ayu^t, on both sides). A process dis- covered some years ago by Sir John Herschel, in which light pro- duces either a positive or negative. A sheet of paper is first prepared with a solution, either of ferro-tartrate or ferro-citrate of protoxide, or peroxide of mercury, and then with a solution of ammonio-tartrate or ammonio-citrate of iron, the latter solution being in excess. On exposure to light in the camera, a negative is produced of more or less vigour, and of a very rich brown tint, when the paper contains a salt of lead. It gradually fades in the dark, but may be restored as a black positive, by immersing it in a solution of nitrate of mercury, and ironing it with a very hot iron. Angular Aperture. The angular aperture of a lens is the angle which its diameter subtends at its principal focus. Anhydrous. Many substances in chemistry have such a strong affinity for one or more atoms of water, that they seem incapable of existing without these atoms, or a base of some kind as a substitute. When the water is replaced by a base, or is otherwise entirely removed, the substance is anhydrous (a, not, and vltop, water). Glacial acetic acid contains one atom of water, which, when it com- bines with oxides or other bases, it liberates. Aniline. C 12 H 7 N. This very powerful base, which should more properly be called Phenylamine, is derived from indigo, 22 ANI nitro-benzole, coal-tar, from the destructive distillation of animal matter, and from other sources. When pure, aniline is a thin, colourless, highly-refracting oil, of a burning taste and aromatic flavour- Its specific gravity is I '020. With acids it forms a remarkable variety of salts which crystallize with great beauty and facility. In combining with oxygen acids it acts very analogously to ammonia, by taking up one equivalent of water : like ammonia, it also combines directly with hydracids. In almost all other respects they behave similarly, so that the two may be considered analogous bases. Aniline is remarkable for the beautiful colours which may be formed from it, most of which have been patented. With chromic acid it gives a deep green or bluish black colour, which has been taken advantage of in one of the photographic printing processes. Aniline Process. Starting from the well-ascertained fact that aniline with chromic acid strikes a deep blue colour, Mr. Willis, after long and patient experiment, has lately discovered a photo- graphic process which is designated by the above name. The process is still in its infancy, but it is already very extensively employed in the copying of engineers' plans and pencil drawings, for which it is eminently adapted. Take any sample of good photographic writing or drawing paper, pin it down on a flat board, and with a broad camel's-hair brush, or a tuft of cotton wool, moisten its upper surface evenly with the following sensitizing solution. Of course, this must be done in the dark room : — Bichromate of ammonia or potash 30 grains. Phosphoric acid solution . . 1 fluid drachm. Water 1 fluid ounce. Hang up the paper in the dark to dry. The phosphoric acid to be used in the above formula is the tri- basic form, usually sold at the shops under the name of dilute phosphoric acid, which contains 100 parts of the acid dissolved in one ounce of water. If the acid obtained is too weak, the picture will be of a reddish tint ; if too strong, it will be green. The best colour obtainable is a deep purplish black, which can easily be got by regulating the quantity or the strength of the acid. When dry, place the sensitized paper in the usual way in the pres- sure frame, under a translucent positive, if a positive copy be required, or under a negative if vice versa, and expose to light. The pro- per time for exposure cannot readily be detected by lifting up part of the paper and examining it, except by a practised eye, because ANI 23 scarcely a trace of a picture is visible after over-prolonged ex- posure. About one-fourth of the time for a silver print similarly lighted is needed. The exposure completed, take the embryo picture into the dark room and develop it in the following manner. The development should be commenced within a few hours after exposure, because bichromate of potash combined with organic matter is a very unstable compound ; and any decomposition arising from heat or otherwise may vitiate the whole results. Development. For a picture 8x5 inches : pour into a flat porce- lain or gutta-percha vessel, the bottom of which has been pre- viously covered with a sheet of bibulous paper, one drachm of the ordinary commercial aniline, thoroughly mixed with two ounces of benzole. Attach the exposed paper by wafers to a piece of glass, or by pins to a flat board, the exposed face being upwards, and now place it downwards over the mouth of the tray in such away that the embryo picture may receive and intercept the whole of the aniline vapour. The image quickly begins to show itself, and, as a general rule, if the exposure has been correctly timed, will be complete within an hour. The same solution will develop many prints, but the develop- ment is quick or slow according to the amount of aniline present in the developer and the temperature of the air. For large prints, Mr. Willis recommends that the exposed paper should be pinned to the bottom of a wooden box or chest, and the aniline mixture poured, to the point of saturation, into blotting- paper attached to the inside of the lid, which is then shut down and the developing vapour allowed to descend. When the development of the image is complete, which can easily be seen by inspection, wash the print in plain water for two or three minutes, afterwards in water acidulated with a few drops of sulphuric acid. Remove the sulphuric acid by several washings in water, and dry the picture. It may then be mounted on thick cardboard if necessary ; but in this printing process there is no reason why the picture should not be printed direct on the stiffest or coarsest cardboard, provided the upper surface is covered with a good white or tinted paper which suits the character of the print. The washing needs only to be superficial, to remove the soluble bichromate, which otherwise tints the ground of the picture to a dirty orange tone. There are other modifications of this process now under investi- gation by Mr. Willis, which it is to be hoped may supersede the unsatisfactory silver processes. 24 ANI AEGr Animal Charcoal. The product obtained by the carboniza- tion of muscle, horn, blood, bones, and other animal matters. It is distinguished from vegetable charcoal by its lustre and spongi- ness. It is used in Photography to remove colour and organic impurities from solutions of nitrate of silver, &c. In commerce it contains phosphates and other salts derived from the animal organism, which, when it is added to nitrate of silver solutions, produce a yellow precipitate of phosphate of silver, and this often to such a degree as greatly to weaken the solution. Nitrate baths have been so weakened by it as to cause the iodide of silver entirely to peel off the plate on attempting to excite it. When purified it often con- tains hydrochloric acid, which precipitates white chloride of silver. It is called in trade ivory black, bone black, &c, and its impurities are always such that it is better to employ kaolin. Animal Oil of Dippel. Has been employed as a solvent of bitumen in M. Niepce's processes. See "Bitumen." It is rectified oil of hartshorn, and is itself discoloured by solar light. Animal Substances. The animal matters used in Photography are gelatine, albumen, gluten, isinglass, and a few others; they are of service both in forming transparent films, and in them- selves combining with silver compounds for the production of images. They are substances with which English papers are sized, and hence pictures on English papers are redder in tone than those on the French and German papers sized with starch. See "Organic Matter." Aperture. See " Lens." Aplanatic (from a, without, and irXavr), deviation). Is a term used in Optics to denote a lens so constructed that spherical aberration is entirely destroyed. Aqua Regia. Nitro-hydrochloric acid, q. v. Aqua Fortis. Nitric acid, q. v. Argentometer. Is a name given to a little instrument for mea- suring the strength of solutions of nitrate of silver. It acts on the principle of the hydrometer, and is so constructed and graduated that when allowed to float in a nitrate solution, the number of grains of nitrate of silver to the ounce of water is indicated by the number on the scale at the surface of the liquid. This instrument is accurate enough for the wants of the photographer. But a more correct method of determining the strength of a nitrate of silver solution consists in precipitating an accurately- ARE, ART 25 measured portion of it by means of a carefully-proportioned solu- tion of chloride of sodium, and then from the tables of atomic weights, calculating the percentage of silver present by comparing it with the weight of the sodium salt required to precipitate the whole of the silver as a chloride. Cheap apparatus, and the accurately-weighed pure reagent, with instructions how to use them, are sold at most of the dealers in photographic chemicals. Arrow-root. Is the common name for the starch derived from the tubers of the Maranta arundinacea, by a process very similar to that by which starch is obtained from potatoes. It is often much adulterated with potato farina, and other more objectionable substances. In its pure form it constitutes an excel- lent subsidiary sizing for photographic paper, giving a surface almost equal to ivory, and being in other respects useful in improv- ing the tone of the image. Artificial Light. In a photographic sense, artificial light is one by which negatives may be taken in the camera, or by which the different printing operations may be performed. The photo- grapher may therefore avail himself of this actinic property of various kinds of artificial lights to work at night, or when day- light is too feeble for his purpose. Of all artificial lights, that produced by the ignition of mag- nesium is one of the most actinic (see " Magnesium"). This metal can now be obtained of a pure quality, and at a cheap rate. It is generally sold in the form of wire, flat ribbons, or filings for illuminating purposes, and is used to a considerable extent in Photography. For the method of using it, see " Magnesium Lamp." The electric light produced by a powerful galvanic battery, having wires tipped with charcoal brought into close proximity, is also highly actinic ; but the light is very expensive, and is scarcely used. A burning jet of the mixed gases, hydrogen and oxygen, under pressure, and brought to bear on a ball of lime, is much used in the enlarging process by development. See "Enlarge- ments." Common gas burners, Argand, oil, and paraffme lamps, are also used in photo-micrography, for printing on dry collodion plates, and for some other photographic purposes. Various chemical compositions have been employed for pro- ducing, by their rapid combustion, an intense actinic light, acting during a few seconds only. They need not be particularized here, 26 ASP BAC because they have now been almost entirely superseded by those mentioned above. The effect of all artificial lights is greatly increased by placing them in the focus of a parabolic reflector, so as to throw the whole of their illuminating power on the object to be photographed. Asphalt, Asphaltum, or Bitumen of Judasa. Is an indu- rated pitch, first found on the shores of the Dead Sea, in Judaea. It is now obtained in more abundance from Texas, Trinidad, Bar- badoes, and other places. It is somewhat heavier than water, is easily melted, and very inflammable. It may be purified by boiling in water, when the pure asphalt floats on the surface, and the im- purities subside ; by the action of hydrochloric acid, which dis- solves out the carbonate of calcium with which it is associated, or by dissolving the pure substance from the crude material by oil of turpentine. It is the basis of most black varnishes, being soluble in several substances. It has also been used in some photographic processes, first by M. Nicephose Niepce, in his heliographic process, after- wards by M. Negre, of Paris, and Mr. Macpherson in photo- lithography, and by Mr. Pouncy in his carbon-printing process on paper. Astigmation. (Greek, a privative, ariy\xt], a point). A term used in Optics, to denote the want of a sharp point, or focus, in a pencil of rays. Astro-photography. A convenient name for the application of Photography to the delineation of solar spots, the moon's disc, the planets and constellations. Avoirdupois Weight. See " Tables of Weights and Mea- sures " at the end of the volume. Background. This is a screen placed behind the sitter in a photographic studio, or behind any object to be copied, in order to cut oil" the view of surrounding objects. In photographic portraiture, the background is usually made of a frame of wood covered with canvas, painted in distemper, either plain, shaded, or scenic, or with a broad sheet of woollen stuff, without a seam, dyed with some colour. Scenic backgrounds are now manufactured to suit every conceivable variety of taste in the photographer, whether good or bad. On a matter which must be so purely one of taste, no rules can be laid down beyond this general one, which is pretty self-evident ; viz., that no colour except some shade of grey should be used either for painting or dyeing the back- BAL BAT 27 ground ; for where can be the use of introducing tints of red, yellow, blue, &c, which are not rendered truthfully in Photography by the corresponding shade, and which can only have the effect of misleading the eye of the artist ? Of course, the background should on no account present a glazed surface. Balance. A machine for weighing bodies. For practical photo- graphic purposes a balance, in either of the scales of whicli a two grain weight will throw bhe beam, is sufficiently fine. Balances of this kind may be purchased at the photographic warehouses very cheaply. Barium. A white and lustrous metal, which gradually oxidizes in the air. Barium, Chloride of. BaCl + 2 H 0 = 122. Is sometimes used for salting positive paper. It may be prepared by neutralizing diluted hydrochloric acid with the native carbonate of the same metal. By evaporation crystals of the chloride are obtained in flat, transparent, four-sided tables. It has been stated that positive paper salted with chloride of barium gives a redder tone to the print than the other chlorides. This opinion may have arisen from the fact that it contains less chlorine, in the same weight of salt, than some others. Chloride of barium is used as a test for sulphuric acid, with which it forms an insoluble compound. It will also detect two very common impurities in iodide of potassium ; viz., a carbonate and a sulphate, by forming a precipitate in the solution. Base. When a compound body possesses the property of neutralizing acids, whereby the distinguishing characteristics of both disappear, it is called a base. Oxygen is an element in almost all bases. A metal with oxygen forms an oxide often more than one of which are bases, and unite with acids. When a compound so formed is neutral, it is called a neutral salt ; when not neutral, the salt is called basic or acid, according as the acid or base may be in excess. The salt is named from both the ingredients. Bath. The vessel in which solutions are contained, when in use in photographic operations, is sometimes called a " bath." For all home purposes, glass is the best material for such vessels, because it does not contaminate the chemicals, and can be easily kept clean. The next best material is the " vitreous ware," manufactured by Messrs. Edwards & Son, of the Dalehall Potteries, Burslem. This is much cheaper, and at the same time much stronger than glass, and almost equally free from the risk of contaminating the 28 EEA BEE chemicals. A material is used for baths in America which some- what resembles Wedgwood ware, and appears to give general satis- faction ; it is called " Photographic ware." The common white earthenware baths and dishes very soon acquire abraded surfaces ; and in those spots where the glaze has been rubbed off or eaten away, the chemicals are acted on by the unprotected material beneath. For the purposes of the travelling photographer, pure gutta-percha seems to be the best material for baths and dishes, on account of its not being liable to be broken. The Gutta-Percha Company, in the City Koad, supply a great variety of baths, funnels, &c, for photographic purposes, made of a sufficiently pure kind of gutta-percha. Ebonite is another material which has been used for baths. Messrs. Silver & Co. are the patentees, and have a manu- factory of this substance on the banks of the Thames, opposite to Woolwich. It makes a hard, light, portable vessel, which does not seem to affect the chemicals contained in it, but is somewhat fragile under rough treatment. Papier-mache has also been used for baths, but is not to be recommended. Wood coated with shellac is a cheap substitute for a better material, when the vessel is of large size. All baths and trays should have the corners properly rounded, and no sharp angles left, in which dirt can accumulate. Glass baths should be solid, and not built up of pieces cemented together with marine glue, as these are very liable to come apart. Messrs. Edwards & Son manufacture many very useful kinds of baths and trays, of novel and ingenious design, to meet the varied wants of the photographer. They can be procured from any of the principal dealers in photographic goods. Beaufoy's Acetic Acid. Is the acetic acid fortiss. of the Phar- macopoeia. It is much cheaper and weaker than the glacial acid, and often contains impurities which do not unfit it for adding to the developer. It should not, however, be used in the paper processes for making aceto-nitrate of silver, or, if no other can be had, three times as much must be added as the formula for glacial acetic acid would indicate. Beer Process. A collodion process, wherein the plate, after having been sensitized and washed in the usual manner, is dipped in or washed over with an infusion of malt or with beer. Plates so prepared may be preserved in the dark for a considerable time, and afterwards exposed in the camera, or they may, when dry, be used for producing positive transparencies by contact with a negative. The process has little to recommend it, except its simplicity. BEL BIN 29 Bellows Camera. A camera made to expand in the same manner as an accordion. The principle is good for some purposes, such as copying, but is not to be recommended for out-of-door work, because the instrument becomes liable to serious injury by being accidentally exposed to rain ; the motion of its bellows side in the wind is also liable to detach particles of dust from the in- side which may settle upon the film. The proper adjustments of a view camera are more difficult to apply when it is made on the bellows principle ; and steadiness is thereby sacrificed. Neverthe- less, the bellows form of view camera is a favourite with many amateurs ; and Mr. Kinnear, of Edinburgh, has devised a form of it in which he seems to have made the most of the principle. It can be obtained from any of the leading camera makers. Benzole, or Benzine. Is obtained in a pure state by distilling one part of crystallized benzoic acid with three of slaked lime. Com- mercially it is prepared on a large scale from coal naphtha, which contains it in abundance. It is a limpid, colourless liquid of an agreeable etherial odour, insoluble in water, but mixible with alcohol. Benzole is used in dissolving many gums and resins for varnishes in Niepce's heliographic and Pouncy's carbon photographic processes, for dissolving bitumen of Judaea, and for some other purposes. It also dissolves caoutchouc and gutta-percha, forming solutions much used as a substratum for the collodion film in the dry processes, and for filling up the pores of positive paper previous to applying the albumen. The latter application has been patented by Mr. Sutton. Berlin Ware. This is a kind of pottery marked with a blue stamp, and of such a quality as to resist the action of nitrate of silver, even when fused in it. Ordinary ware will not do for this purpose. Binocular Camera. This is the name absurdly given to a stereoscopic camera fitted with a pair of lenses which are equal and similar in all respects, and have their axes parallel, and their centres from 2 \ inches and upwards apart. The principle on which such a camera is constructed is perfectly sound ; as will be seen by reference to the article on the " Theory of the Stereoscope." Binocular Perspective Portraits ; Binographs. This term has been applied to photographic portraits produced in a camera which is made to revolve in a horizontal plane through an angle, its axis being always directed to the same part of the sitter. It wa3 supposed that in this way a single portrait could be obtained, which would exhibit the same effects of solidity as duplicate pictures 30 BIN BLI viewed in the stereoscope. This notion is, however, erroneous, as will be found explained in the article on the " Theory of the Stereo- scope." Binocular Vision. Vision when both eyes are employed. The term is used in contradistinction to monocular vision, in which only one eye is employed. The effects produced by binocular vision are very remarkable. The subject will be found discussed at some length in the article " Vision." Bitumen of Judaea. See " Asphalt." Black Varnish. See " Varnish." Blacking for the Inside of Cameras, &c. Of all things which have been tried for covering the inside of lens tubes, cameras, and other pieces of apparatus, nothing has been found so suitable as black cotton velvet. Tubes are sometimes blackened by a solution of bichloride of platinum in water, which precipitates platinum as a smooth black layer over the metal on which it is brushed. Some- times bone black or ivory black is ground up with water and a little glue to a proper consistence, and applied as water colour to the surface. But both the colour and texture of black velvet are infinitely superior for this purpose to either platinum or charcoal ; the pile of the velvet acting as so many little welb into which the light enters and is lost. Nothing takes a longer exposure than this, if it be desired to photograph it. Blisters ; Blistering. The formation of blisters in the film is a rather serious trouble, which occurs in some of the dry processes. It happens when the film, to which certain kinds of preservative have been applied, and allowed to get dry upon it, is wetted a second time in the development or subsequent washings. The blisters vary in size from a pin's head upwards, and are often very numerous. They do not occur with all kinds of preservative, and are most troublesome when albumen or gum arabic is employed. When a blister is formed in a film during development, it generally gets filled with the black liquid, and when dry leaves a black spot ; but when formed during the subsequent washing or fixing, it merely leaves a small dark ring, denoting its boundary line, where it disappears on drying. One very good remedy for blistering of the film is to give the plate a preliminary coating of india-rubber dis- solved in kerosolene, chloroform, or benzole. But it is better, if possible, to avoid the evil by going directly to the source of it, and so modifying the mechanical condition of the preservative BLU 31 as to prevent its occurrence. The addition of ammonia to the albumen, in the Taupenot process, seems to have a beneficial effect, by rendering the albumen more limpid, and less horny when dry. Blistering is evidently owing to the expansibility of the film when wetted after being dried, and its non-adhesion to the glass in the spots where the blisters occur. A dried collodion film never blisters ; nor does it when coated with tannin. It is some peculiarity of other preservatives which causes it to blister when impregnated with them. Heating the prepared plate strongly before the fire, as some persons have recommended, has no effect whatever in the prevention of blistering. Blue Glass. The question whether blue or white glass is the best for the portrait-room has been frequently discussed. Blue glass increases the time of exposure more or less, according to the depth and shade of colour, but, on the other hand, it acts beneficially in modifying the glare of light which is sometimes injudiciously admitted into the glass room, and in this way relieves the eye of the sitter from a painful annoyance, which causes an unpleasant expression. Blue glass stained with cobalt is said to be very per- manent in colour. White glass containing manganese is acted on by light, and becomes of a reddish tint, which is very objection- able. Blue Meniscus. It has been proposed to use a blue meniscus instead of an achromatic lens for taking views, partly on the ground of economy, and partly because it was believed that a better picture might be produced. But these supposed advantages of a blue lens have not yet been demonstrated practically ; on the contrary, a blue lens appears to have the disadvantage of increasing the time of exposure, and of yielding an inferior picture to that produced by an achromatic lens. This might have been expected ; for an achro- matic lens not only combines two of the coloured foci, but com- presses together, so to speak, all the other coloured foci, some of which are more or less actinic. It seems impossible to obtain a satisfactory picture with either a colourless or blue meniscus, in consequence of their having no single actinic focus in which the principal actinic rays are collected. Besides this objec- tion, no single lens can have its spherical aberration reduced in the same way as a compound lens can. Blurring. A spreading of the dark parts of a negative over the lights. It is produced by pencils of light passing through the film into the body of the glass, and then suffering internal reflection at the back of it, and thus acting again upon the back of the film in a 32 BOD BUO different part. It occurs, of course, in both the wet and dry pro- cesses ; but more generally in the dry, because the film is then more transparent, and does not so effectually impede the passage of actinic rays. The remedy for blurring is to paint the back of the dry plate with a non-actinic water-colour, which dries sufficiently transparent to allow you to watch the development of the picture through it ; or, in the case of a wet plate, to lay a piece of wet red or orange coloured bibulous paper against the back of the plate, before putting it into the slide. If this precaution be taken, a class of subjects which, on account of the strong contrasts of light and shade which they exhibit, have offered much serious difficulty to the photogra- pher, are brought more under his control. Body. This term is applied to substances, in allusion to the quantity of stuff or substance which they contain ; in meaning, it is opposed to thinness, weakness, transparency, and flimsiness. Hence we have papers without body, and with body ; body colour in oppo- sition to transparent colour ; and photographs or pictures with no body, in opposition to those which have plenty of reduced material and of vigour. Bowl Negatives. Negatives can be taken, by means of the panoramic lens, upon the segment of a sphere, including a hundred degrees of angle of view in all directions. Prints can be obtained from such negatives, either by a process of copying upon another bowl, by means of a panoramic copying lens placed at the common centre of both, or by laying a sensitive piece of silk upon the convex side of a suitable cushion, and pressing the negative into contact with it. In the latter case distortion is introduced in the picture when flattened out. Bromides. Under this term is included a class of metallic salts which are of great importance in Photography. The most useful bromides will be described under the heading of their distinctive metals. Bromine. Br = 80. Is usually extracted from bittern (the mother liquor which remains after crystallizing salt from sea water or salt springs) by the action of chlorine. Bromine at ordinary temperatures is a brownish-red liquid, and very volatile, the vapour being poisonous. It is slightly soluble in water, more so in alcohol, and still more in ether. In the daguerreotype process, it is used to give a much greater sensitiveness to the iodized plate than it would otherwise possess. For this purpose it may be used in an aqueous solution, or, prefer- ably, in the form of bromide of lime. See " Daguerreotype." BRO BUP 33 Bromised Collpdion Process. See "Collodion Bromised." Bronze in Photographic Printing. In many processes, both of sun -printing; and development, the darkest parts of the impression assume after a time an olive-green colour, lighter than the dark brown which immediately preceded it. This has been called bronze, and its production bronzing. It occurs when the paper is rich in silver salts and organic matter, and especially when there is abundance of free nitrate and the exposure is prolonged. With very vigorous negatives to print from, this may be excessive, but there are means of pre- vent ing and also of remedying the extreme action. The paper must be rendered less sensitive, or the bronzing can be removed by chloride of gold and other means. See "Printing." Brunswick Black. For the composition of this black varnish see "Varnish." Buckle's Brush. The little article which passes by this name is made by pulling out some fine cotton wool by the fingers, which is then placed in a hook formed at the end of an annealed copper or silver wire, and drawn tightly into a suitable glass tube to serve as a handle. It may be used on albumen or paper surfaces, and is, in many respects, the most convenient method of applying chemical solutions. The solutions, when it is used, remain constant in their action, and the papers are more uniformly of the same strength ; camel's-hair and other brushes are immediately destroyed by nitrate of silver ; this brush may be hourly renewed. Buff, and Buffing. A buff is used in the daguerreotype process for polishing the silver plate, and in the glass processes for polishing the glass plate. In the former process, it may be either a hand-buff or a buffing-lathe. The hand-buff is made of a piece of deal about sixteen inches long and five inches wide, having a handle at one end, and covered with velveteen, the fine ribs of which are laid across the buff. The buffing-lathe is a wheel in the shape of the frustrum of a cone, the axis of which is the axis of rotation, covered with wash- leather or doe-skin, and turned by the loot, lland-buffs are gene- rally kept in pairs, face to face, to prevent particles of dust from settling on the velveteen. The hand-buff used for polishing glass plates has a handle on the back, like a blacking brush, the front being padded, and covered with wash-leather. To use the buffing- lathe, the wheel is put into rapid motion by means of a treadle worked by the foot, and the plate held against the leather. In using the hand-buff, the operator puts an old kid glove on the right hand, takes the handle of the buff in the left, l) 34 BUR CAD rests the end of it on the edge of a table, and lays the face of the plate on the velveteen ; then, putting the fingers of the right hand on the back of the plate, he rubs it briskly backwards and forwards until it has received a sufficient polish ; the plate being turned occasionally diagonally and crossway3 on the buff, and considerable pressure being employed. A glass plate should first be rubbed dry with a cambric handker- chief, then laid on a pad of paper, and rubbed in every direction with the buff, until the dew from the breath condenses on it in an even sheet, without exhibiting marks or patches. The plate, whether metal or glass, should be polished immediately before use, and the clean surface should on no account be touched with the fingers. Burnishing Photographs. This operation is performed by rubbing the photograph, when mounted on cardboard, with a polished piece of agate set in a stick ; the object being to give a finer surface and more brilliancy to the print. Cadmium. Cd = 56. This metal in appearance much resembles tin, but in its chemical qualities it still more nearly resembles zinc, in the ores of which it is found. Zinc is the only metal more electro- positive than cadmium, and therefore the only one which will pre- cipitate it from its solution in the metallic state. It is a scarce metal. Air and moisture scarcely act upon it, except it is heated ; and this quality makes it valuable in Photography, because its salts in collodion are extremely stable, and in the crystalline state they have little tendency to deliquesce and oxidise. Cadmium, Bromide of. Cd Br = 136. Occurs in acicular crystals or nacreous scales, and is formed by mixing bromide and filings of cadmium in water. The acicular crystals contain water, but when sublimed the bromide condenses in anhydrous pearly scales. It is preferred by many to the other bromides, for the same reasons as the corresponding iodide ; viz., its stability, solubility, and purity. Cadmium, Iodide of. Cd I = 183. This salt is obtained by heating filings of cadmium with iodine, or mixing them in a moist state. It crystallizes in large white six-sided tables, of a pearly lustre, which are fusible, and decomposed at a high temperature. Iodide of cadmium is very soluble in water and alcohol. When pure, its alcoholic solution is permanent, and becomes discoloured very slowly and slightly by exposure to sunshine. Collodion iodised with iodide of cadmium does not become discoloured, or undergo any visible alteration by time, if kept in a cool dark place. This is CAL 35 a great advantage ; but, on the other hand, the nitrate of cadmium formed in the nitrate of silver bath is a salt which has an acid reaction, and its effects are similar to those produced by free nitric acid. The nitrate bath, therefore, gets gradually out of order when this iodiser is employed. The permanence of collodion iodised with iodide of cadmium is probably due to the difficulty with which cadmium is oxidised. The common impurity of iodide of cadmium is iodide of zinc. When this is present the collodion becomes gradually discoloured ; zinc being an easily oxidisable metal. Iodide of cadmium impairs the fluidity of collodion. Calcium. Ca = 20. An extremely oxidisable metal of which lime is the oxide. Little has been ascertained of its properties by actual experiment. Its compounds with the halogens are generally • deliquescent, and prone to oxidise. Calcium, Bromide of. Ca Br = 100. Obtained by digesting hydrate of lime in a solution of protobromide of iron, by heating lime in bromine vapour, or by dissolving carbonate of lime in hydro- bromic acid. It is white and deliquescent, and its aqueous solution yields silky hydrated crystals. It is not so good as bromide of potassium for preparing sensitive papers for keeping, because the nitrate of lime formed is deliquescent, and keeps the paper moist, and therefore more prone to deterioration. Calcium, Chloride of. Ca CI =55*5. This salt is generally prepared by dissolving chalk in hydrochloric acid. It is extremely soluble in water, its attraction for water being so great that it soon deliquesces in the air. On this account it is employed, when freshly fused, to deprive gases of aqueous vapour, and ether, alcohol, &c, of water. It is copiously soluble in alcohol, ten parts of anhydrous alcohol, sp. gr. '794, dissolving seven of the chloride, the solution in cold weather affording crystals which contain sixty percent, of alcohol instead of water of crystallization. It is not well adapted for salting- positive printing paper, because the nitrate of lime which is formed is very deliquescent, and keeps the paper moist. Calcium, Iodide of. Ca I = 147. This salt is obtained by dissolving carbonate of lime in hydriodic acid, evaporating to dry- ness, and fusing the residue in a close vessel. W hen this is dis- solved in water and evaporated, it furnishes white deliquescent crystals. Iodide of calcium is extremely soluble in water, and alcohol, even when absolute. The alcoholic solution is discoloured by light, and D 2 .36 CAL collodion iodised with it becomes gradually reddened, as with the potassium salt. It is not much used for iodising paper or collodion, being inferior to some other iodides. Calomel, or Subchloride of Mercury. Hg 2 CI = 235 5. This salt is formed in the positive collodion process when chloride of mercury in solution is poured over the picture. Ammoniacal gas blackens it, but heat restores the colour, and calomel remains unchanged; liquid ammonia decomposes it, and black suboxide of mercury is one of the results. Calorific Rays. The heat rays of the spectrum. See " Spec- trum." These rays, which reside principally at and beyond the red end of the spectrum, bear a strong analogy in some of their proper- ties to the luminous and actinic rays. They proceed in straight lines, can be reflected from mirrors, refracted through lenses of rock salt, collected into foci, and polarized by various substances. The calorific rays which proceed from terrestrial bodies are almost entirely stopped by glass ; therefore the lenses used to produce the image upon a sensitive photographic tablet would, by intercepting those rays, prevent any action, injurious or otherwise, which they might produce if allowed to fall upon the chemicals composing the sensitive surface. Calorific rays, however, which proceed direct from the sun are not intercepted by glass ; therefore the above remark does not apply to the case of obtaining a photograph of the sun's image formed by a lens. The examination of images formed by calorific rays, refracted through lenses of rock salt, and received upon various chemically-prepared surfaces, would be an interesting- study, and might lead to the discovery of a new science, which might aptly be termed Thermography. Calotype Process. (Gr. koKoq, beautiful, tvttoq, an impression). This is a negative process upon paper, and a very convenient one for the tourist, in which the paper is first prepared with a coating of iodide of silver, and then excited at any convenient subsequent time, by washing over it a weak solution of gallo-nitrate of silver. The following directions for performing the various operations are the result of considerable experience on the part of the writer, and will lead to good results, with much certainty and uniformity, if they are carefully carried out. But it must be remembered, as a ride, that the calotype process, in which no bromide of silver occurs, is generally considered better adapted for well-lighted architectural subjects than for views in which much dark foliage occurs. The waxed-paper process is no doubt better adapted for the latter class of subjects than that now under consideration. CAL 37 There are two different methods of preparing the paper with a coating of iodide of silver. The first is called the method by the single wash ; the second, that by the double wash. To argento-iodise the Paper by the Double Wash— Float the face of the paper upon a bath containing 20 grains of nitrate of silver to the ounce of distilled water. Let it remain a minute or two on the bath, then hang it up to dry. Next immerse it in a solution containing 25 grains of iodide of potassium to the ounce of distilled water. Let it remain a minute or two in this solution, the exact time depending on the kind of paper employed, and requiring to be ascertained by experiment. If too short a time is allowed, the whole of the nitrate of silver is not decomposed, and the paper darkens in the light ; if, on the contrary, too long a time is allowed, the iodide of potassium in the bath dissolves some of the iodide of silver in the paper. The time of sojourn in the iodide bath is therefore rather critical. On removing the paper from the bath, let it drain, and then immerse it in a pan of water, in which it must be allowed to soak, the water being changed several times, until the whole of the free iodide of potassium is removed. This soaking operation is rather troublesome, and the texture of the paper is decidedly injured by it. Should any excess of iodide of potassium remain in any part of the paper, it would decompose the weak exciting solution, and produce insensitive iodide of silver, and con- sequently a white patch in the negative. When the paper has been sufficiently washed, hang it up to dry. It is of a yellow primrose colour. When dry, it may be kept for use in a portfolio. It is not sensitive to light unless it has been excessively washed. To argento-iodise the Paper by the Single JVash. — Lay the paper upon a board, with a piece of blotting-paper under it, and brush over it a solution called "double-iodide;" i.e., a solution of iodide of silver in iodide of potassium {see " Double-iodide"). The best kind of brush is a large round one of camel's hair, bound with string or silver wire. Apply the solution copiously, both longitudinally and transversely, inclining the board, and keeping a flowing edge. Hang up the paper to dry in a room having a pure atmosphere, free from sulphurous and acid vapours, which bleach the reddish tint that the paper ought to assume. When very nearly, or even quite dry, immerse the paper in a pan of water as before, in order to remove completely the excess of iodide of potassium which it contains. Two or more papers should not be soaked in the same pan, but each paper should have a separate pan. When sufficiently washed — an operation which requires several hours — dry the paper, and put it away for use. 38 CAL Argento-iodised paper may be kept for some weeks without losing its good qualities. Some persons affirm that it may be kept inde- finitely in a dry place. It is said, on good authority, to be much improved by exposing it for an hour or two to strong sunshine. In comparing the two methods of iodising the paper which have been described, it will be seen that the first is the most economical, but at the same time the least certain. A considerable quantity of iodide of potassium is wasted in the second mode of iodising, and the washing operation is rather more tedious. The paper having been iodised by either of these methods, the remaining operations are the same, and are as follow : — To excite the Paper. — Make two solutions, one a saturated solu- tion of gallic acid in cold distilled water — which call solution A ; the other, by dissolving 50 grains of nitrate of silver in 1 ounce of distilled water, and adding 1 drachm of glacial acetic acid — which call solution B. Immediately before use, mix, in a chemically-clean measure, 1 ounce of distilled water, 15 drops of solution B, and about as many drops of solution A, the number of the latter depend- ing on the temperature and the kind of paper employed. Lay the paper upon a board, with a piece of blotting-paper beneath, and apply this mixture copiously to it with a clean Buckle's brush (q. v.) Hold up the paper to drain for a minute, then blot off the surface moisture with clean blotting-paper, and put the sensitive paper into the dark slide. The Exposure. — The average time of exposure, in a good light, with a lens of fifteen inches focus, and a half-inch stop, is seven minutes. To develop the Picture. — Lay it upon a board, and brush over it, with a clean Buckle's brush, a mixture composed of 3 parts of so- lution A, and 2 parts of solution B. The picture, the darker parts of which are at first faintly visible, soon comes out of a fiery red tint. At this stage of the development it is necessary to check it, in order to obtain dense blacks instead of feeble reds in the darker parts of the negative. To accomplish this, brush over the picture, and complete the development with a solution of gallic acid alone. Under this treatment the reds soon darken and intensify, and become eventually opaque blacks. The entire development should occupy about twenty minutes. It is an excellent plan, after having brushed on the gallic acid, to lay the paper face downwards upon a horizontal glass slab, on which a quantity of gallic acid solution has been previously spread. To fix the Picture. — When the details are fully out, and the blacks of the proper intensity, wash the negative with water, and then im- CAL 39 merse it in a solution containing 1 part of hyposulphite of soda to 4 parts of water. Let it remain until the whole of the yellow iodide of silver is removed from the paper. Then wash and soak it in water for several hours, changing the water several times, in order to remove the whole of the hypo from the paper. Lastly, hang it up to dry. The negative is now finished ; but, before printing from it, it is advisable to render it semi-transparent, either by waxing it (see " Waxing"), or by immersing it for a few hours or days, in almond oil ; then wiping it and hanging it up for a week or two to dry. There are one or two important points to be observed in this process : — If the iodised paper is excited with a weak solution of aceto-nitrate of silver containing no admixture of gallic acid, it is quite as sensitive, or even more sensitive than before, but the negative is devoid of density, and the dark portions are grey, feeble and metallic. This shows the importance of introducing organic matter with the exciting solution. But the above effect does not take place equally with all kinds of paper. There is a coarse spongy kind of Whatman's paper, sized perhaps in a peculiar way, in which the gallic acid may be advantageously omitted in the exciting solution. If, on the other hand, too much gallic acid is added to this solution, the paper is liable to become brown all over, particularly in very hot weather. A good test of the proper state of the sensitive paper is to take a strip of it into the light. If it darkens instantly to a cold grey tint, incapable of being intensified by the further action of light, the negatives will probably be grey and feeble ; but if it darkens to a red tint, which gets still darker by continuing the exposure, there is sufficient organic matter present to give a very intense picture. It is of the utmost importance, in the calotype process just de- scribed, to use a paper which is not alkaline ; and also to use pure trebly re-crystallised nitrate of silver, which is absolutely neutral to test paper. The English papers of Turner and Hollingworth are the best ; and of these the thinnest sorts should be selected. They are sized with gelatine and alum, and have therefore an acid reaction. The foreign papers of Canson Freres, Bive, Marion, &c, are sized with resin dissolved in potass, and are therefore alkaline ; the effect of which is, that from there not being enough acid in the feeble sen- sitising wash to counteract the alkali, they blacken all over in the development. A process by which Mr. Stuart has taken some of the most charming photographs upon paper which the writer has ever seen, and which has been erroneously called a calotype process, will be found described under the head of " Waxed Paper Process." 40 CAM Cameo Portraits. Small portraits about the size of a cameo broocb, printed upon albumenised paper, mounted upon a card, and then passed under a press, which renders the surface of the print slightly convex. When four such portraits are mounted upon one card, diamond fashion, they are called " diamond cameos." Messrs. Window & Bridge are the patentees of this novelty. Camera. (Latin, camera, a chamber). A dark box, in which the real image of an external object, formed by a convex lens, can be received upon a sensitive photographic tablet, which is at the same time protected from the action of all actinic rays which proceed from other sources. There are many different kinds of photographic camera, adapted to the various requirements of the photographer, but we are only concerned here with discussing the principles on which such instru- ments should be constructed ; the reader will then be able to judge for himself how far auy camera which may be offered to his notice fulfils the required conditions. The cameras treated of in the present work will be found described respectively under the follow- ing heads: — "Camera for Views;" "Bound-front Camera;" "Pano- ramic Camera ;" " Pantascopic Camera ;" " Stereoscopic Camera ;" "Binocular Camera;" u Beflecting Camera;" "Camera for Por- traits ;" " Copying Camera ;" and " Solar Camera " (q. v.) The leading principles to be generally observed in the construc- tion of a camera are, that it should be perfectly steady during the exposure, and that no light save that which actually forms the image should be permitted to fall upon the sensitive plate. The exceptions to the above general rule of perfect steadiness occur in a copying camera, in which the object to be copied is attached to the instrument ; in a camera which is intended to follow the motion of a heavenly body ; and in the pantascopic camera, in which either the plate or lens, or both, are moved by certain mechanism during the exposure. With these exceptions, a camera which cannot be made to stand steadily upon a firm support, or which can be easily made to vibrate perceptibly by a slight disturbing cause, is to be con- demned. With respect to stray light falling upon the sensitive tablet, not only should every camera be light-tight, and have its in- terior surface lined or coated with some black, light-absorbing pig- ment, such as lamp-black ground with glue, or black velvet, which is still better, but it should be fitted with one or more perforated diaphragms, placed between the lens and the sensitive tablet, in order to intercept the light which, in spite of every precaution, would otherwise be reflected from the inner sides of the camera. CAM 41 It may be well, perhaps, to mention that cameras have been devised for out-of-door work, in which the whole operation of taking a wet collodion picture is performed within the body of the instrument; and others in which the camera itself contains the nitrate bath, with the sensitive plate immersed until ready for use. Some good work has been done by Mr. Archer, and subsequently by others, in a camera of the former kind, but it was working under" difficulties ; and as for the latter kind of camera, the too prolonged sojourn of an iodised plate in the bath in which it is excited is a certain cause of failure. It is hardly worth while, therefore, to do more than briefly allude to such instruments in the present article. When the conditions of steadiness, and the prevention of stray light from falling upon the plate, have been satisfied, the other con- ditions of mechanical suitability for the purpose intended must be left to the photographer himself to decide on, according to his own taste and intelligence. In purchasing a camera, let him not be misled by its outward appearance, the beauty of its workmanship, the smoothness of its French-polished surfaces, &c, but let him rather consider whether judgment has been shown in the plan of it, and whether it will bear the unavoidable wear and tear of use, under a variety of trying circumstances not easy to foresee. The material of which cameras are commonly made is well- seasoned Honduras mahogany, such as is used for coach panels. For the tropics, teak, however, appears to be still better. French polish is a great protection to the wood, and ought never to be omitted ; but joiners rarely make allowance for it, and new work is too often affected by " sticktion." The perfection of good cabinet work is to fit well, and yet slide easily ; but, above all things, the camera should be true to square and parallelism ; and the plane of the ground glass should correspond exactly to that of the sensitive plate when in position. A patent was taken out some time ago for metal cameras, but they do not appear to have been largely used. Camera for Portraits. This is generally made with a sliding body, when intended for half-plate or larger portraits. It is some- times also provided with a swing back, so as to bring the feet as well as the head of the sitter into focus. There is no objection to its being made rather heavy, as it is not intended to be carried about. For small portraits of the card or cameo size, a special camera should be used to suit that purpose only ; and it need not have a sliding body, as the focussing can be done by the screw in the mounting of the lens, or by a rack and pinion. Every portrait camera ought to be fitted with an internal diaphragm or screen, so as 42 CAM to cut off reflected light from its inner sides. A great gain in the brilliancy of the image is effected by this simple means. Extreme steadiness is necessary in all small portrait cameras. The motion of the camera during exposure is much more injurious to the sharpness of the image than the motion of the object itself. For large portrait cameras the reflecting principle is strongly recommended. See " Eeflecting Camera." In the following figure is represented a portrait camera, with a sliding body, and projecting sides in front, in order to shade the lens, and give greater brilliancy to the image. Camera for Views. Opticians are not to be blamed if they manufacture a great variety of view-cameras, in order to suit the whims of their customers ; but it will be found that professional photographers, who take the largest number of good views, generally use a very simple kind of camera, suitable for the particular purpose which they require, and for that only. Amateurs should endeavour to follow, more than they usually do, the same sensible plan, and not haul about with them on their travels cumbrous and costly pieces of apparatus, which are contrived to answer many purposes, but to do no one thing properly. The view-camera, for pictures about 12 inches diagonal, should be rigid and strong, and made for plates about 9 inches square, with a holder, also, for plates 9x7, which can be inserted with the longest side either horizontal or vertical. There will then never be any necessity for putting the camera upon its side, to take upright views. In the top of the camera should be inserted a spirit-level, by means of which it can be adjusted exactly horizontal ; and at each corner of the triangle, upon which the camera rests, there should be a screw by means of which the levelling of the in- strument can be effected, after the legs of the tripod have been securely planted in the ground. The camera front should be fitted with a slider, which carries the lens, so contrived as to work either up or down upon the segment of a cylinder, the central axis of which is the line drawn across the centre of the picture. Inside the CAM" 43 camera there should be a screen or diaphragm for cutting off reflected light from the sides. The lens used should be one suited to the re- quirements of the case. There should be a single slide for wet plates, and three or more double slides for dry ones. The instrument will then be the most perfect that has yet been devised for taking photographic views of the size indicated. An exceedingly useful form of view-camera for artists, for taking pictures upon a plate 7 inches square, which will either include a single view, covering the whole plate, or four small ones, each 3^- inches square, may be made on the same principle as the former one, but witli the addition of the necessary removable internal partitions, and an extra flat front with four flanges. Instruments on this plan can be seen at Mr. Boss's manufactory. Only one lens is necessary, viz., a wide-angle doublet of about 5^ inches equivalent focus. With two such lenses, however, two pairs of stereoscopic views can be taken upon the same plate. The addition of a portrait lens for instantaneous bits, or portraits 3& inches square, will render the instrument as complete as an artist could desire to take with him on his rambles. Camera for Copying. This camera is used for taking copies of photographs, prints, pictures, &c, either by reflected or transmitted light, and either of the same size as the original, or of a different size from it. When the copy is to be less than one half the linear dimensions of the original, the ordinary camera, with a portrait or view lens, may be employed; but when the copy is to be nearly as large, or even larger than the original, some modifications must be introduced in the form of the ordinary camera, and also in the lens. The principle to be borne in mind in copying an object on a dif- ferent scale is this — that the linear dimension of the copy bears to the corresponding linear dimension of the original the same ratio that the distance of the copy bears to the distance of the original from the lens. For instance, let C be a certain linear dimension of the copy, and O the corresponding linear dimension of the original, and let U be the distance of the original from the lens, and V the distance of the copy from it. Then C : 0 : : V : U. (assuming, of course, that a single view lens is employed). Hence we arrive at the following important conclusion, viz., that so long as the ratio V : U remains constant, U may be increased as much as you please. Now, as the principal defects of photographic lenses proceed from the obliquity of the lateral pencils, and as the obliquity of these pencils is diminished in proportion as U is in- 44 GAM creased, it is evident that the lens should not be brought so near to the object to be copied as to introduce very oblique pencils, but that it should be placed at a distance from it equal to at least three times the longest dimension of the object to be copied, and be of sufficient length of focus to give an image of the required size. In ordinary photographic work, pencils having an obliquity of from 17° to 30° are introduced, and the lens has to be so con- structed as to meet this difficulty ; but the form of lens best calcu* lated for giving a flat field when pencils of great obliquity occur, is not that which at the same time gives the greatest distinctness of focus of the central pencils ; that is to say, if the central pencils alone had to be corrected in the best possible way for spherical aberration, the form of lens which this condition would impose would not be such as to satisfy at the same time the conditions necessary for flatness of field when very oblique pencils are introduced. When, therefore, the necessity for obviating the defects due to obliquity is in some measure removed, the form of the lens may be such as to remedy more perfectly the defects of central pencils. The best form of copying lens for enlarging purposes has yet to be devised. It will probably be found that a triple cemented lens resembling the object-glass of a microscope is the best form. In copying a small photograph or print on a larger scale, the pro- jecting front of the camera should be continued until it nearly touches the picture, and this should be illuminated as strongly as possible, either by the sun or strong diffused daylight. When artificial light is employed (the oxycalcium light, for instance), it may be brought very near to the end of the camera, and its light con- centrated on the picture by means of a convex lens, as shown in annexed figure. CAM 45 Whenever the light is sufficiently intense to allow of a stop being used, it should certainly be introduced as a remedy for the unavoid- able defects of the lens. When an achromatic convex lens is used, like that in the figure, the stop should be placed immediately in con- tact with it, either in front or behind. Small bas-reliefs may be copied in this way very successfully, by throwing the light obliquely on them, and using a reflector to diminish the intensity of the shadows. It is generally found difficult in practice to place the plane of the sensitive surface accurately parallel to the plane of the picture to be copied, but unless this is done, the lines of the copy are distorted. Where the original is small, it maybe placed on a holder connected with the bottom of the camera, as shown in the figure ; but, if this is not possible, the end of the sliding part of the camera should be fitted with the arrange- ment employed in the ordinary camera, for allowing the plane of the picture to be inclined at any small angle to the axis of the lens. By means of these adjustments, any distortion of the image on the ground-glass may be easily remedied. When the picture to be copied is transparent, it should, if pos- sible, be inserted within the camera, the front of which should be sufficiently lengthened to receive it ; and the light which is transmitted through it should either proceed from the sky, or a large white illuminated disc, or an artificial light placed behind a condenser. When practicable, the sky should always be used as a luminous background, in preference to any other kind of light. When the sky is used as the source of transmitted light, the instrument may be mounted on a stand, in such a way as to turn on an axis, to permit of its being directed to any part of the heavens. When the copying camera is placed with its axis vertical, the free nitrate on a sensitive plate does not drain towards the lower part, and the picture is consequently more uniform in density. Whenever it is practicable, therefore, to point the instrument towards the zenith, this should certainly be done. Having now explained the principle of the construction of a copying camera, it will be unnecessary to enter further into detail, as the operator will find no difficulty in modifying his arrangements to suit any particular case. Camphor. A concrete essential oil obtained by distillation from the camphor laurel of China and Japan. It is sometimes added to albumen, gelatine, tannin, and other organic solutions used in Pho- tography, to prevent them from decomposition. This effect which it possesses is probably due to its absorbing the oxygen which would otherwise act on such solutions. 46 CAN CAR Canada Balsam. This is a kind of turpentine obtained from the Abies balsamea, which grows in Canada and the State of Maine ; it is used to cement the lenses which form an achromatic combination, in order to diminish the reflections at the inner surfaces. When thus employed its colour is not found appreciably to stop the actinic rays. It is also used for mounting microscopic objects, and for some other purposes. Caoutchouc. Common india-rubber, the inspissated milky juice of certain trees growing in America and the East Indies. Various plate-holders, and other pieces of apparatus, are made in part or wholly of this material ; but if it is to be brought into contact with nitrate of silver solutions, it should be remembered that the very flexible and elastic variety called vulcanized india-rubber, contains sulphur, imparted to it in its manufacture. It is soluble in ether, chloroform, benzole, and some volatile and fixed oils. Naphtha also dissolves it, with the aid of heat. In some brittle black varnishes, where these solvents are resorted to, caoutchouc, in small quantity, is a useful ingredient. Cap of the Lens. The brass, or, what is better, pasteboard cover, which is used to cover and uncover the lens at the time of exposing the sensitive plate to light. Capsules. Small shallow basins made of berlin ware, platinum, &c, for evaporations, solutions, &c. Carbon. C=6. An elementary substance appearing in a great variety of forms, of which the diamond is the purest and charcoal the most common. A stick of charcoal put into a solution of nitrate of silver and placed in the sunlight, will reduce pure white silver around and adhering to itself. It has hence been said that carbon, with the aid of light, can reduce silver in solution to the metallic state ; but the reduction is due rather to the peculiar physical pro- perties of carbon in certain forms, than to an ordinary chemical reaction. This effect of carbon rather resembles the catalytic action of spongy platinum. It is present in all organic bodies. Carbon, Bisulphide of, C S 2 =38. Is a volatile, transparent, and inflammable liquid of great refractive and dispersive power. It is a solvent of sulphur, and is therefore sometimes used to determine whether the yellow colour of a photograph is due to sulphur. Carbonates. Are salts which contain carbonic acid. The carbonates used in Photography, when soluble, are alkaline in their properties ; and this must be remembered, because an excess added to any acid solution containing a stronger acid, in order to CAR 47 neutralise it, renders the solution alkaline. When hard water con- taining carbonate of lime is used for making nitrate of silver baths for negatives, the solution will be alkaline ; and positives should not be washed in such hard water immediately after passing through solutions containing carbonate of soda — a toning-bath, for instance — or the lime will be precipitated on the surface. It is better to place them at once into the hyposulphite of soda fixing-bath. Carbonic Acid. C0 2 = 22. Is a very heavy and feebly acid gas, most readily prepared by displacing it from any of the car- bonates by a stronger acid, when it escapes with effervescence. Its effect in restraining a developer is almost inappreciable. Carbonized Plates. Copper plates have been evenly covered with hydrocarbon, in the form of finely-divided powder, and by ex- posure to heat, have been superficially converted into carburet of copper. These plates, covered with nitrate of silver and exposed to light under a negative, will receive an image in pure milk-white silver; but there is always too much tendency in the metallic plates themselves, to reduce the nitrate, to render the process available. Carbon-Printing Process.— Beauregard's. M.Testudde Beaure- gard took out a patent in England, bearing date December 12, 1857, for an invention which consists in producing photographic prints by means of carbon or other colouring matter, applied by superposition to a coating sensitive to the action of light. The following are the particulars stated in his specification : — "Paper is immersed in or floated upon a warm solution of bichromate of potass or ammonia, mixed with gelatine. It is then dried, and its surface covered with the pigment. The pigment may be rubbed over the dry surface with a pad of leather or other suitable material ; or it may be ground very fine with nut-oil, or other oil, and rubbed over the surface, which is subsequently im- mersed in a bath of ether, to which a little collodion may be added ; or the paper may be immersed in a bath of Indian ink or other pigment, ground up very fine with water and mixed with gelatine, and a little gum or dextrine, and used hot ; or rollers, or presses, or other apparatus may be employed to apply the pigment or assist the operation. The paper having been prepared in the dark, is exposed to the action of light, and washed in hot water. This dis- solves the gelatine which has not been acted on by the light, but does not dissolve that which has been rendered insoluble by the action of light, and which insoluble gelatine retains the pigment, and thus produces the image. Glass, or other substances, may be substituted for paper. By employing carbon, pigment, or finely- '48 CAH divided gold or silver, photographs or pictures of the greatest per- manence and durability may be obtained." Carbon-Printing Process, Pcitevin's. M. Alphonse Louis Poitevin took out a patent in England, bearing date December 13, 1855, in which he vaguely describes a process by which a carbon print may be obtained. There is, however, no evidence to prove that he ever exhibited a carbon print taken by his patented method, before Mr. Pouncy had exhibited prints taken by a similar method, reduced to an exact formula. The following is an extract from M. Poitevin's specification : — " I apply various liquid and solid colours upon paper, cloth, glass, and other surfaces, by mixing such colours with a mixture composed of equal parts of a concentrated solution of albumen, fibrine, gum arabic, gelatine, or similar organic substance, and a concentrated solution of bichromate of potass, or of any base which does not pre- cipitate the organic matter of the first solution, and applying this new mixture or combination to the paper, or other fabric or surface. " The photographic impression is produced upon this prepared surface by the action of light passing through a negative photogra- phic picture, or an engraving or other suitable object, or screen, or in the camera obscura, and it is then washed with a sponge and a large quantity of water. The albumen, or other organic matter, is rendered insoluble at the parts where it has been acted on by the light, and the design is thus produced in the colour which has been employed. Mixtures containing different colours may be applied to different parts of the surface, corresponding to different parts of the negative or screen employed to produce the photographic impres- sion. A design in several colours may thus be produced. The proportions of the materials may be varied. " What I claim as new is the mode of printing upon paper, cloth, glass, or other suitable surfaces, by applying to them a mixture of liquid or solid colours, with the aforesaid chromatized albumen or other organic matter, and exposing to light as hereinbefore men- tioned, and afterwards washing away those portions of the mix- ture which have not been acted upon by the light, as hereinbefore described." Carbon-Printing Process.— Pouncy's. The following are the printed directions for the above process, which were first published by Mr. John Pouncy, of Dorchester, through the medium of Photo- graphic Notes, of January 1st, 1859. It appears to be the first definite process by which presentable carbon prints were produced ; CAE 49 and Mr. Pouncy received for it a silver medal, and a prize of four hundred francs from the French Photographic Society : — " 1. Prepare a saturated solution of bichromate of potass. 94 2. Prepare a common solution of gum arabic, about the con- sistency of thin varnish. "3. Prepare vegetable carbon, by grinding it with a muller on a paint stone or slab, in the same manner that a painter grinds his colours ; and be careful that it is ground very fine. It is to be ground with water. "4. Mix together equal parts of solutions (1) and (2), say four drachms of each to the ounce, and then add one drachm of No. 3. " 5. Stir the whole well together with a glass rod, and strain it through the finest muslin that can be obtained. " 6. Now apply the prepared solution in the following manner : — Lay the paper, face uppermost, on a glass slab, or a very level and smooth board ; the glass is the better of the two. Commence coating freely with a broad camel's-hair brush, laying on a copious supply over the whole surface; and then allow the paper to absorb for about two minutes. " 7. This done, remove the superfluous liquid thus : — Take a painter's four-inch hog's-hair 'softener,' and work it regularly over the paper, with an alternate vertical and horizontal motion, until the whole presents a smooth even surface, partially dry. The drying may then be completed by the fire. " [Operators will of course have inferred that the whole of these operations must be carried on in a dark room. They should also be informed that any other method of application, including floating, &c, will prove ineffectual.] " 8. Expose in the usual way, varying the time according to light, say about four or five minutes in the sun, and from ten to fifteen in the shade. This, however, will be affected by the intensity of the negative, time of year, &c. ''9. On removal from the pressure-frame, lay the picture, face downwards, in a flat dish of clean water, taking care to exclude all air-bubbles. It will be found advisable to place some slight weight upon the picture, that the back may thus be retained wholly under water and kept free from stains. The time of soaking may be roughly stated at five or six hours ; though in some cases of over- exposure, pictures may remain in the water for days, and come out equally good. " It may be observed here, that when the high lights of the pic- ture appear soon after immersion, the operator may conclude that he has unde?'-ex_po$ed } or that his gum arabic is too thick, which 50 CAE last fault may be corrected by the addition of a little more bichro- mate. It is preferable to find the picture developing evenly all over. Each picture must be in a separate dish, and finally washed under a gentle stream of clean water from a tap or a lip-cup. Should the margin be not quite clean, pass a camel's-hair brush carefully over it before rinsing from the tap, and if needful, any parts of the picture ; but the best results are obtained by soaking only." More recently Mr. Pouncy has entirely remodelled his system of carbon printing, which in his, as in all other attempts at that period, consisted in laying the blackened side of the paper against the negative, instead of the white back. This was pointed out by Mr. Blair, of Perth, in a long and interesting letter to Photo- graphic Notes of February 1st, 1859. It is almost self-evident that, in order to reproduce the gradations of shade in the negative, the light must act first upon the back of the sensitive film, and not upon its face. Mr. Pouncy now uses bitumen of Judsea, instead of a bichromate and gum, as the sensitising agent. He dissolves this substance in benzole, and intimately mixes it with lithographic ink, or other greasy colouring matter. He applies this, by means of a brush, to a piece of thin " bank post," rendered transparent with poppy oil. When dry, the bach of the paper is placed in contact with a negative in the printing frame, as usual. The time of exposure to light varies from five to twenty minutes, according to its intensity. The picture is then developed by immersing it in turpentine or benzole, which washes away all the superfluous colouring matter, and the bitumen which has not been affected by light. The developed picture is then coated on the carbon side with a sticky varnish, and attached by pressure to a mounting board. The thin paper support of the bitumen is now uppermost ; but it may be removed by damping it with a sponge dipped in warm water, leaving the pig- ment firmly attached to the mounting board. Instead of carbon, ceramic pigments, or metallic oxides, may be mixed with the bitumen, and the whole transferred to " potters' biscuit," and burnt in as usual. Carbon-Printing Process. — Salmon and Gamier. This pro- cess consists in first applying to a sheet of paper, or any other suitable surface, a sensitive mixture composed of organic matter and bichromate of potash, then exposing it to light under a negative—* then applying carbon, or other pigment, in powder, to the exposed surface — and then immersing the whole in water. Wherever the light has acted the powder sticks ; but from the parts where it has not CAE CAS 51 acted, the powder comes off along with the unaltered sensitive mixture. A picture is thus produced by means of the pigment employed. Carbon-Printing Process. — J. W. Swan. In this process, by which the finest carbon prints that have yet been seen were taken, a glass plate is coated with plain collodion, and to this film is then ap- plied a sensitive layer, composed of gelatine, bichromate of ammonia, sugar, and finely ground Indian ink. When this is dry, the film is stripped from the glass, and the collodion side exposed under a ne- gative for about one-third the time required for a silver print. You cannot, however, tell by inspection whether the exposure has been long enough or not. The exposed film is then fastened, by a solution of starch, to a piece of paper or card, collodion side down- wards, and the picture is developed by placing it in warm water, which clears up the lights by removing the gelatine layer that has not been acted on by light, and with it the carbon which it contains. The picture is, of course, reversed, and the negative must be taken with its back towards the lens ; or, if this is not done, a double transfer is required, and india-rubber paste must be used instead of starch. The finished print is, of course, glazed with the film of collodion in front. Carbon prints taken by the above process are as fine in gradation, vigour, and sharpness, as the best silver prints. There seems, how- ever, to be some risk of their destruction, in consequence of the collodion film cracking. The process is patented, and the patent (No. 503) bears date February 20, 18G4. Some specimens sub- mitted to the public by the patentee are not inferior to the very finest photographs upon paper that have ever been seen. Carboy. A large globular vessel of glass, surrounded by wicker- work to protect it. Card-Portrait. A small portrait mounted upon a card, measur- ing 4 J- by 2 J- inches. Carrageen, or Irish Moss. Is a kind of seaweed containing a jelly, which has been used for re-sizing positive photographic paper, and for mixing with the iodizing solution for the waxed paper process. Carte-de-Visite. An absurd name given to a small portrait. Caseine. Constitutes the chief nitrogenized substance in milk. It resembles albumen in most of its properties, and has been used with good effect in the waxed paper negative, and in some printing- processes on plain salted paper. E 2 52 CAS CATJ Castor Oil. This oil is extracted from the seeds of Ricinm communis, or Palma Christi, which is cultivated in warm climates. In Photography castor oil has been used conjoined in small propor- tions with collodion, for the purpose of conferring better flowing properties and greater adhesiveness ; but this is only necessary when the collodion contains a considerable quantity of water. Catalysis, or Chemical Action by Contact. Is a term applied by chemists to that process in which a substance, by its mere presence, and without being itself changed, induces decomposition of another body with which it is brought in contact. Tor instance, when starch is boiled for some time in water acidulated with sul- phuric acid, the sulphuric acid itself remains unchanged, but the starch is converted into sugar. This effect would not take place without the presence of the acid. It has been attempted to explain some obscure photographic phenomena by catalysis. Catalysotype. A process invented by Dr. Woods, in which the paper is first prepared with syrup of iodide of iron, instead of the ordinary iodide of potassium. As the picture developes itself after exposure by merely keeping the paper moist without using the usual gallic acid developer, it was supposed that light set up a " catalytic" action on the silver salt, which then operated on the iron salt to produce a picture. The truth was, that when the paper was excited on the nitrate of silver, protonitrate of iron was formed by the same decomposition which produced iodide of silver, and this protosalt is a still more energetic developer than gallic acid. The process is not good, because the solution of nitrate of silver used for exciting is in a few hours quite blackened by the iron, and the iron-syrup is itself very inconstant in its composition ; but with smart manipula- tion and fresh solutions it is extremely quick. Caustic. (Gr., Kavatg, burning). Chemical substances which de- stroy the vitality of any part of the body, or produce effects like burning, are called caustic. The name is generally applied to cer- tain alkaline oxides and acid salts, which destroy animal structure. The most commonly used caustics are nitrate of silver, and potassa. The fixed alkalies and earths are all caustic, as lime, baryta, potassa ; also such salts as chloride of zinc and chloride of gold, Caustic Curve; Caustic Surface. These are terms used in Optics. When a pencil of rays, after refraction through a lens, or re- flexion from a mirror, is affected by spherical aberration, the locus of the ultimate intersections of the rays which cross each other con- stitutes what is called the "caustic surface;" and a plane section CEL CHA 53 of that locus, passing through the axis of the pencil, is called the "caustic curve." The "primary focal line" is situated on that curve. Cellulose, or Lignin. This substance constitutes the walls or the solid framework of plants. The purest natural form in which it can be obtained is cotton, linen, or hemp, and in this shape it is used in Photography for the manufacture of pyroxyline. Cement. This term is applied to any substance having the pro- perty of holding together the surface of two bodies by means of adhesion. Many glutinous and resinous cements are used in Photo- graphy ; but it would occupy too much space to particularise them and their uses here. Centigrade Thermometer. This is the thermometer generally used on the Continent, and is so called because the space between freezing and boiling water is divided into 100 degrees. The ther- mometer used in England is Fahrenheit's, where freezing water is marked 32°, and the boiling point 2 1 2°. In the two scales, therefore, 32 and 0, 41 and 5, 59 and 15, 68 and 20, 86 and 30 are corre- sponding points. To convert centigrade into Fahrenheit, multiply by 9, divide by 5, and add 32. Centigramme. A weight often introduced into England with French formulae. It is the hundredth of a gramme. A gramme is equal to 15 432 grains troy, and a centigramme to *1543 gr. Centilitre. The hundredth part of a litre, or of 35 oz. 2dr. Umin. English fluid measure. Centimetre. The hundredth of a metre, or of 39*7 inches English ; it is nearly A or •£■ of an English inch. Chalk. A soft form of carbonate of lime. It is sometimes used in the photographer's laboratory for polishing glass plates, and for neutralising substances which contain an excess of a free acid more powerful than carbonic. Chamois or Wash Leather. Is extensively used for cleaning glass plates ; but before it is fit for this purpose it must undergo a preliminary preparation as follows. Soak the skin in a cold solu- tion of common washing soda for twelve hours. The object of this is to neutralize the oil used in the preparation of the skin. Then soak the skin in many changes of cold water, wringing out between each change, till the water is no longer discoloured when wrung out. Hang up to dry, but not in too warm a place. When dry, the cleansed leather will be somewhat hard and uupliable. Roll it up U CHA CHL and beat it with a wooden mallet, after which it will remain soft and pliable. Nothing can be better suited for giving a finishing polish' to glass plates than wash-leather so prepared. Keep the leather when not in use in a dry cotton bag. Charcoal. See " Animal Charcoal." Chiaroscuro. Literally, light dark. Is a term applied by artists to the proper balancing of light and shade in a picture. Bembrandt's works are often masterly illustrations. China Clay. See " Kaolin." China Ink. Is a finely-divided carbon made into sticks with gelatine. The finest kinds come from China, and the preparation ; is kept secret. Chlorates. Are compounds of chloric acid with a base. They are chiefly interesting as sources of oxygen. Chlorides. This term is generally used to designate those com- pounds of chlorine which are not acids. There are, however, per- chlorides of some of the metals which are decidedly negative to the alkaline protochlorides, and form with them definite and permanent, salts. The terchloride of gold, for example, forms with chloride of sodium, the auro-chloride of sodium, much used in the photographic toning processes. The different chlorides useful in Photography will be described under the headings of their respective metals or bases. Chlorine. CI = 35 5. Is a greenish coloured gas, discovered by Scheele, in the year 1774. It occurs in very large quantity in common salt, but it is generally isolated by the action of peroxide of manganese on hydrochloric acid. The gas itself has not, as yet, been found of much practical use in Photography ; but, as some of its combinations are extensively employed, we have given it a passing notice here. Chloroform. C 2 HC1 3 = 119*5. Was discovered in 1831, by Goubeiran. It can be prepared by several methods ; but the most economical, and the one generally employed, is by the distillation of alcohol with chloride of lime. It has been used to enhance the limpidity and sensitiveness of collodion ; but it is very doubtful whether it really confers any benefit on that substance. Chloroform, however, is a very powerful solvent of several resins, gutta-percha, caoutchouc, &c. It also extracts from amber a resin which forms a varnish for collodion negatives or positives. This varnish does not CHO CHE 55 require the aid of heat for its application ; but it does not form so protective a surface as some other less expensive preparations. Chondrine. Is a form of gelatine prepared from cartilages. It differs from ordinary gelatine, in being precipitated from its aqueous solutions by almost any acid. From the fact of its being prepared from animal organisms, like the other, this substance is often found conjoined with gelatine, and, photographically speaking, constitutes an impurity. Chromates and Chromic Acid. Chromic acid is a teroxide of chromium Cr 0 3 . It is decomposed instantly by the contact of organic matter, which takes part of the oxygen with alkali metals ; it forms two kinds of salts, the neutral and the acid chromates. The latter are called bichromates, and they possess powerful photo- graphic properties when conjoined with certain organic substances, such as gelatine, &c. The rationale of the action of chromic acid, and of the bichromates, in the presence of organic matter, is shortly this : — Light sets free oxygen, which unites with the gelatine, &c, thus forming a resinous and insoluble compound ; it also deepens the colours, because of the formation of a sesquioxide. The practical application of their properties to photographic purposes will be described under the headings of the different processes in which ohromic acid is used. Chromatic Aberration. See " Aberration." Chromatype. A name given to that class of photogenic de- compositions in which chromic acid is partially deoxidized. The following are some of the modes adopted for getting photographs by the chromium salts : — (a) Soak the paper in a saturated solution of bichromate of pot- ash, and dry it by rapid agitation in front of a brisk fire out of the light. It is now of a bright yellow colour, but exposure to the sun under a negative will produce a positive, by darkening the exposed parts to a deep orange colour. Washing well in water removes the unchanged yellow salt from the lights, but the reduced, sesquioxide in the shadows remains combined with the paper. The paper should be well sized, or the bichromate will be only feebly decomposed. (7;) Brush a sizing of starch very uniformly over the paper, and then steep it in a weak alcoholic solution of iodine, and if the coating of blue iodide of starch be not uniform, repeat the operation. Steep it in bichromate and dry it as before. The print will be negative, even, from a negative, and positive from a positive, if after exposure 56 CHE, and washing it is again steeped in the solution of iodine, which renders the unexposed parts of a dark violet colour. (c) To a saturated solution of bichromate of potassa, add a satu- rated solution of sulphate of nickel, in quantity more than sufficient to decompose the whole of the bichromate, or 1 drachm of the latter (the potassa) in crystals to 2 drachms of the former, also in crystals. Apply this to the surface of the paper and expose under a negative. The exposed parts become brown and the rest remain yellow, and if now washed in water the result is a positive picture. But if the exposure be continued under the negative beyond the dark stage the browning disappears and the exposed parts are white, a colourless double salt being formed there. When nitrate of silver is applied as a developer, the chromate of silver is deposited in the unsunned por- tions, where the original solution remains unchanged, but not on the whitened parts. Pure water will remove nearly all except the precipitated chromate, which, if thought desirable, may be converted into chloride, and exposed again, and developed as in the usual silver processes. Sometimes the colourless double salt formed will de- compose the nitrate of silver, owing to its imperfect formation. The chromate of silver, by which the shadows are represented, is itself decomposed by light, and, therefore, must be changed in some way, before permanence is attained. (d) Dissolve neutral chromate of copper in ammonia. The solu- tion, which is now a mixture of chromate of ammonia and of ammo- niacal solution of oxide of copper, is of a green colour, formed by the mixed yellow and blue of the separate solutions. Papers prepared with this behave like those soaked in the chromate of copper. Chrysotype Process. (Gr., xp v(T0 £> gold). This is one of Sir John Herschel's ingenious processes. The operations are as follow : — First. Immerse a sheet of paper in a moderately strong solution of ammonia-citrate of iron, and dry it in the dark. The strength of the solution should be such as to dry into a good yellow colour, not at all brown. Second. Expose the paper to light, either in the camera or pres- sure frame, under a negative, until a very faint impression is ob- tained. Third. Brush over the paper a neutral solution of chloride of gold, of such strength as to have about the colour of sherry wine. The picture immediately appears, and is rapidly developed to a purple tint. Fourth. Wash the developed print in several changes of water, CIR CLE 57 and fix it with a weak solution of iodide of potassium : then wash again thoroughly, and dry. The picture is now finished. Circle of least confusion. The nearest approach to a focus of a pencil after oblique reflexion or refraction. See " Focal Lines." Citric Acid is derived from the juice of lemons and other fruits. The juice is made to undergo a fermentation ; it is then neutralized with lime, by which citrate of calcium is formed. From this the acid is liberated by means of sulphuric acid. Citric acid is a powerful retardent to the reduction of silver salts by means of a developer. Hence it is found serviceable to add it to the pyrogallic solution used as an intensifier for negatives, developed with protosulphate of iron. It tends to keep the deep shadows very clear, and prevents abnormal reduction of the silver. A small proportion of citric acid may be added with great advantage to the nitrate bath for positive paper, when there is too much tendency exhibited by the paper to print of a slaty tone. Cleansing. The cleansing of bottles, papers, stirring rods, and apparatus in general, is a matter of essential importance in many photographical operations ; and the great rule must be to clean immediately after use, when purity is obtained with much more cer- tainty and much less labour. All dirty glasses, &c, should be immediately put in a particular place and attended to at the first leisure moment. The following directions will be found useful : — (a) To clean Albumen from Glass Plates. Use a solution of caustic potash, or ammonia, for albumen is soluble in alkaline solu- tions ; then wash with water ; and lastly, with dilute nitric acid and plenty of water. (b) Collodion Bottles. Leave the stopper out until the ether and alcohol have evaporated, and the film is hard and horny, when it will be easily removed, without any adhering to the glass, by means of cold water and a bottle brush. Drain and rinse out with a little alcohol. (c) Developing Measures and Trays. Wash well with tow, or a rag and common water, then with a little strong nitric acid if the black precipitate has dried in the vessel. The acid should remain some time in contact with the blackened parts, even after they appear clean : lastly, plenty of water and a clean dry cloth. (d) Gallic Acid and Gallo-Nitrate. Bottles in which gallic acid has been allowed to become discoloured or mouldy require the use of nitric acid, which decomposes most organic matters, before they are fit to receive solutions of other compounds. CLE (e) Glass- Plates and Vessels. These should be washed and rinsed, if possible, as soon as they are done with, and before they . have got dry : even common water, if-allowed to dry on them, will "leave matters which often require considerable force to remove. If plain water will not clean them, a little tow and the ashes from the, fire will generally remove everything. Sand or gravel should not be used with white glass vessels, since it always scratches the soft flint glass of which English vessels are made. They must after washing be rinsed, drained, and wiped with a clean coarse cloth. New window glass or crown glass requires particular care in clean-' ing, especially in hot weather. Glass plates which have received pictures before are, contrary to the general opinion, better than new, if carefully cleaned after each experiment. Nitric acid with or without tripoli is the best detergent for new glass : better without tripoli, if possible. If a plate be cold, and then take the breath in ; an even film, it is clean. If a collodion picture has been developed with pyrogallic acid, it requires only water, but if with an iron salt, 1 nitric acid will be necessary. (f) Grease. Greasy glasses should not be washed, but, in the first place, wiped with tow, to remove as much as possible of the; grease, and then a dry cloth should be used until the surface appears clean. It should afterwards be washed with nitric acid, or caustic potassa and tow, which removes the thin film of grease remaining, and it may then be rinsed, drained, and wiped as before. A special duster should be appropriated to remove grease. {cj) Lenses. Do not use silk, for it is apt to scratch : a soft wash- leather, free from the powders used in cleaning it, is best ; if the lens be greasy, soft tissue paper will clean it quickly, especially if mois- tened with a little alkali. Rub oft' the alkali with fresh paper, and; finish with the wash-leather kept for this purpose. ill) Nitrate of Silver Stains. Nothing is so good as nitric acid for porcelain or glass. For linen, or the hands, mix together alcohol 10 ounces, iodine i ounce. Apply a little to the stain, and when it has become yellow, dissolve it out with cyanide of potassium, and wash well. (i) Papers containing Metallic Spots. Make two solutions, one of one ounce of tartaric acid in 10 of water, and the other of one fl. ounce of liquor ammonia? in 20 ounces of water. Let the papers soak in the first for a quarter of an hour, one over the other, then place them for a few minutes in the second. Einse in plain water and hang up to dry. The metal is removed as ammonio-tartrate. . (j) Resinous, Bituminous, and Tarry Varnishes. As much as possible should be scraped off with a knife; then^ use tow, with a CLE 59 little strong caustic potash or sulphuric acid ; rub the glass well, and after a few minutes the resin, &c, will wash off with water, and the glass may be cleaned in the ordinary way. Wood spirit is a clean solvent of these substances. (h) Spots of over-development in Photographs. The solution given above for nitrate of silver stains has been found to answer, but it requires care in using. (I) Sulphate of Iron Stains, Iron Moulds, and Ink Stains. To remove these from linen make use of a solution of oxalic acid, and from glass vessels hydrochloric. (m) Salphuret of Silver Films in Hypo-bottles. Remove all that can be rubbed off with tow and water, and then use nitric acid, which will form nitrate of silver : finish with plenty of water. (n) Turpentine. Strong alkali and tow will soon soften it, so that it will be moveable by water, or perhaps better, sulphuric acid, which will decompose it. (o) Famishes and Varnished Collodion Pictures. These are all easily removed by wood spirit, even if asphalt or brunswick black has been applied : or, if time be not pressing, let them soak for some days in water, when the varnish will peel off. Clearness. This quality in a photograph implies the absence of any action except what is induced by the impact of light, the exact amount of exposure, both as to the intensity of the light and the duration of its action, the exclusion of all light except that reflected by the object which is photographed, perfect optical arrangement so that the image is sharp and evenly illuminated, a uniformly sensitive surface to receive the impression, the arrangement of light so that the illuminated object to be taken appears perfectly modelled, and clean, uniform, and smart manipulation. The things to avoid are dirty plates, impure chemicals, too weak or too strong solutions, or those of unknown or inconstant strength, too much heat or cold, bad water, dirty hands or frames, and unclean vessels which cause action independent of the light ; lenses with foci not coincident, or with no sharp focus, and of .too short focus, and cameras shaky or not light, tight, or not adjusted to the focussing glass, or open in front to rays from any objects beside the one to be photographed, or deficient in means of absorbing the light scattered by the lenses or the mount- ing, which cause an imperfect image ; illuminated smoke or vapours between the object and lens, bright back-ground or sky to the picture, and dew or grease on the lens, which introduce into the camera diffused light that veils the picture ; feeble light, or too intense, or not falling upon the object so as. to throw out the relief, light coming 60 CLI COL too obliquely from parts of the object, and exposure too long or too short, which injure the purity of the modelling ; and careless, or unequal, or dilatory manipulation which makes one part of the plate more sensitive or develop more quickly than another. Cliche. The French word for negative, or mould. Clips. Little clasps made of wood for hanging up papers to dry. They are best when they are tipped with shellac, or other varnish that the solutions cannot penetrate. A separate set should be kept for each operation. They should be provided with S hooks of wire, so that they may be hung over a line. Such hooks, made with black pins or silver wire, are very useful substitutes for the clips themselves. Coating Fluid. A solution used in the dry-collodion processes in order to prevent the dry film from splitting, blistering, or wrinkling, on being wetted by the developer. It is composed of two grains of gutta-percha dissolved in an ounce of chloroform. The glass plate is coated with it, and then held for a minute before a red fire before applying the collodion. Cobalt. A metal resembling, in its photographic applications, iron, nickel, and chromium. It is the basis of smalt, and the blue colour of writing papers. Its compounds with sulphur and arsenic have been found to be affected by light. Collodio- Albumen Process— Mudd's. Coat the cleaned plate with collodion in the usual way. After allowing the film to " set " well, sensitize in the ordinary nitrate of silver bath. If the collodion should give a very thick and creamy film, it must be reduced by adding ether. After sensitizing, the plate must be well washed, and then placed in a dish containing a weak solution of iodide of' potassium and water (about one grain to the ounce of water) for two or three minutes, gently moving the dish the while. Rinse with tap water, and drain a minute. To the whites of 10 eggs add — 50 grains iodide potassium. 10 „ bromide „ 100 minims liquid ammonia. 1\ ounces water. Dissolve the iodide and bromide in the water, then add the ammonia. Mix all together with the albumen, and beat the whole into a froth. Let it settle. It is then fit for use. While the plate is still wet, pour over its surface the albumen. Pour off again. Repeat this twice. Now allow the plate to drain COL 61 live or ten minutes ; then dry it rapidly before a clear bright fire, and make it quite hot. To make the plate sensitive, it is only necessary to dip it for one minute into the aceto-nitrate bath : — 40 grains nitrate of silver. £ drachm glacial acetic acid. 1 ounce water. Warm the plate slightly, immerse it, drain a moment, and then wash as before in the dishes, and finally under the tap. The plates may be dried artificially, but will dry without heat in about ten minutes. Plates so prepared will keep good, in cool weather, six or eight weeks, but in J uly or August it is better not to trust them longer than a fortnight. The method of developing at present is with pyrogallic acid. Take the exposed plate, and, after placing it upon the stand, pass over the surface a little common clean water ; then take a plain pyro solution without acid, say two or three grains to the ounce of water, and pour it on the plate. This mixture must be made just before use, as it does not keep without acid. The sky and high lights will appear almost immediately, and ultimately, without either silver or acid, the whole picture comes out. It now requires intensity. Now take — Pyro . . . . . .2 grains, Citric acid 2 grains, Silver, two or three drops of 20-grain solution, and pour on the plate. If necessary, add more silver until sufficient intensity is gained. A warm solution ought only to be used when the picture is under- exposed. It should always be used in the winter months, when the temperature of the water is low, and in pictures which would not appear with coid water. Fix in hyposulphite of soda, about six ounces to the pint of water. Cyanide of potassium must not be used for this purpose. Collodio-Chloride of Silver. This is a sort of emulsion, consist- ing of collodion holding in suspension for a time chloride of silver. Tt is used chiefly for the purpose of printing upon opal glass, by contact with the negative. The addition of a small quantity of citric acid to the emulsion is said to impart vigour to the print. The compound is poured over the plate in the same manner as collodion, and allowed to set and get dry thoroughly. It may then be exposed in a suitable printing frame. About double the ex- posure is required as that for a common silver print. To make the 62 COL- emulsion, first dissolve the soluble chloride in the collodion, and then add a solution of nitrate of silver dissolved in alcohol. An ounce of collodion will take up about seven grains of nitrate of silver. The chloride of silver only makes an imperfect mixture with the collodion, and does not enter into combination with it, for in whatever manner .the emulsion is prepared, it will be found, after a few weeks of repose, that the whole of the chloride is precipitated. This method of printing upon glass is the discovery of Mr. Wharton Simpson, and when properly executed the results are very beautiful. Collodion. (Gr., KoXXa, glue). A term applied to a solution of pyroxyline in a mixture of ether and alcohol. The manufacture of collodion suitable for the positive and nega- tive process is a matter of some difficulty, and requires considerable experience in the operator before he can always be successful. The first and most important point is to manufacture a kind of gun cotton called pyroxyline (see " Pyroxyline "). The rest of the operation is easy when the alcoholic and ethereal solvents are ob- tained of the requisite strength and purity. Formula for Positive Plain Collodion. Ether, sp. gr. '725 to *730 . 12 fluid ounces. Alcohol, sp. gr. -810 to -820 .• 4 Pyroxyline (see "Pyroxyline ")„ . 110 grains. In a clean and dry bottle first shake up the pyroxyline with the alcohol, then add the ether, and again shake till the whole of the pyroxyline is dissolved. , Formula for Promo-iodising the Positive Collodion. Iodide of ammonium . .40 grains. Iodide of cadmium . . 40 „ Bromic.e of ammonium . . 20 „ Bromide of cadmium . . 10 „ Alcohol, sp. gr. *810 . . : ' . 4 fluid ounces. Shake up in a clean bottle with the alcohol till dissolved, and add the whole to the above proportion of plain collodion. Allow to settle for a few days, and decant off the clear portion for use. Positive iodised collodion should be of a pale sherry colour or a little deeper. Should it not assume this colour after being for a few days iodised, add tincture of iodine drop by drop till the required depth of colour is obtained. ! Should the solution turn red im- mediately after the iodiser ha? been added to it, the inference to be drawn is, that the ether has been kept for some time and become ozonised or acid, from the action of light and air. But this may be no drawback to the successful working of such collodion. Should, COL 63 -again, the red coloration be absorbed, and the solution become almost colourless, this may be considered a proof that the ether or alcohol contains methyl or wood spirit. In this case it may not be necessary to add tincture of iodine, for the collodion has already shown itself to be slightly acid. The chief object in adding free iodine is to give an acid reaction to the bath. Formula for Negative Plain Collodion. Ether, sp. gr. '725 . . 10 fluid ounces. Alcohol, sp. gr. -805 . 5 „ Pyroxyline {see " Pyroxyline ") . 120 grains. Mix in the same way as described above for positive collodion. Formula 1st. To bromo-iodise the plain negative collodion, take — Todide of ammonium . .40 grains. Iodide of cadmium . . 40 „ Bromide of ammonium . . 10 „ Bromide of cadmium . . 10 „ Alcohol, sp. gr. '805 . .5 fluid ounces. Shake in a clean bottle till dissolved, and add to the above pro- portion of plain collodion. After settling, decant off for use. This iodising solution is specially meant for use with an iron developer. For a pyrogallic developer the following iodiser is preferable — Iodide of potassium or cadmium . 50 grains. Iodide of ammonium . . 50 ,, Alcohol, sp. gr. '810 . . . 5 fluid ounces. In the dry processes a larger proportion of bromide in the col- lodion than is desirable in the wet should always be employed. Four grains of iodide to two of bromide in each ounce of the collodion is a fair average proportion; but occasionally, especially in the tannin process by alkaline development, the amount of bromide may be considerably increased with advantage, and the iodide cor- respondingly diminished. Collodion, whether iodised or plain, should be kept in well- stoppered or corked bottles, nearly full, and as much as possible in a cool and dark place ; otherwise it rapidly becomes ozonised, and shows great tendency to decompose some of the iodides, which thus disengage iodine, rendering the collodion slow in action. Collodion Positive Process. This is a method of taking a direct positive, upon a collodionised surface, without any subsequent printing operation, and by developing the image that is obtained in the camera. 64 COL The collodion positive and negative processes differ in the follow- ing impprtant particulars ; viz. : — In the negative process the object is to obtain a picture, in which the material of the image shall be more or less opaque when looked through ; — in the positive process, to obtain a picture in which the material of the image shall be a dead white when looked at. In a negative, density of various gradations is what is required, without any reference to the appearance of the surface deposit. In a posi- tive, whiteness of the surface deposit is what is required, without any reference to density. In the negative process, the object is to obtain density in the image by combining organic matter with the -reduced silver, so as to obtain sufficient opacity, and not a grey, feeble, metallic image. In the positive process, on the contrary, the object is to avoid, as much as possible, the introduction of organic matter in any part of the formula, first by acidifying the nitrate bath with nitric instead of acetic acid, and secondly, by developing with an inorganic developer, such as a mixture of the proto-salts of iron with nitric acid, and in this way endeavouring to produce, not an opaque organic image, brown on the surface, but a thin, white, metallic deposit. The nitrate bath should be rather stronger in silver than that for negatives, and should be acidified with nitric acid. The formula is as follows : — Distilled water . . .1 ounce. Nitrate of silver 40 grains. Nitric acid . . . .1 minim. The time of exposure is about half that required for negatives. The Developer is made thus : — Dissolve one ounce of powdered nitrate of baryta in 16 ounces of distilled water, and add 2 drachms of nitric acid, s.g. 1-4. Next add 1^ ounces of powdered proto-sulphate of iron. Shake well until the iron salt is dissolved. The mixture becomes white and turbid, in consequence of the formation of sulphate of baryta. Let it stand a few hours until this has settled to the bottom of the vessel ; then decant and filter the solution, which, if right, will be of an apple green colour. Add two ounces of alcohol, to enable it to flow freely over the collodion film. It is now ready for use, but gradually deteriorates by keeping. It may be kept about a month in a cool place. The nitric acid slowly oxidises the proto-salts of iron and the solution turns yellow. In this state it is much slower in its action, and a little fresh proto-sulphate should be added to it. An ounce of proto-sulphate of iron decomposes about an ounce of nitrate of baryta, forming insoluble sulphate of baryta, and soluble proto- COL 65 nitrate of iron. The remaining half ounce of undecomposed proto- sulphate of iron forms the energetic part of the developer. The nitric acid should be added as stated, and not after the iron salt, as some peroxidation of the iron might then occur, which would occasion a browning of the solution, and be injurious. The mode of developing the picture is quite different from that in the negative process. It must be done almost at a blow, and is completed as soon as there is a visible indication of the details in the shadows when the plate is laid upon a black ground. These details come out very quickly, and the development generally occu- pies only a few seconds. If it be carried too far, the boldness and vigour of the contrasts are destroyed. Over-exposure produces a blue solarized appearance, as in the daguerreotype process. The picture is to be washed, and fixed with cyanide of potassium ; then washed again, dried and varnished in the Same way as a nega- tive ; after which, the back of the plate is to be coated with black varnish. Some operators varnish the picture itself with black varnish, because then it is non-reversed on looking at it through the glass ; but this is a bad plan, because the varnish injures the tone of the whites, and is very liable to crack and destroy the picture. When applied to the back of the plate, it can easily be rubbed off and re- newed if it cracks, and no harm is done to the picture. See " Varnish." Positives should be taken in a non-reversing slide whenever the reversion of the image is objectionable. Some operators take positives upon purple glass, which forms an excellent background to the picture, and renders the use of black varnish unnecessary. This is an excellent plan. Positives may be taken upon a variety of substances, such as black- varnished paper and card, black- enamelled iron tablets, black patent leather, black-glazed canvas, &c, &c. Some of these processes are very ingenious and useful, and deserve a particular description. To take Positives uvpon Paper, or Card. The paper or card must first be gelatinized by floating the side on which the picture is to be taken on a warm strong solution of gelatine, and drying it. It is then cut a little smaller every way than a glass plate, to which it is to be attached in the following manner : — Lay the back of the paper upon the glass, heat the blade of a penknife in a spirit lamp, rub it upon a lump of wax, and then pass it quickly all round the edge of the paper so as to fasten it to the glass plate by an edging of wax, without allowing any liquid to get behind it. Next coat the paper with black varnish, and let it dry. Then take the positive in the ordinary way, and when finished cut off the edges of the paper and detach it from the glass. Positives taken on paper or card in this I 66 COL way may be safely transmitted by post. The object of the gelatine is to prevent the black varnish from soaking into the paper. To take Positives on Patent Leather, Glazed Canvas, Enamelled Iron Tablets, &c. The leather or canvas is to be attached to the glass by an edging of wax, in the manner described for paper, and the positive taken on the glazed surface at once, without black-var- nishing it. Enamelled iron tablets may be treated in precisely the same way as a plate of purple glass. The back of the tablet should be varnished so as to prevent its injuring the nitrate bath. Collodion positives may be transferred to a great variety of tablets. Collodion Processes (Negative). The collodion negative pro- cess with a pyrogallic acid developer is now rarely practised ; but it is deemed advisable to give a succinct summary of it in this Dic- tionary. A proto-sulphate of iron developer is that which is generally used. 1st. Negative Process with Pyrogallic Acid Developer. Select a collodion iodised by the second formula ; see " Collodion. " A bromo-iodized collodion will answer, but it is not nearly so sensitive. It may, however, be occasionally found preferable under circum- stances of high temperature, and when the light is very actinic. Formula for nitrate bath. Eecrystallized and neutral nitrate of silver . 300 grains. Distilled water . . . . .10 fluid ounces. Glacial acetic acid 1 drop. Dissolve and filter. Pour into the glass trough, then coat a clean glass plate (say 5x4 inches for the above proportion of solution) with collodion, and allow all the iodide of silver, which at first is formed in the film, to be dissolved out by the nitrate of silver. It may occupy more than a quarter of an hour before the glass resumes its original transparency. The bath is now said to be iodized — that is, its tendency to dissolve iodide of silver is considerably diminished — and it is ready for use. To coat the Plate with iodized Collodion. The plate must first be cleaned in the manner described in the article " Cleaning," and then well polished with a cambric rag, or leather buff, immediately before pouring on the collodion ; for, unless the plate is wiped thoroughly dry and well polished before use, it will be covered with streaks or marks where the damp rag last touched it. The breath condenses upon a clean, dry, polished plate in an even sheet, without exhibiting marks or irregularities. Hold the plate horizontally, by one corner, between the finger and thumb of the left hand, if a small plate, or place it on a plate holder COL 67 (see " Plate Holder"), if too large to be conveniently held in this way, and pour upon the middle of it rather more collodion than is sufficient to cover it with a good thick layer. Then tilt the plate so as to let the collodion flow towards the thumb, but without touch- ing it, and afterwards to the other corners in succession, and pour off the surplus into the bottle from the corner opposite to that by which you hold it. This done, keeping the corner of the plate still resting upon the neck of the bottle, and holding it vertically, rock it three or four times quickly through a wide angle, in order to prevent the formation of lines in the collodion ; then place it upon the dipper ready to be plunged into the nitrate bath. Be careful to wipe the neck of the collodion bottle occasionally, as bits of dry collodion which are formed there are liable to become detached and deposited on the plate ; also, avoid dust in the dark room, and blow off, or better, brush off with a camel's-hair brush any floating particles which settle upon the plate, before coating it. To excite the Film. The ether evaporates very quickly from a coated plate, and the fluid collodion speedily gelatinizes. This is called " setting." When the collodion has sufficiently set — that is, as soon as it ceases dropping from the corner of the plate — and is safely placed upon the dipper, immerse it, without pausing, in the nitrate bath ; a pause during the immersion producing a line across the plate. Then move it from side to side in the bath, for a few seconds, in order to prevent the formation of streaks in the direction of the dip, and leave it immersed for a couple of minutes or so. Then raise it out of and lower it into the bath gently two or three times, in order to wash off the ether and get rid of the greasy, streaky appearance of the film ; let it drain for a few seconds over the bath ; wipe the back of the plate with blotting-paper, and place it in the slide, taking care never to invert the slide while the plate is in it, which would allow the free nitrate which drains towards the bottom to flow back over the partially dry upper surface of the plate. Immediately expose in the camera for what may be considered the requisite time. The effects of over and under exposure can only be seen during development ; but experience is the best preliminary guide.^ To develop the Image. Pour over the plate a solution containing: — Distilled water .... 1 ounce, Pyrogallic acid 1 grain, Glacial acetic acid . . . .20 minims ; or Distilled water . . . . 2 ounces. Pyrogallic acid 2 grains, Citric acid 3 grains. f 2 68 COL It is convenient to make a small quantity of developer of three times the above strength, and to dilute it when wanted. You have, then, at hand a little strong solution to use, should occasion require. The developer will not keep longer than three or four weeks. A little alcohol added to it makes it flow more freely over the plate. Tt should be filtered if any floating particles appear in it. Citric acid is much cheaper to employ than acetic acid, and it answers very well, giving negatives of a more inky tint than acetic acid ; but it must be carefully weighed, as a grain or two more or less may make a considerable difference, citric being a very powerful retardent in the developer. Fix the negative, when fully developed, with a saturated solution of hyposulphite of soda, not with cyanide of potassium, which will dissolve out all the unimpressed iodide of silver. Afterwards wash the plate in a stream of running water for three or four minutes, to remove the excess of hyposulphite or cyanide, which, otherwise, would gradually destroy the image. 2nd. Negative Process with Proto-sulpkate of Iron Developer. In this case the plain collodion should be bromo-iodised by the 2nd formula (see " Collodion"), and the nitrate bath prepared in a slightly different manner. Mix and iodize the bath as before, but, instead of one drop of acetic acid, add to it one drop of nitric acid for each 10 ounces of solution. The plate is to be cleaned, coated with collodion, sensitized, and exposed also in the same manner, the only difference of treatment being in the mode of development, which is as follows : — Developing solution. Proto-sulphate of iron . . . 15 grains. Water (common) . . . . 1 fluid ounce. Glacial acetic acid . . .20 minims. Alcohol (not always required) . . 20 minims. The method of applying this solution to the exposed film is of great consequence. The plate should be held in the left hand, nearly level, and the developer flushed over it as speedily and evenly as possible ; at the same time care must be taken to prevent it from flowing over the edges of the plate. When a sufficiency has been poured on, rock the plate gently with a circular sort of motion, till the image has been developed U*t x sooner or later, lade. The more modern method of toning photographic prints after thorough washing, in an alkaline solution of chloride of gold, and then fixing them in a slightly alkaline solution of hyposulphite of soda, is not open to this objection ; because there is no acid nor free nitrate of silver present to decompose the hyposulphite, which, being very soluble in water, can easily be removed by long and careful washing. There seems, therefore, good reason to suppose that, when photographic prints are thus treated conscientiously and carefully by the operator, they will not fade unless exposed to sul- phuretting fumes applied intentionally or derived from the atmo- sphere. The fading of photographs is often hastened by the nature of the paste used in mounting them, which is apt to become acid, and thus decompose any trace of hyposulphite which the print may con- tain. The best of all pastes for mounting is gelatine. Starch and gurn arabic, both of which are frequently used, turn acid after a time, and the former, when the picture is kept in a damp place, generates the starch fungus, which is equally injurious. I 2 116 PAD FEU Photographic mounting-boards often contain a considerable quantity of hyposulphite of soda, used as an antichlor by the paper- makers and not carefully removed. This salt is a valuable agent in its proper place, but, when a finished silver print is mounted on any material containing it, the picture must very soon fade from decom- position of this very unstable hyposulphite. In every case, the fading of photographic prints can be traced to sulphur, for which silver has a powerful affinity. The object of every one should therefore be to guard them from its influence by all means in his power. Fahrenheit's Thermometer. (Greek, dipfirj, heat, /ucrpov, a measure.) In this thermometer the freezing point of water, or temperature of melting snow, is marked +32°, and the boiling point of water, in a thin clean metallic vessel, at an atmospheric pressure of 30 inches, 212°. The space, therefore, between the freezing and boiling points of water is divided into 180°. In the Centigrade thermometer this space is divided into 100°, the freezing point being 0 and the boiling point 100°. There is a table at the end in which Fahrenheit's thermometer and the Centigrade are placed side by side, and the scales compared. Fermentation. This is a peculiar metamorphosis brought about in solutions of certain organic substances, such as sugar, by the introduction of a decomposing azotised body, called a " ferment." Ferrogelatine Developer. This term is used to designate a new and very powerful developing agent, recently discovered by Mr. M. Carey Lea, of Philadelphia, U.S. Plain acidulated proto- sulphate of iron solution, it is well known, will, in very few instances, develop an image of full printing density by the first application. It therefore becomes necessary to reinforce the negative by intensi- fying solutions of either pyrogallic acid or protosulphate of iron mixed with a little nitrate of silver. To obviate this inconvenience and waste of time, Mr. Lea has hit on the happy idea of combining gelatine with the iron, so as to give sufficient density by the first developing operation. The method he proposed has found much favour in this country and in America, and is now extensively prac- tised. The following modification of the original formula by Mr. Lea will be found most effective, and at the same time economical. Take half a pound of the finest Scotch or Russian glue ; break it into small fragments, and allow to soak for a night in cold water. Mix six ounces of commercial sulphuric acid with twenty-four of FEE, 117 common water in a large porcelain or glass jar capable of holding at least one hundred ounces of solution. When the mixture has cooled, pour away the excess of water in which the glue has been soaking, and add the latter to the diluted sulphuric acid. In the course of twenty-four hours, more or less, the glue will be entirely dissolved by the acid, if the mixture is occasionally stirred with a glass rod. When the solution is complete, add ten more ounces of water ; tie up a number of small round bundles of clean iron wire, and throw them into the solution till they rise above the sur- face. Then tie up a long sheaf of the same wire to be used as a stirring rod. Place the vessel in a warm but not hot place. Hydrogen will soon commence to be evolved, showing that the iron is being converted into the protosulpliate. This action will go on briskly for a few days, but if it happens to be violent remove the jar to a colder place. After a few days add ten more ounces of water, and stir well. Continue adding water or more wire as re- quired, till the whole bulk of solution reaches eighty ounces. The operation is complete, when no more hydrogen is evolved, or when a scum ceases to rise to the surface. The solution should now be filtered through common filtering paper into stock bottles, and well corked up. But previous to filter- ing, it is a wise precaution to dissolve in the solution half an ounce of acetate of soda, which will destroy the last traces of sulphuric acid which may remain, and which would materially interfere with the proper action of the developer. The solution so prepared will keep well for at least twelve months, without throwing down much sedi- ment. Should a sediment be thrown down, it only shows that tin- glue was impure and, so far, the solution weakened. No other bad effects follow. To use the ferrogelatine developer, make a solution of plain pro- tosulpliate of iron of any desired strength, and for every ten fluid ounces add one of the ferrogelatine. No acid is required, but in very hot weather it may be necessary to add more than one ounce of the gelatine solution to prevent fogging. The image comes out more slowly than is usual with plain acidulated protosulpliate, but the action of development is more uniform, and may be continued till almost any density can be secured. The chief thing to be avoided is carrying it too far, and thus getting hard negatives. Another modification of the gelatine developer, which answers very well, was suggested by a clergyman in England. It consists in soaking for several hours from sixty to ninety grains of gelatine in about six ounces of cold water, then adding two ounces of glacial acetic acid, and dissolving the gelatine by means of a gentle heat. 118 FER FLA A quantity of this, proportioned to the temperature and other cir- cumstances, is to be added to the plain protosulphate of iron solu- tion instead of acetic acid. In most instances this developer gives quite sufficient density by the first application. Ferrotype. A name given by Mr. Hunt to a process in which an argentine photograph is developed with proto-sulphate of iron. The term is no longer in use in this country, but sometimes in America. Filtration. This is a process for separating a liquid from the insoluble matter which it may contain. The liquid to be filtered is made to pass through a porous substance, such as unsized paper, porous earthenware, cloth, sand, &c. When common blotting-paper is used, it should first be washed with dilute muriatic acid, in order to remove some lime and iron which it generally contains. Filter papers are generally cut; round, and the sides folded in puckers like a fan. They are then' placed in a glass funnel, the diameter of which should be about three-fourths of its height, measured from the neck. The liquid should be poured into the funnel very gently along a glass rod. A filter covered with sediment may be con- veniently washed by squirting water against it from a small syringe. Linen or calico should be used for filtering weak alkaline liquids, and flannel or felt-stuff for weak acid ones. These filter bags are made like a fool's cap, and have a wooden hoop at the top. Cottonwool, put into the neck of a glass funnel, makes a good filter for many purposes. Strong acids and alkalies should be filtered through a layer of pounded glass, quartz, clean sand, or bruised charcoal. Sometimes the liquid is made to ascend in the filter by hydrostatic pressure. This is often a very good plan. Volatile liquids, such as ether, collodion, benzole, &c, should be filtered under an air-tight vessel, or in a suitable filter sold for the purpose. Fixing. By this term is meant, in the daguerreotype process, the real fixation of the image to the plate by means of a boiling hot solu- tion of sel d'or. The image can then no longer be rubbed off by the finger. Tn the ordinary collodion and paper processes the term "fixing" is generally used to denote the removal of the sensitive material from the tablet, when the picture itself is in other respects completed. Flame. The combustion of an inflammable vapour mixed with air; or, according to some, "luminous gaseous matter." Thelumi- FLA 119 nosity of a flame depends chiefly on the presence of particles of solid matter. For instance, the flame of burning hydrogen is intensely hot and very feebly luminous ; but if a little lime be dusted into it, the particles become intensely luminous. In general it appears that the greater the heat of a flame the less the light, and conversely. If the top of the glass chimney of an oil lamp be contracted, there is less escape of smoke, more combustion of solid matter, and the light is increased with a diminution of the oil consumed. In the case of the flame of a candle or spirit lamp, combustion only takes place at the outer surface of the flame and not in the centre ; this may be proved by inserting a tube into the hollow of the flame, when the inflammable vapour will pass up it, and may be lighted at its other extremity. In the Argand lamp the wick is cylindrical, and the inside of the flame is supplied with air. Flame can only exist at a very high temperature. If a piece of wire gauze be laid across a flame, it conducts away some of the heat, and the combustible vapour, cooled by passing through the gauze, passes off on the upper side without flame. This is the principle on which Davy's safety lamp is constructed. Flare. Stray light falling upon the sensitive plate during its exposure in the camera. When certain forms of double or triple compound lenses are used, and the camera is turned towards a strong light, such as the sky, a circular spot of flare is sometimes seen in the centre of the ground glass. The cause of this has been a great puzzle to opticians and photographers. Some persons have supposed that it is the image of the round central stop formed by the back lens ; but that idea is absurd, because, if a bright object were placed so much nearer to the lens than its focal length, no real image of it could be formed, for the rays would still be divergent, and would cover the entire screen. Others have supposed that the round spot of light is pro- duced by rays which have suffered internal reflexion at the front lens ; but that supposition is open to some fatal objections. The probability is that it is really produced by reflected light from the bright edges of the lenses, since, when these are blackened and pro- perly covered with an annulus of metal, the flare spot disappears. No one should purchase a lens without first submitting it to the following test: — Mount it in the camera, and point the instrument towards the sky. Throw a black cloth over your head and shoulders, and look into the inside of the camera from behind. If you then see a ring of Light round the circumference of the lens, reject it, for it will never give a clear picture of a view in which any portion of the 120 FLI FLU sky is introduced. The cheap French portrait lenses are generally subject to the above defect. It never exists in the double and triple lenses manufactured by the best English opticians. Flint Glass. The composition of a good flint glass, sp.gr. 3*2, is as follows : — 120 parts of fine clear white sand, 40 purified pearl- ash, 35 litharge (red oxide of lead, or " minium"), 13 nitre, and a small quantity of black oxide of manganese. Flint glass is distinguished from crown glass by the absence of colour, and its higher refractive power. The lead, which is absent in crown glass, renders the flint glass more fusible, and increases its refractive power, giving it great brilliancy when cut ; but it has some disadvantages, for glass containing lead is softer and more easily scratched ; and it is difficult to obtain it of equal density throughout, and free from wavy marks or striaB. When borate of lead is added to glass, the density and refractive power are raised to the maximum at present known. See " Glass." Flowers, Coloured Juices of. Some of the coloured juices of flowers have been shown by Sir John Herschel and M. Chevreul to be sensitive to light, but none of them have yet been employed in Photography. The reader is referred to Sir John Herschel's memoir on this subject in the "Philosophical Transactions," Part 2, for 1842, for further information. Fluorescence. This term has been introduced by Professor Stokes to denote a remarkable property possessed by certain sub- stances with respect to light, and it has been adopted from the fact that fluor spar exhibits the phenomenon in a marked degree. Suppose a trough, the sides and ends of which are made of plate glass, to be filled with a solution of sulphate of quinine. A ray of sunshine is admitted through a small hole in a shutter, and passed through a prism so as to be decomposed into rays of the prismatic colours. The trough, with its solution, is then placed so as to receive and transmit the solar spectrum. On looking through the ends of the trough, the luminous and least refrangible rays are seen to be transmitted, while the extreme violet rays are absorbed, and seen to penetrate only to a certain depth in the liquid ; and in addition to this, rays beyond the violet, which were before invisible, are now rendered visible, and appear of a celestial blue colour, penetrating to a certain depth, and then disappearing. If a piece of sensitive photographic paper be placed so as to receive the spectrum transmitted through the fluid, it is found that the usual darkening at and beyond the violet end of it is wanting. FLU FOC 121 A block of yellow uranium glass possesses a similar property to the solution of sulphate of quinine. So do aesculine and other substances. For further particulars of this curious phenomenon, the reader is referred to Mr. Stokes's original paper in the " Philosophical Trans- actions " for the year 1852. It has been said that fluorescent bodies have the property not only of rendering the invisible rays visible, but of imparting chemical action to the luminous rays. This assertion must, however, be received with caution, and will probably turn out to be erroneous. Mr. Stokes is of opinion that " the phenomena of internal dis- persion oppose fresh difficulties to the supposition of a difference of nature in luminous, chemical, and phosphorogenic rays, but are per- fectly conformable to the supposition that the production of light, of chemical changes, and of phosphoric excitement, are merely different effects of the same cause." Fluorine. F=7. This is a hypothetical elementary body, which has not yet been isolated. Some of its compounds are well known. Focal Lines. "When a small oblique pencil is reflected or refracted at a spherical surface, or refracted at a plane surface, the reflected or refracted pencil has not a M geometrical focus," or " least circle of aberration," but all the rays composing it pass through two straight lines (or elongated figures of 8), situated in planes at right angles to each other, and called respectively " primary " and "secondary focal lines." For instance, suppose the circle at A, situated in a plane perpen- dicular to that of the paper, to be the base of a small pencil which has suffered oblique reflection or refraction. Then all the rays composing this pencil will first pass through the " primary focal line" q lt which is perpendicular to the plane of the paper, and after- wards through the " secondary focal line"; LEN 183 BAO= (j>' OC = OA=r ; and let fx be the refractive index from air into glass. Now when a pencil of parallel rays is incident upon a hollow sur- face, such as we are considering, the pencil becomes divergent within the glass. When direct and axial, its geometrical focus is at q ; when oblique, its primary focal line is q l5 and its secondary focal line q 3 .' We are going to prove in what follows that q x is nearer to the point A than q 3 ; and also that Aq 3 is less than Cq. Conse- quently the oblique pencil will be more divergent within the glass than the direct axial pencil ; and after emergence the oblique pencil will therefore have its focus (or circle of least coufusion) further from the lens than the geometrical focus of the direct axial pencil, so as in fact to flatten the field. Using the common notation, and bearing in mind that our pencils are small cylinders, the formula for the direct axial pencil becomes fi fi — l - = , « . .(1) Cq r and the formulae for the oblique pencil become fl fX cos 0' — cos Aq 2 r LL COS 2 0' fl COS 0' COS 0 (2) .(3) A qi r We have to ascertain first whether Cq is greater, equal, or less than Aq 3 ; and next, whether Aq Y is greater, equal, or less than Aq 3 ; bearing in mind that sin

another at /, and so on. If then we draw a curved line through the instantaneous positions of the atoms, epoqf, that curve will represent an undulation, or wave of light ; and the particles e and /, q and s, are said to be in the same "phase" of undulation; the length of the wave being the distance ef, or qs. A ray of common light is composed of undulations which are propagated in the manner described in all possible planes passing through AB. A ray of " plane polarized light" is one in which the undulations are propagated in only one plane which passes through AB. A ray of " circularly polarized light" is one in which the curved outline of the undulation, instead of lying on a plane, forms a spiral round AB like the thread of a corkscrew, and called a " helix." A ray of " elliptically polarized light" is one in which the spiral, instead of being coiled, so to speak, round a circular cylinder, as in the former case, is coiled round an elliptical cylinder. The subject of polarized light will be discussed presently. The effect produced by a ray of light is due to the blow of the last vibrating atom against the material substance upon which it is incident. As the undulations arc propagated by the luminous body continuously, these blows follow one another in rapid succession, and a vast number of very small blows thus administered produce an appreciable effect in a finite time. This effect is, moreover, con- 200 siderably increased when a number of rays are brought to a focus, and act upon the same point. Light, therefore, is motion ; or shall we say that light is the means by which a blow is transmitted from the luminous body to the body upon which light is incident ? Light travels in vacuo with uniform velocity; but there are different kinds of light, that is, light which exhibits different colours, viz., red, orange, yellow, green, blue, indigo, violet. These different colours are produced by the different lengths of the waves of light, as exhibited in the following table : — Extreme red ..... line A in Spectrum . C „ . . D E „ . E (x „ H I Extreme violet .... •00075 millimetres. •00074 •0006879 •0006559 •0005888 •0005265 •0004856 •0004296 •0003963 •00037 •00036 Hence it appears that the waves of red light being the longest, the number of undulations in a given time are the fewest ; and the waves of violet light being the shortest, its undulations are the quickest. When light passes from vacuum into a transparent medium, or from a rare medium into a denser, the velocity of the waves is diminished, and vice versa. The index of refraction, "ft," in geometrical optics, expresses in physical optics the ratio which the velocity of a wave of light in vacuo bears to its velocity in the medium into which it passes. This quantity " fi " is greater for violet than red light ; it would appear, therefore, that their velocities being equal at incidence, the red ray travels faster through a refract- ing medium than the violet ray. There would consequently appear to be a connection existing between the length of a wave and the velocity of its propagation. This circumstance is stated as a diffi- culty at page 285 of Professor Airy 's tract on the Undulatory Theory of Light. The difficulty has, however, been since removed by Pro- fessor Powell, of Oxford, who has demonstrated that within a refracting medium there is actually a difference between the velocities of red and violet light, the condition being that the intervals between the vibrating molecules of ether should bear a sensible ratio to the LIGr 201 length of an undulation, which condition is fulfilled within the refracting medium, although apparently not in space, where the velocity of light of all colours is the same. All material bodies are supposed to be more or less elastic, their particles not being in actual contact, and the interstices between them filled with lumeniferous ether. It is easy to conceive therefore that the chemical phenomena of light, and we may add of heat, and probably electricity, are produced by motion among the particles of the ether within the interstices of bodies, which communicates motion to the material atoms of the body itself, and alters their mutual arrangement. On this supposition, there can be no such thing as latent heat, latent light, or latent electricity, any more than there can be latent motion, which is a contradiction in terms. If we suppose light, heat, actinism, and the various forms of electricity, when developed in any body to be nothing more than the motion of an ether pervading all space, and filling the interstices of every substance, but varying in the length, velocity, and species of its undulations, we may explain by one general hypothesis a vast variety of astonishing phenomena due to agents between which many strong analogies are found to exist. Heat, for instance, is proved to be the undulation of an elastic medium, and its rays can be reflected, re- fracted, polarized, and made to exhibit interference just in the same way as rays of light. In short, there is a high degree of probability that the actinic, calorific, and luminous properties of the sunbeam are due simply to the different lengths of the undulations which are transmitted — a long wave (comparatively speaking) like the red exhibiting in a marked degree the effects due to heat, a short wave like the violet, those due to actinism, and a wave of medium length, those due to light. The laws of the reflexion and refraction of light can be easily explained on the undulatory theory, but not without having recourse to a mathematical demonstration which is not sufficiently elementary for the present work. The reader is referred for this demonstration to Professor Airy's Tract, pages 277 to 296, and also to Herschel's " Treatise on Light." We shall now consider some of the phenomena of polarized light. We have said that a ray of common light is composed of undu- lations which take place in all possible planes passing through the direction of the ray. Now the internal structure of certain sub- stances is such, that when a ray of common light is incident upon them, only the undulations which take place in a certain plane or jplanes can be propagated through the substance, and the others arc 202 LIG arrested. An instance of this occurs in the case of Iceland spar, the crystals of which are rhombs, and are said to be " doubly refracting." ft Br is a ray of common light incident at r upon a crystal of Iceland spar. On entering the crystal, the ray is divided into two, and suffers what is called " double refraction." One part of it, ro, is refracted nearly according to the usual law, and emerges in a direction oo', parallel to Br. This is called the " ordinary " ray. The other part of it, re, suffers refraction according to a new law (which is somewhat complicated, and need not be enunciated in this place), and takes the direction re, after which it emerges in the direction ee' , parallel to Br, the direction of the incident ray. This is called the " extraordinary " ray. It is evident that after emer- gence the ordinary and extraordinary rays are parallel to one another. On examining their properties by methods which will be described presently, it is found that both the rays oo' and ee' are what is called " polarized," that is to say, the undulations take place in one plane only, the plane of the undulations of the ray oo' being perpendicular to that of the ray ee' . These planes are called the " planes of polarization," and the rays are called " polarized rays," the term being derived from the idea entertained by Newton, that a ray of light has sides or poles. In the case of the Iceland spar, both the polarized rays are trans- mitted ; but in that of a thin plate of tourmaline, cut parallel to the axis of the crystal, only one of the polarized rays is transmitted ; and if this polarized ray be received upon another plate of tourma- line placed parallel to the first as regards its plane, but cross ways LIG 203 to the other in that plane, the ray will be altogether stopped. This effect may be popularly explained in the following way : — A ray of common light, consisting of undulations in all possible planes, is incident upon a plate of tourmaline, which is to all appear- ance a transparent substance, but its internal structure is such as to resemble the parallel bars of a grating, or wires of a cage. If then we consider the undulations of the ray of common light as taking place upon a number of cards, all passing lengthways through the direction of the ray, it will be only one of these cards that can be pushed between the bars of tourmaline and the other cards will be stopped. Again, if we receive this one card, which is the polarized ray from the first tourmaline, upon a second plate of tourmaline, with its bars placed crosswise to the first, it will be completely stopped ; but if the bars of the second tourmaline be placed parallel to those of the first, it will pass through readily enough. This ex- planation will perhaps convey a sort of popular idea of what is meant by polarized light. A ray of common light may be con- sidered as round, like a ruler ; a ray of polarized light as flat, like a riband. Light is polarized by reflexion as well as by refraction, and all reflecting surfaces have the property of polarizing light more or less, according to the angle at which it is incident upon them. Sir David Brewster discovered that when the tangent of the angle of incidence is equal to the refractive index of the medium upon the surface of which light is incident, the reflected ray is completely polarized, and therefore its undulations take place in one plane only. For instance, the surface of plate glass is a reflecting surface, and the refractive index of plate glass is about 1*54, which is the tangent of an angle of 57°. If, then, a ray of common light is incident upon the sur- face of plate glass at an angle of 57°, it will be completely polarized by reflexion, and the reflected ray will not pass through tourmaline placed in a particular position ; nor will it be reflected by another plate of glass placed in a particular position with respect to the first. Polarized light consists of the same colours as common light, and the waves interfere in the same way. Photographic pictures may be taken by it. Photographic experiments with polarized light have not yet, however, received much attention. In the process of copying negatives by light transmitted through thein alter having suffered reflexion at the surface of a plane reflector, the whole of the light which falls upon the sensitive plate would be polarized if the reflector were placed at a particular angle with the incident rays. 204 By means of the law of the tangent, discovered by Sir David Brewster, the refractive index of opaque bodies may be ascertained by finding the angle of incidence at which complete polarization by reflexion takes place. Although the subject of polarized light is one of great interest, yet the scope and object of this work do not permit us to say more about it. The phenomena next to be described are those of Inter- ference. Keturning to the figure at page 198. If we suppose another undulation to be propagated along the line AB, in such a way as to combine with the first, the elevations and depressions of the first would be increased and the effect at the ex- tremity B would be doubled ; but if the second undulation were such that its highest point came exactly over the greatest depression of the first, the undulations would exactly counteract each other, and no effect would be produced at B. Between these two extremes there would be of course an infinite number of mean effects, accord- ing to the way in which the waves were superposed. In the same way, by letting a stone fall into a pond of still water, undulations are produced ; and if a second stone be dropped in the same place, the undulations occasioned by the first may be either increased or diminished, or even altogether destroyed, and smooth water pro- duced, according to the state of the first undulations at the instant of time when the second series were propagated. It would appear, therefore, that two rays of light falling upon the same spot might either produce increased or diminished brightness, or even absolute darkness ; and this is found experimentally to be the case. For, let two rays of homogeneous light, red light suppose, emitted from dif- ferent sources of light A, B, be transmitted through two pin holes in a darkened box, and received upon the same spot C, of a white v screen. If the length of the beam AC be equal to that of the beam BC, or if the difference between AC and BC be any multiple of the length of a ray of red light, i. e., any multiple of -000025 8ths of an inch, the undulations will exactly combine at C, and the intensity of the red spot produced by either ray singly will be doubled. But if the difference between AC and BC be any odd multiple of half the length of a wave of red light, so that the crest of the undulation of one ray may be superposed on the depression of the undulation of the other ray, darkness will be produced at C. Two rays of light incident upon the same spot may therefore produce darkness, and in the same way two rays of heat may produce cold, and two rays of sound silence. This remarkable effect cannot possibly be explained on the corpuscular theory, for on the theory of the emission of LIGr 205 particles, more particles ought to produce more light instead of darkness. The phenomena produced by the interference of waves of light are among the most beautiful in optics. Nothing can exceed the splendour of the coloured images exhibited in many of the experi- ments which are illustrative of interference; but in this place we can only briefly allude to the fact that the colours produced by thin films are caused by the interference of the rays reflected from the inner susface of the film, with those reflected from the outer surface, while the iridescence of mother of pearl, and the varied hues of iridescent ornaments, are occasioned by fine lines existing in the surfaces of these bodies, which cause the interference of waves of light. Newton's rings, for instance, and the varied colours of soap- bubbles, are produced by interference. We now pass on to the Diffraction or Inflexion of Light. It is assumed in geometrical optics that light can only proceed in a straight line, and therefore that a body which intercepts it must necessarily cast a shadow of definite form, sharp outline, and uniform intensity of blackness. This assumption may have its uses in geo- metrical optics, but in physical optics it is found to be not strictly correct, for it appears that a ray of light, or line of undulations, is actually bent round the corner, so to speak, in passing close to the edge of an opaque body ; so that when an opaque body which inter- cepts the light proceeding through a small orifice into a darkened chamber is sufficiently narrow, and at a proper distance from the opening, the rays which are bent round the opposite sides of it, in- terfere and produce alternate bauds of light and darkness across its shadow ; and in every case, whatever may be the shape or size of the intercepting body, it is found that interfering waves of light produce a series of dark lines and coloured spaces round the edge of its shadow. To explain this phenomenon it is supposed that when light is admitted through a small hole into a darkened chamber, the central rays of the pencil pass straight on and produce a light spot upon the opposite wall, while the undulations which immediately touch the sides of the opening have the property, like those of sound, of communicating undulations obliquely to the ether within the box, and thereby of producing refracted rays which travel with diminished velocities, and, by interfering at the edge of the shadow, produce the dark lines and coloured spaces in question. It has been thought by some persons that the inflexion of light round the edges of dark objects which intercept it might tend to produce indistinct positives, when these are taken in a copying camera by light which is transmitted through a transparent negative. 206 LIG But this idea is erroneous. We have shown in the article on the " Condenser " that the light parts of a transparent negative which has either the sky, or a luminous background, or a light and con- denser, behind it may be considered as made up of a system of bright points, each of which is the origin of a divergent pencil of light, so that the bright point immediately adjacent to the edge of any dark part of the negative is the origin of a divergent pencil of light which is refracted by the lens to a focus. Now it is evident that any rays of light which may be bent by inflexion round the edge of the dark part of the negative would only add so many more rays to this divergent pencil, and that the lens would refract them to the same focus as the other rays of that pencil. They could not, therefore, produce indistinctness in the picture. It is important that this should be clearly understood. The indistinctness produced by copying-lenses is occasioned by spherical aberration not being properly corrected in them. We must now draw to a close our remarks on the interesting subject of the physical nature of light with a few observations on the colour of natural objects, and the theory of the decomposition of light by absorption. The colour of a natural object (when its surface is not iridescent and the colours produced by interference), is due to its absorbing all the rays of light which fall upon it, and reflecting or radiating only those of its particular colour. The colour of a substance is therefore due to some peculiarity of its structure. A black substance absorbs all the rays of light and reflects none, becoming at the same time heated, or, to speak more correctly, radiating heat rays, which seem to indicate that the absorbed light becomes heat. A white substance, on the contrary, reflects all the rays of light, and absorbs none, and does not become heated (comparatively speaking). This again bears out the idea of the conversion of absorbed light into heat. The reader may inquire what, in general, becomes of absorbed light, or lost light. We cannot do better than quote the following reply of Sir John Herschel to this question : — " The question, ' What becomes of light V merges in the more general one, ' What becomes of motion?' and the answer on dyna- mical principles is, that it continues for ever. No motion is, strictly speaking, annihilated ; but it may be divided, and the divided parts made to oppose, and in effect destroy one another. A body struck, however perfectly elastic, vibrates tor a time, and then appears to sink into its original repose. But this apparent rest is nothing else than a state of subdivided and mutually destroying motion, in which every molecule continues to be agitated by an indefinite multitude LIGr 207 of internally reflected waves, propagated through it in every possible direction from every point in its surface on which they successively impinge. The superposition of such waves will, it is easily seen, at length operate their mutual destruction, which will be the more complete the more irregular the figure of the body, and the greater the number of internal reflections." The theory of the decomposition of light by absorption, brought forward some years ago by Sir David Brewster and since advocated by numerous popular writers, has not been adopted by men of science, but, on the contrary, condemned by Professors Airy, Helmholtz, and others. The idea that the seven colours of the spectrum can be decomposed by absorption into three, viz., red, blue, and yellow, has been shown to be an illogical inference from experiments imperfectly conducted. There must be considered to exist as many different kinds of light as there are different lengths of waves within the limits of the visible spectrum ; and when the undulating ether ceases to produce the phenomena of light and colour, its undulations may be called heat, actinism, or as the case may be, according to the effects they produce on grosser matter. With respect to the chemical action of light, this must be consi- dered as due, in all probability, to the vibratory motions of the ether within the interstices of a body establishing a mechanical disturbance amongst its particles, which either enables or compels them in certain cases to form a new arrangement, or enter into new com- binations with each other. By adopting the notion that there is no absolute contact between atoms, ami that all bodies are more or less elastic, we can readily imagine that the undulations of the universal ether may act as a mechanical force in promoting chemical combina- tion, or decomposition, determining cystallization, and so on. We need not in this place enumerate any of the principal phenomena of actinism, but it may be well to define clearly what the term actinism means. Assuming, then, as highly probable, that the phenomena of heat, light, actinism, and the various kinds of electricity are produced by undulations of the same universal ether, which differ only in the lengths of the waves and the velocities of their propaga- tion, we should define actinism to include all such chemical changes as are effected by waves of ether varying in length between thai of a ray of yellow light and of an invisible ray of the greatest ascertained ref'rangibility. According to this definition, whatever the chemical effects of the red and yellow rays may be, they would be attributable either to heat or light, and not to actinism, for both heat and light are known to produce important chemical changes. But, after all, a definition of this kind is only of use until, by some fortunate 208 LIG generalization of causes, we are enabled to substitute a better one f01 Tlie question of " Latent Light" is discussed with that of" Latent Heat;" (g.v.) Lignin. C qB Ho 2 Oo 2 . This substance may be considered as a compound of carbon and water. It is of great importance in photo- graphy, first, because pyroxyline is made from it ; and secondly, because paper is a nearly pure form of it. . Li-nin forms the solid framework of plants. It is obtained m a pure form by removing from sawdust, or any other kind of finely divided woodv fibre, all soluble matter, by steeping it m hot and cold water, boiling it in alcohol, water, solution ot potash, weak hydrochloric acid, and lastly in distilled water, and then drying the residue at 212°. Or in addition to the above treatment, it may be bleached by chlorine, and rinsed in acetic acid. The cleansed and bleached fibres of linen or cotton are tolerably pure lignin. Purelio-nin is white, tasteless, and insoluble in water, alcohol, ether, audthe oils, or hydro-carbons. Its sp. gr. is 1-5. When acted on by a cold concentrated solution of sulphuric acid, it is converted into dextrine and grape sugar ; cold concentrated nitric acid converts it into xyloidin, having nearly the same properties as that obtained from starch; the continued action of hot nitric acid on it produces oxalic acid; hydrochloric acid blackens, but does not dissolve it, and the acid becomes red or brown; a hot and strong aqueous solution of potash produces oxalate and acetate ot potash. It is evident, therefore, that in the process of making pyroxyline by acting on lignin with nitro-sulphuric acid, other compounds may be formed which would in general be injurious in collodion. Lio-nin combines energetically with various salts and metallic oxides, and this property is very important in the arts ot dyeing and calico printing, in which colouring matters are made to combine, with textile fabrics; and also in the preservation of timber from dry rot, and of canvass from mildew, &c. This property lies also at the foundation of the photographic processes on collodion and paper ; for had lignin, in its natural form of paper, or in its altered form as pyroxyline, no power of combining chemically with metallic oxides the photograph would merely lie upon the surface of the film, and could be blown by a breath or removed by a touch from it. It may, however, be the presence of the organic matter that is necessarily associated with the material of photographs which either causes or assists them to fade. Wood may be preserved from dry rot by Kyan s patented process LTM 209 of steeping it in a solution of bichloride of mercury ; or in one of sulphate of iron, sulphate of copper, or chloride of zinc. The latter salt is especially useful in protecting sail cloth from mildew. Alu- mina combines energetically with calico and linen, and is much used as a mordant in dyeing. Woody fibre appears to be permanent in dry air, or completely under water, but not when exposed alternately to the action of air and damp ; the ultimate effect of the gradual process of decay being the removal of all the elements but a portion of the carbon from the lignin. Hence it is that some forms of coal, as anthracite, are nearly pure carbon. One of the products of the decomposition of lignin is " fire damp another, carbonic acid. The beds of coal in different parts of the world are supposed to have been formed by the decom- position of the forests of monster ferns which at one time covered a large portion of the earth's surface, in all latitudes, before it cooled down to its present temperature, and when its atmosphere was too much impregnated with carbonic acid to be fit for the support of animal lffe. Lime, Carbonate of. CaO, C0 2 = 50. This substance forms one of the most abundant compounds of rocks in almost every part of the world, in the shape of marble, limestone, chalk, &c. In Photography, precipitated chalk is often used for neutralizing acid solutions of silver, gold, &c. It is particularly useful in depriving chloride of gold solutions of their excess of hydrochloric and nitric acids, and thus suiting them for toning photographic prints ; for this purpose, pounded, or better still, precipitated chalk, is shaken up in a bottle with the acid gold solution ; the weak carbonic acid is expelled by the stronger acids and escapes ; chloride of calcium and nitrate of lime are formed, and the solution after a time becomes neutral. Lime, Chloride of. The exact chemical composition of this substance is a subject of dispute among chemists. It emits the peculiar odour of hypochlorous acid when exposed to the air, and at the same time absorbs carbonic acid. This constitutes its value as a disinfecting agent. Its bleaching properties are well known. In photography it is sometimes used in the gold toning bath to neutralize the acidity of the solution, but it must be used with great discretion, otherwise the chlorine which escapes attacks the silver image, converting it into the white chloride of silver. In the case of over printed pictures, a weak aqueous solution of it might occasionally be found useful in reducing excessive vigour. On the other hand, there is considerable disadvantage attending its use, p 210 LTM LTN both in the toning bath and in plain aqueous solution, viz., that it attacks the half tones more strongly than it does the deep shadows. On account of its powerful oxidizing properties, this substance has also recently been recommended for eliminating the last traces of hyposulphites from washed photographic prints. This it effects by converting the hyposulphites into innocuous sulphates. It is doubtful, however, whether the advantage gained will counter- balance the disadvantage of an enfeebled print. By means of chloride of lime, silver stains on the hands, &c, may easily be removed, and this is a more convenient method than the one generally adopted with cyanide of potassium. Make up a little of the dry chloride into a paste with water, acidulated with any acid, and apply it to the stains, by hard rubbing. If the stains have been recently formed, they will quickly disappear, but if they are old, a little friction with pumice stone may be required. Lime-light. When a jet of mixed oxygen and hydrogen gases is ignited, the flame is scarcely visible although intensely hot ; but by introducing solid matter into it, by causing it to play upon a ball of lime, a most intense white light is produced. (See "Flame.") This is called the " Drummond Light." It is highly actinic, but less so than the light produced by the charcoal points of a voltaic bat- tery. The Drummond Light is employed at public institutions for the exhibition of dissolving views and microscopic specimens. Another form of lime light, invented by Messrs. Home and Thorn- thwaite, of Newgate Street, consists in urging a jet of oxygen through the flame of a spirit lamp, and causing it to play upon a ball of lime. The light is very white and intense, although not equal to that of the Drummond Light. It is called the " Oxycalcium Light" (q.v.), and is very convenient for exhibiting the magic lantern to a small assemblage of spectators, as well as in certain photogaphic operations when conducted by artificial light. The incandescent lime is gradually dissipated, or sublimed, and the lime ball must be renewed from time to time. Lime-toning Bath. See " Calciochloride of Gold." Line. The one-twelfth part of an inch. Linseed Oil. A drying oil, obtained by expressing the seeds of common flax, which yield from 20 to 25 per cent, of their weight. Its sp. gr. is -9395 at 52°. It may be cooled down to 40° without solidifying. It is soluble in 40 parts of cold, and 5 parts of boiling alcohol and in 1*6 parts of ether. A small quantity of the alcoholic LTQ LIT 211 solution is sometimes added to spirit varnishes to diminish their brittleness. The drying quality of linseed oil is increased by boiling it from three to six hours, and then stirring into it from 7 to 8 hundredths of its weight of litharge ; (q. v.) In this operation the lead is par- tially reduced, and a little oleate and stearate of lead are formed, which the oil holds in solution. A mixture of boiled linseed oil and mastic varnish forms a gela- tinous substance much used by artists, and called "Magilp." Linseed oil is extensively used in paints and varnishes, and also in printer's ink. Liquor Ammonise. Aqueous solution of Ammonia ; q. v. Liquor Potassae. Aqueous solution of Potash ; q. v. Liquorice Sugar. See " Glycyrrhizine." Litharge; Massicot. Protoxide of lead, Pb. 0 = 112. The " galena," or native sulphide of lead, from which lead is commonly obtained, sometimes contains silver. When this is the case a blast of hot air is passed over the fused mixed metals. This oxidizes the lead, but has no effect upon the silver. The oxide of lead, or litharge, is therefore blown off, and collected in a suitable chamber. Litharge is a heavy yellow powder, slightly soluble in water, to which it gives an alkaline reaction. It melts at a red heat, and tends to crystallize on cooling, but on reaching 212° falls into a powder. When melted it combines energetically with siliceous matter, and would destroy an earthen crucible. Litharge is much used by painters as a dryer. (See " Linseed Oil.") Dark red litharge is called " Litharge of Gold ;" the pale variety, " Litharge of Silver." Water which has been filtered through litharge increases the energy of the developer which is dissolved in it, but this should be used immediately, for it will not keep. There is also danger of its producing fog upon the plate. Lithium, Iodide of. LI +6 aq. = 188. The iodide of lithium has been recommended for photographic use because of its greater solubility in alcohol than many other iodides. It is, however, so very deliquescent that it can with difficulty be preserved in the dry state. It may be prepared by mixing, in concentrated aqueous solution, equivalent weights of sulphate of lithia and iodide of cal- cium, then evaporating in vacuo over sulphuric acid, and exhausting the dry product with strong alcohol. p 2 212 LIT LOG Litmus; Tournesol; Lacmus. A violet-coloured paste, sold in the form of blocks or pyramids. It is made, like archil, by treat- ing certain lichens which grow on rocks by the sea side, in the following manner : — They are cleaned and ground into a pulp with water ; then, am- moniacal liquors from the gas works are added, and the mass fre- quently stirred and exposed to the air as much as possible. In this way a peculiar colouring matter is produced, which when perfect is pressed out, and mixed with chalk or plaster of paris so as to form a paste. This is the "Archil " of commerce, much used as a purple dye. Another variety, made in the same way in Holland, from the lichens called Roccella tartarea, and Lecanoro ta?'tarea, is called " Litmus." It has a violet-blue colour, is easy to pulverize, and is partially soluble in water and dilate alcohol, leaving a residuum composed of carbonate of lime, clay, silica, gypsum, and oxide of iron combined with the dye- The colour of litmus is reddened by acids, and afterwards restored by alkalies. Litmus is much used for making Test-papers ; g.v. Liver of Sulphur. The compounds obtained by fusing potash or its carbonate with sulphur, have been designated livers of sulphur, on account of the colour which they assume. This substance is very useful to the photographer in reducing silver residues from old hyposulphite baths, &.c It may be prepared by heating in a covered crucible eight ounces of carbonate of potash with from four to six of sulphur, till the whole forms one uniform mass. To use it, when the mass is cold, break it into fragments and throw them into the waste solutions. After a time the whole of the silver will be preci- pitated as a sulphuret, which may be reduced to the metallic state by fusion. It is better to have always a large excess of liver of sulphur in the waste jar. Logwood. The heartwood of the Hamatoxylon Campechianum, of the West Indies ; brought to Europe in logs about 3 feet in length. The infusion of this wood is of a dark red or purple colour, and is used in dyeing and staining. It gives purples and blues, and also blacks of various intensities by means of iron and alum bases. Its colouring matter is called 4 1 Hematin." When chipped logwood is for some time exposed to the air it loses a portion of its dyeing property. Decoction of logwood absorbs oxygen from the atmosphere, and will then precipitate gelatine, a property which it did not at first possess. LUN MAG 213 Lunar Caustic. Nitrate of silver fused and moulded into sticks. Commercial lunar caustic is sometimes adulterated with the nitrates of potash, zinc, lead, and copper, and should not be used in Photo- graphy. Lutes. Used for securing the junctions of vessels, and prevent- ing the escape of their contents when submitted to various chemical operations, as distillation, &c. The lutes used for ordinary purposes are slips of bladder, linseed meal made into paste with gum water, albumen and quick lime, putty, and a fat lute composed of pipe clay and drying oil. Windsor loam is used as a lute to withstand a high temperature ; this is made by mixing clay and sand into a stiff paste with water. If intended to vitrify, borax or red-lead should be mixed with it. Magic Lantern. An instrument for exhibiting magnified images of transparent pictures upon a screen. The arrangement of the different parts of the apparatus will be understood from the following figure. A is the source of light ; which may be either an argand lamp with a reflector behind it, or better still, a lime-ball rendered incan- descent by passing a jet of oxygen through the flame of a spirit lamp, and causing the flame to act upon it. (See " Oxycalcium Light.") BC are the lenses of the condenser (see " Condenser"), which collect all the rays of light which proceed from the lamp, within the angle FAB, and cause them to converge in such a way as to pass through the transparent picture ED. The front lens FB is plano-convex, the back lens C a " Crossed lens q. v. ED is the transparent picture, which must be placed in an inverted position. L is a combination of lenses having short focal length ; and de the screen on which the magnified image of ED is received, and rendered visible. 214 MACx The distance of the slide ED from the lens at L is rather greater than the principal focal length of that combination, and then the screen, which is in the other conjugate focus of the lens, is at a much greater distance from it. A pencil diverging from E is brought to a focus at e, and a pencil from D at d ; and so on. The combination of lenses at L which the photographer will find the best is the common portrait combination ; and when this is used, the posterior lens of the combination should be placed next to the picture, and a stop should be placed between the lenses, as occasion may require. The focus of the condenser should fall pretty nearly in the lens at L which is nearest to the screen. In order to obtain a perfectly flat field upon the screen, the picture ED should be painted, or photographed, upon a curved surface like a watch glass. Magic lantern slides are painted in transparent colours ground in Canada balsam. Photographic slides for the magic lantern may either be printed upon dry collodionized or albumenized glass plates, by superposition of the negative, or they may be printed by the wet collodion process, by means of a Copying Camera. The glass should be perfectly transparent in the light parts ; but stereoscopic slides backed with ground glass may be exhibited in the magic lantern, and produce a tolerably good effect. Sometimes the screen, or sheet, on which the image is thrown is wetted in order to render it semi-transparent, and the spectators are then placed behind it, and consequently see nothing of the apparatus. Dissolving views are produced by means of two lanterns, the axes of which are directed towards the same part of the screen, and each of which contains a slide. When a view is to be changed, the cap which closes one lantern is gradually opened, while the other lantern is being shut off. This is done by turning a handle which puts in motion a piece of mechanism contrived for the purpose. Magic Photographs. These amusing toys may be made thus : — Print from a negative as usual, either on albumenized or plain paper, but do not tone in gold. Fix in hyposulphite of soda, and thoroughly wash. Then immerse the prints in a saturated solution of chloride of mercury till all trace of an image has disappeared. (If the solution be warmed the action is much more rapid and com- plete.) Afterwards wash and dry. When it is required to redevelop them, have a piece of blotting paper previously soaked in hyposul- MAG 215 phite of soda and dried, moisten it with common water, and while still wet press it down on the invisible image. Instantly the picture will reappear with more than its original vigour. If it be again washed, the image will probably be very permanent. Sir John Herschel discovered this process many years ago. Magilp. A gelatinous compound made by mixing boiled oil and mastic varnish, in about equal proportions. It is much used by artists for thinning oil colours, and " glazing " on delicate tints in finishing of the picture. Magma. When a mixture of substances forms a thick pasty mass it is called a " Magma." Magnesium. Mg. = 12. This is a silver-white metal of crystal- line structure and somewhat brittle. It is obtained by acting on chloride of magnesium with metallic sodium, and is afterwards purified by distillation through a current of hydrogen. Afterwards it is formed into wire by forcing it while hot through small apertures in a steel block. Its chief use in Photography is for the purpose of illumination, possessing, as it does, the power of emitting a most dazzling bluish white light when burnt in air or in oxygen. This light is strongly actinic, and hence it is sometimes used by photographers in dull weather, or at night, for taking negatives when they could not otherwise be obtained. Several ingenious contrivances for lamps with reflectors have been proposed in order to facilitate the production of a regular and constant flame from the burning wire, but all of them are more or less liable to objection. The light emitted is more actinic than that produced by any other artificial means. Magnesium, Iodide of. Mg. I -f aq. ? This salt is sometimes used for iodizing collodion, but it is generally considered inferior to some other iodides. To prepare it, heat crystallized sulphate of magnesia in a porcelain vessel on a sand bath, until all the water of crystallization has been expelled and a white mass remains. Rub up 50 grains of this mass with about 65 grains of powdered iodide of potassium. Then agitate the whole in a bath with four ounces of strong alcohol for about a quarter of an hour. Filter out the insoluble sulphate of potash, &c, and add two drachms of the filtered solution to each ounce of plain collodion. Magnesium, Oxide of ; Magnesia. Mg. O=20. A white, 216 MAL MAR heavy, insipid powder, nearly insoluble in water, and having a very feeble alkaline reaction on vegetable colours. It absorbs carbonic acid and water from the air, but these may be driven off again at a red heat. Its salts have in general a bitter taste, and many of them, in particular the chloride of magnesium, and nitrate of magnesia, are deliquescent. The affinities of magnesia for the acids are in general about equal to those of ammonia. Malt Process. By this dry process good negatives can be obtained as follows : — Sensitize and thoroughly wash the colodionized plate, as usual in all dry processes. Then pour on and off the plate several times good sound Burton or Edinburgh ale, and set aside in a dark cupboard to dry, or dry by artificial heat. Another method for making the preservative is this : infuse in four ounces of hot, not boiling, water, one ounce of ground malt. Let it stand for an hour or two in a warm place, with occasional stirring, then filter. This solution will not keep longer than a few days after it has been pre- pared. Conduct the development as in the Fothergill process. Manganese, Black Oxide of ; Mn. 0 2 . This is a mineral which occurs native in Devonshire, Somersetshire, and Aberdeenshire. Its chief, and perhaps only use to the photagrapher, is as a source of oxygen gas for the oxycalcium light. When heated in a retort to a full red heat it gives off oxygen, and becomes converted into a sesquioxide, Mn 3 0 3 ; it should be well dried before being put into the retort. When added, in the proportion of about one part of black oxide of manganese to three parts of chlorate of potash, and moderately heated in a retort, over a common fire, oxygen is abun- dantly given off. The presence of the manganese greatly assists the evolution of oxygen from the latter salt, without appearing itself to undergo decomposition. Marine Glue. Digest from 2 to 4 parts of inrlia rubber, cut into small pieces, in 34 parts of benzole, and promote solution by heat and agitation. The solution should have the consistence of thick cream. Add to it 62 parts of powdered shellac, and melt the mix- ture over the fire, stirring it well. Then pour it upon plates of metal, so that it may cool in sheets. To use it, melt it in an iron vessel at about 250°, and apply it with a brush to the surfaces to be joined. Let it get hard ; then moisten the surfaces with benzole, and press them into contact. Marking Ink. First apply the following mordant to the linen : — Carbonate of soda . . . .2 ozs. Distilled water 1 pint. MAS ME A 217 Then write upon it, when dry, with the following ink : — Nitrate of silver . . .1 drachm. Powdered gum arabic . . . 2 drachms. Sap green ... .1 scruple. Distilled water . . . . 1 ounce. Or the following ink may be applied without a mordant : — Nitrate of silver . . .1 ounce. Carbonate of soda . . . . 1% ounce. Tartaric acid .... 2 drachms. Ammonia . . . . . 2 ounces. Archil . . . . . \ oz. White sugar . . . . 6 drachms. Powdered gum arabic . .10 drachms. Distilled water . • . . quantum suff. Dissolve the nitrate of silver and carbonate of soda separately, then mix, wash the precipitate, put it into a mortar, and add the tartaric acid until effervescence ceases. Add the ammonia to dissolve the tartrate of silver, then mix in the other ingredients with the distilled water. Mastic. A species of resin much used in varnishes. It comes from the Levant, and occurs in small drops or tears of a pale yellow colour, which are the produce of the Pintado, leuthcm. It contains about 90 per cent, of a resin easily soluble in alcohol (used for varnishes), and a viscid and difficultly soluble resin. Matches, Congreve, or Lucifer. Make the following ingredients into a paste with water, and dip the matches into it : — Gum arabic . . . . .16 parts. Phosphorus, powdered . . . . 9 ,, Nitrate of potass . . . . . 14 ,, Black oxide of manganese . . . 18 ,, Mealiness of Prints. This term has been applied to photo- graphic prints characterized by numerous small spots of irregular toning, and general lack of vigour. They arise most often from a badly prepared toning bath, but sometimes, also, from the nature of the paper, some kinds being more liable to this defect than others. Measles. When prints are imperfectly fixed, the appearance pre- sented is very similar to that of the same disease in the human sub- ject. Hence the name. They are best seen by looking through the print, because they exist chiefly in the texture of the paper. The specks consist of sulphide of silver, which, when once they 218 MEA MEE are formed, cannot be removed by hyposulphite of soda, nor by washing. They can, however, in most cases be easily avoided by fixing the prints in stronger solutions of the hyposulphite. Measures. See Tables at the end. Melainotype. This positive process is of American orgin, and derives its name from the black colour of the material which sup- ports the picture. "Very thin sheets of iron are coated on both sides, and on the edges, in fact, entirely covered with a kind of black japan varnish. The side which has to receive the collodion must be very smooth and highly polished. When coliodionized, excited, and in every way treated as in the positive process on glass, these plates yield beautiful results, and possess the great advantage of being less fragile, so that they can be sent by post, &c, without danger of breakage. Melting Point of Metals. The folio from Turner's " Elements of Chemistry" :- ving table is extracted Fusible below a red heat, or 980°, the heat of a common fire being about 1200°. ( Mercury Potassium Sodium . Tin . Cadmium Bismuth Lead Zinc . ^ Antimony Silver Copper . Gold Cast Iron Fahrenheit. . —39° . 136° . 190° . 442° . 450° . 497° . 612° . 773° . 1873° . 1996° . 2016° . 2786° Mercury. Hg. = 100. A white metal, fluid at ordinary tem- peratures, and solid at — 40°. It boils and becomes vapour at 660°, and emits vapour at all temperatures above 40°, The prin- cipal ore of this metal is the sulphide, or native cinnabar. Perfectly pure mercury may be agitated in contact with air, oxygen, hydrogen, nitrogen, carbonic acid, and alcohol ; but when impure it becomes coated with a gray powder, which is a mixture of the oxide of the foreign metal and finely-divided mercury. On the other hand, when mercury is agitated with water, ether, or oil of turpentine, it is reduced to gray powder, which is composed of minute globules of mercury blended with .the foreign body. MER MET 219 Mercury combines with several of the metals, and forms !' amal- gams." Mercury, Chloride of. Hg Cl= 135-5. This salt, generally known under the name of corrosive sublimate, is of considerable photographic importance. Tt can be economically prepared by subliming, at a red heat, equal portions of the sulphate of the protoxide of mercury and dry chloride of sodium. The chloride of mercury passes over and forms a transparent and dense crystalline mass, whilst sulphate of soda remains behind in the retort. It is soluble in about sixteen parts of cold, and in about three of boiling water. It is more soluble in ether and alcohol. A little hydrochloric acid added to water increases its solvent powers. Being very poisonous, this salt should be used with great caution. This salt is sometimes used as an intensifier for negatives ; the modus operandi being as follows : — Pour over the negative a cold saturated solution of the chloride, until the image becomes of a whitish gray colour. Thoroughly wash the film, and treat it with a one-grain solution of iodide of potassium till the image assumes a greenish colour, which is very non-actinic. Negatives treated in this way print excellently for a while ; but, after being exposed to light many times, the image becomes too intense for yielding good prints. Mercury, Oxide of. There are two oxides of mercury, viz., the black, Hg. 2 0, and the red, Hg. 0. Both are salifiable. The black oxide is reduced by light into Hg. and Hg. O. The red oxide becomes black when heated, but red again on cooling. It is superficially reduced by light, and becomes black. These oxides form a great number of curious and important salts. Mercury Bath. The box in which daguerreotypes are developed by the fumes of mercury. It should be made of iron, in the form of an inverted pyramid, provided with a thermometer, and supported upon an iron stand. The plate should be placed horizontally over the mercury. This form of apparatus was first used in America, and may now be seen at most photographic depots in this country. It is very simple and inexpensive. Metagallic Acid. An organic substance formed by the oxid- ation of gallic acid. It combines with oxide of silver, and other metallic oxides, and displaces carbonic acid from the alkaline car- bonates. It is produced by rapidly heating gallic acid up to 480°, when carbonic acid and water are evolved, and metagallic acid remains as a black shining substance, insoluble in water, alcohol, and ether. Its equivalent is supposed to be C 12 H 3 0 3 , HO. 220 MET Metagelatine. When a strong solution of gelatine has been boiled and cooled several times, it ceases to gelatinise on cooling, and remains fluid. In this state it is called metagelatine, and may be used successfully as a preservative substance in the collodion process. It is a much less powerful reducing agent than honey, and therefore less likely to fog the collodion plate. The mode of preparing metagelatine, originally described by Mr. Maxwell Lyte, is as follows : — Dissolve 1^ ounce of pure white gelatine in 10 ounces of boiling water. Add 60 minims of strong sulphuric acid diluted with 2| ounces of distilled water. Boil for five minutes, and then let the liquid cool. Then boil it again for five minutes and let it cool. Should it still gelatinise on cooling, repeat the operation. When it remains fluid on cooling, neutralise the acid with powdered chalk, and remove the insoluble sulphate of lime by squeezing the mixture through a cloth. Metagelatine dries to a hard transparent film upon the collodion. When intended to be used as a moist preservative, a little golden treacle (not honey, nor glucose) should be added to it. Methyl. Me. (Greek, pedv, wine, and v\rj, wood.) A hypo- thetical substance, the composition of which is assumed to be C 2 H 3 , and which forms the supposed base of a series of methyl compounds analogous to those of ethyl. Methylated Alcohol. See " Alcohol." Methylated Ether. See " Ether." Methylic Alcohol. Me. 0 + H 0 = C 2 H 8 0 + H 0. This substance, called " wood alcohol," or " pyroxylic spirit," is a hy- drated oxide of methyl, and analogous to the alcohol of the ethyl series. It is one of the products obtained from the distillation of wood. In its properties it greatly resembles alcohol, and as there is no duty on it, it is much used as a solvent for varnishes, &c. Its taste is hot and pungent; sp. gr. *800 ; it boils at 150° at the mean pressure of the atmosphere ; mixes in all proportions with water, alcohol, and ether ; and is neutral to test paper. By a method of purification and re-distillation through charcoal, Mr. Eschwege has been enabled to free this alcohol from its nauseous smell, and in a great measure from its hot, pungent taste. In consequence, the legislature have forbidden this method of purifi- cation ; which is much to be regretted, because Mr. Eschwege's preparation is a capital solvent, without the aid of ether, for pyroxy- line, and forms a collodion admirably suited for hot climates, where MET MTC 221 great difficulty exists in coating a plate evenly with the ordinary collodion, on account of the rapid evaporation of the ether. Methylic Ether. Me. 0 = C 2 H 3 0. When equal parts of "wood alcohol" (or " pyroligneous spirit," as it is termed) and sulphuric acid are distilled together, a gas is evolved, which, when collected over mercury and purified by potash, is found to consist of oxide of methyl, or methylic ether. It has an ethereal odour, with a taint resembling peppermint, and may be taken up abundantly by alcohol, or wood-alcohol, or ether. Mica. A transparent mineral, of a pearly lustre, mostly brought from Siberia and India. It may be divided into sheets as thin as paper, which are hard and flexible. It is an ingredient of granite and gneiss. It is sometimes used in Photography instead of glass. Micro-Photography. This term is now used to designate the re- duction of negatives to a very minute size, and serves to distinguish it from the process denominated "Photo-micrography," (y. v.), which means the enlargement, by means of Photography, of micro- scopic objects. Micro- photography has received considerable attention in this country ; chiefly, however, by amateurs. In France it is carried out commercially on a large scale by several firms. M. "Wulff, Hue Richelieu, Paris, has published a short treatise on the subject, wherein he describes a very perfect apparatus, whereby these bijoux photographs can be taken with the greatest ease by any one conversant with the manipulations of Photography. A translation of this article, with illustrations, was, by permission of the author, published in the " British Journal of Photography," of March 24th, 1865. For very detailed information we refer our readers to that article, or to the original work. On a small scale an apparatus can be extemporised very readily by those who know how to use the microscope ; but if the instru- ment is a valuable one, it is advisable not to run the risk of injuring it by nitrate of silver or other chemicals. In the year 1857, Mr. Hislop communicated to the North London Photographic Associa- tion a description of a very cheap and convenient apparatus, which we have used with much success. It consists of a rigid mahogany board of about three and a half feet long and six inches wide. At one end there are two uprights, between which a miniature camera, fitted with a one-inch microscopical objective, can be raised up or down, so as to place the centre of the lens opposite the centre of the negative, whatever may be its size. The object-glass is screwed 222 MIL to a brass tube projecting from the camera towards the negative, the tube being fitted with stops of different sizes. A micrometer head, for fine adjustments of the lens, is also necessary, because microscopic objectives are only corrected for the visual rays. In focussing, the best glass is one coated with collodion, sensitised and washed. The sharpest visible image must be found with a powerful magnifying glass, and the chemical focus ascertained by repeated trials. When once found it will be the same in every case, provided the negative is placed at the same distance from the objective. The negative itself is placed in a frame at any required distance on the long mahogany board, but its plane must be per- pendicular to the axis of the tube holding the objective. In Mr. Hislop's apparatus the frame is constructed so as to slide along in grooves, cut perpendicular to the axis. The illumination may be either by natural or artificial light, but in every case it must pass through the negative. When using natural light, the whole system, except the side of the negative farthest away from the lens, must be inclosed in a dark box, and that side exposed to the sky. Artificial light is much less trouble- some, and all the operations may be conducted in a drawing- room moderately lighted, without any of the usual photographic " messes." A paraffine lamp behind the negative, with a condenser intervening, and very close to the latter, gives very nearly parallel rays, which, without any covering to shut out extraneous light, will impress the sensitive plate in from five to forty seconds, according to the strength of the illumination and the density of the negative. We believe Messrs. Home and Thornthwaite, of Newgate Street, London, supply the above apparatus at a trifling cost. The collodion used to coat the plates should be absolutely structureless, otherwise a disagreeably reticulated appearance will be presented when such " pin-head " photographs are examined under the microscope. For a similar reason, pyrogallic acid, and not protosulphate of iron, should be used in the development, because the former gives a much finer silver deposit than the latter. M. Dagron, of Paris, w r ho has been most successful in this kind of photographic practice, uses collodio-albumen. Milk. The milk of different animals does not appear to vary much in composition. It is composed of three principles, viz., butter, cheese (or casein), and whey (or serum). When examined under the microscope, milk appears as a transpa- rent fluid in which small white globules are diffused. These rise to MOR MOS 223 the surface when the milk is left at rest, and form cream, which may be removed by skimming. The globules are supposed to be the butter contained in small membranous bags, which are broken by churning. Ether has at first no effect upon these globules, but the membrane is soluble in acetic acid, and when dissolved the butter is liberated and ether readily takes it up. Casein is a substance resembling albumen in its properties. It is soluble in an alkali, and may be coagulated by an acid. See "Ca- sein." Serum or Whey is the watery matter of milk. It contains a sugar called Sugar of Milk, q. v., and also various salts. Milk contains about 14 per cent, of butter, 14 percent, of casein, and the remainder whey. It is either neutral or slightly alkaline, but quickly becomes acid by exposure to air, from the formation of lactic acid. Both casein and whey are useful in Photography, but milk should not be used in any process until the cream has been completely re- moved by skimming. Curd or casein is produced by adding an acid, or better still, a piece of rennet, to fresh milk. See " Rennet." Mordant. A class of substances used in dyeing, their effect being to cause the dyeing material to combine with the fabric to which it is applied, so that it cannot be removed by simple washing, or any ordinary treatment. Mordants are in general metallic oxides which have an affinity for the organic matter of the stuff, and by combining with it, cause the particles of the dye to combine also, and form a species of triple compound. The mordant most likely to be useful in Photography is bichromate of potash when reduced by light. This enters into powerful combination with lignin, and takes down with it any particles of carbon, or pigment, or colouring matter that are mixed with the bichromate. In this way textile fabrics may be printed by Photography. Moser's Images. M. Ludwig Moser has described at different times in Poggendorff's " Annalen," a variety of curious experiments in which images were obtained by contact and developed by vapours in a similar way to the images in the process of Daguerre. Prom these experiments he has drawn certain conclusions, which are by many supposed to be erroneous. For an account of them the reader is referred to Hunt's " Researches on Light," page 248. Mr. Grove, and Professor Volpicelli have also obtained latent images capable of being developed by vapours, by means of contact aided by electricity. The account of these experiments will be found in the 2nd volume of " Photographic Notes," edited by Mr. Sutton. This 224 MOU subject is one of great interest, and no doubt intimately connected with Photography ; it is, however, at present involved in so much obscurity, that we think it better to refer the reader to other sources of information than to occupy space in this work with more than a brief notice of it. Mr. M. Carey Lea has recently found that a raised image on any hard object pressed against a sensitive collodion film can be developed by the usual method with such films. These facts are very curious, and as yet inexplicable. Mounting Prints. This consists in attaching the print, either entirely, or by its edges, to a sheet of cardboard, by means of an adhesive cement. Of all adhesive substances, starch seems to be preferred by photo- graphers. It should not be made too thick, and after being boiled should be strained through a cloth, and used quite fresh, as sour starch would be certain to cause the fading of the print ; and if the pictures are kept in a damp place, they are apt to be destroyed by the generation of the starch fungus. Gelatine or the finest glue is undoubtedly the best cement for mounting, although its application may be a little more troublesome than other adhesive solutions. The face of the cardboard is first damped with a moist sponge, to cause it to expand in the same degree as the print when the cement is applied to it. If this be not done, the print contracts on drying, and draws the cardboard out of shape. The print is then laid with its face upon a slab of glass, and the cement spread thinly and evenly upon the back with a stout hog-hair paint-brush. The print is then applied carefully to the damp cardboard, and lightly pressed into contact, and air bubbles pressed out with a linen rag. A sheet of cardboard is then laid upon it, and the rag rubbed over this pretty forcibly in order to ensure the perfect adhesion of the print to the mount in every part. The cardboard upon which the print is mounted is then pinned up by one corner to dry, and afterwards placed under a book-binder's press, or rolled. See " Rolling Press. " French cardboard is mechanically the best for the purpose, and the thicker it is the easier the operation becomes ; but the bluish tint of French cardboard is very objectionable, as it contrasts disagreeably with the tint of most photographs. Cream colour is far better ; and there is no doubt that cardboards might be manufactured of a great variety of suitable tints, and that the general effect of photogra- phic prints would be greatly improved by being mounted upon such. Stereoscopic subjects should be mounted upon cardboards nearly if not absolutely black. As a general rule, a photograph should never MUR NAT 225 be contrasted with anything absolutely white, or what is even worse, of a bluish white like French. paper. Muriatic Acid. See " Hydrochloric Acid." Naphtha ; Rock Oil. C 6 H 5 . A combustible and volatile liquid, resembling oil of turpentine. It occurs naturally, and may also be made artificially. As a natural product it exists in the soil at Baku, on the north-east shore of the Caspian, at Amiano in the duchy of Parma, at Zibio in the duchy of Modena, at Neufchatel in Switzerland, at Clermont in France, at Val di Noto in Sicily, at Trinidad, Barbadoes, Rangoon, and recently in great abundance in several districts of North America. As an artificial product it is obtained from the distillation of petroleum, or the coal-oil of the gas works. Naphtha does not congeal at 0° ; it is not soluble in water, but communicates a smell and taste to it ; it dissolves in absolute alcohol, ether, and oils, and is a solvent of the resins, as well as of phosphorus and sulphur in small quantities. It softens and gela- tinizes india rubber, and this glairy varnish is spread upon textile fabrics to render them waterproof. The boiling point of naphtha or rock oil varies from 180° to 600°. It is not acted on by potassium and sodium, and is used for pre- serving those highly oxidisable metals from the action of the air. The naphtha above described is sometimes called mineral naphtha to distinguish it from wood alcohol or "wood naphtha/' which is a totally different substance. Naphtha may be economically used for burning in spirit lamps. Natural Colours. To obtain photographs in the natural colours is, of course, a grand problem ; but the probability of its ever re- ceiving a solution is not great ; and, so far, nothing has been done to inspire much hope that it may ever be accomplished. The different colours produced upon various sensitive tablets by coloured images seem, in every case, to arise from the different state of decomposition of the sensitive material, by different degrees of actinic power in the light which do not correspond with the different tints of the natural colours. In order to produce a photograph in the natural colours, the coloured image formed in the camera must be received upon a tablet so constituted as that every colour may, where it acts, produce such an effect as that when the tablet is afterwards exposed to white light the same coloured rays may be emitted at the particular spot impressed. To discover the means of producing such a tablet really 226 NEG NIT seems to be somewhat hopeless, although certainly not impossible. {See also " Heliochromy." Negative. A photograph in which the lights and shades are reversed, and the opposite of those in nature, is called a " negative." The value of a negative consists in the means it affords of multiply- ing positive prints in which the lights and shades are true to nature. The best negatives are those which are taken upon glass coated with a uniformly transparent and structureless film of collodion, albumen, &c. ; but as glass is a costly, heavy, and fragile substance, negatives are sometimes transferred from glass to a film of gutta percha ; and are frequently taken upon paper, waxed or oiled, in order to render it more evenly transparent. For a certain class of bold artistic subjects paper may be considered suitable. The various negative processes upon collodion, albumen, paper, &c, are described under their respective heads. Neutralization is a term used to denote the reduction of an acid or alkaline solution to that state in which it exhibits no tendency either way. Nitrate Bath. This term is most commonly applied to the nitrate of silver solution, used for exciting collodionized plates. It should always be prepared with the purest materials, and preserved with the utmost care, otherwise failures are sure to ensue. See " Collodion Process." Nitrates. These salts are compounds of nitric acid with bases. Those used in Photography will be found described under the heads of the various metals to which they belong. Nitre. See " Potash, Nitrate of." Nitric Acid; N0 5 =54. Nitric acid is manufactured in large quantities by mixing equal weights of nitrate of potash and sul- phuric acid and distilling the mixture in a retort by means of heat. Nitric acid passes over and is condensed, while sulphate of potash remains behind. By this process a liquid nitric acid, having a specific gravity of about 1*5, is obtained, which possesses a yellow colour due to nitrous or hyponitric acid. It is purified and strengthened by redistillation with an equal weight of sulphuric acid, and allowing air to act on the distillate in a warm vessel, whereby the nitrous acid is com- pletely removed. Nitric acid is a powerful oxidizing agent, and forms with bases a very extensive series of salts, which are all soluble in water. The most common impurities found in it are chlorine and sulphate of NIT 227 potash or sulphuric acid. Neither of these, however, are of any importance when the acid is used for the manufacture of pyroxyline, but they may be detrimental when it is used for acidulating the bath, and for making nitrate of silver, chloride of gold, &c. In such cases the acid should be pure, although not necessarily strong. To de- tect the impurities, dilute a portion of the acid with distilled water and add a drop or two of solution of nitrate of silver. If a white cloudiness appears, chlorine is present. To detect sulphates or sulphuric acid, dilute another portion and add a few drops of solution of nitrate of baryta, or chloride of barium. If no change ensues sulphates are absent. Nitro-Glucose. This substance is made by acting on finely powdered cane sugar with nitro-sulphuric acid; the proportions being 2 ounces sulphuric acid, 1 ounce nitric acid, I ounce cane sugar. The pasty mass at first formed is stirred for some minutes, and it then separates from the liquid in lumps. When these are kneaded in warm water until the acidity is removed, they acquire a white and silky lustre. This organic substance, when added in very small quantity to collodion, increases the density of the negative, and renders the film less sensitive to light. Nitro-Hydrochloric Acid. This is a mixture of nitric and hydrochloric acids, which is sometimes called aqua regia, from the property which it possesses of dissolving the noble metals gold and platinum. When the mixture is heated both acids are decomposed, hyponitric acid and chlorine being evolved. The latter attacks the me- tal, forming chloride of gold. The best proportions for dissolving gold seem to be nitric acid 1 part, hydrochloric acid 3, and distilled water 3. After the gold is placed in it, a gentle heat is necessary before solution commences ; but on no account should the tempe- rature be raised to the boiling point, because then a great portion of the chlorine escapes instead of acting on the metal. Nitro-Sulphuric Acid. This term is applied to a mixture of nitric and sulphuric acids, which is used in the preparation of pyroxyline, Nitrogen. N = 14. An elementary gas. It is obtained by burning phosphorus in a closed vessel containing air, and passing the gas through lime water ; or by agitating a liquid amalgam of lead and mercury with air in a closed vessel for two or three hours, when the lead abstracts the oxygen. It has neither smell nor taste, nor any action upon vegetable colours ; nor is it a supporter of 228 NON combustion. Atmospheric air contains principally 1 atom of oxygen to 2 of nitrogen, in a state of mixture, not combination. Nitrogen is a little lighter than air. Nitrogen, Oxides of. There are 5 oxides of nitrogen, viz. : — Nitrous oxide, (laughing gas) . . . NO Nitric oxide, (binoxide of nitrogen) . . N0 3 Nitrous acid N0 3 Peroxide of nitrogen N0 4 Nitric acid N0 5 Nitrogen, Peroxide of. N0 4 . "When two volumes of binoxide of nitrogen (N0 3 ) are mixed with one of oxygen, in an exhausted vessel, they combine with the evolution of heat, and form vapour of peroxide of nitrogen. This condenses into a liquid at 0°, and crys- tallizes at a lower temperature. The liquid is pale yellow at 32°, and deep orange at 60°; it boils at 82°; and when exposed to the air at common temperatures evaporates in yellow fumes. It is produced in nitric acid by exposure to light. The vapour of peroxide of nitrogen is composed of 1 volume of nitrogen and 2 volumes of oxygen, condensed into one volume. Nitrous Acid. N0 3 = 38. This acid is by some chemists called hyponitrous acid. It is not easily isolated. It forms salts called nitrites, none of which have any interest in Photography except the nitrite of silver ; q.v. Noble Metals. Gold, platinum, silver, and a few other metals, are called " noble metals," on account of their feeble affinity for oxygen, for they may remain in fusion for many hours in contact with air without becoming oxidized. Non-reversing Slide. This is a camera slide, so contrived that collodionized glass plates may be exposed in it with the back of the plate next to the lens, and the film next to the back shutter. The best plan is to make an ordinary slide deep enough to hold two glass plates, with a space at least equal to the thickness of a plate between them ; a glass plate is then to have a small triangular piece of glass cemented to each corner ; the collodionized plate is laid, film upwards, in the slide, and the other plate laid with the corners upon it ; the back shutter is then closed with its spring pressing against the upper plate. A non-reversing slide should always be employed for taking col- lodion positives, and also for negatives when the prints are intended to be viewed in the reflecting stereoscope. NOR OIL 229 Normal. The normal to a surface at any point, is the straight Jine perpendicular to the tangent plane at that point. . Object Glass. In a telescope, or microscope, the glass placed next to the object to be viewed is called the " Object Glass." The arrangement of lenses at the opposite end of the instrument, through which the spectator looks, is called the " Eye Piece." The glass next to the eye is called the " Eye Glass ;" and that next the object glass the "Field Glass." In an astronomical telescope, or compound microscope, there are only these three lenses. Oils. Oils are divided into two great classes, viz., Fixed, and Volatile (or essential). The Fixed oils are distributed largely through the animal and vegetable kingdoms. In the former the fatty matter is inclosed in membraneous cells, existing in various parts of the body of the ani- mal ; in the latter they are obtained by expression from the seed, kernel, root, bark, and other parts of plants. Fatty substances may be classified under the heads of Stearine and Oleine, the former solid and resembling suet, the latter liquid at ordinary temperatures. They may be again classified according to their property either of drying, or becoming rancid by exposure to air and light. Most oils, whether fixed or volatile, absorb large quantities of oxygen by exposure to air and light ; — in the case of drying oils the effect pro- duced is the formation of a skin or resinous varnish ; — in the other case the oil is decomposed and becomes rancid and acid. Drying oils are much used in paints and varnishes. Volatile oils are contained principally in various parts of odori- ferous flowers, and shrubs. They are obtained in general by distil- lation of the dried leaves, &c, with water, and sometimes with salt and water, which raises the boiling point. The volatile oil and steam go over together, and when condensed in the receiver, the oil in most cases separates and floats upon the surface of the water. A small quantity is also dissolved in the water, to which it com- municates its peculiar smell. Rose water is an instance of this. A drop of fixed oil leaves a permanent stain upon paper, a drop of volatile oil does not. Volatile oils are mostly soluble in alcohol, fixed oils are not ; both kinds are, however, freely soluble in ether. When water is added to a mixture of a volatile oil and alcohol it renders it turbid. The odoriferous spirits called " Lavender water," "Eau de Cologne," " Eau de jasmine," &c, are solutions of a volatile oil in alcohol. Volatile oils combine with acetic and 230 OPA OPT oxalic acid ; but with the exception of oil of cloves, they do not combine with alkalies to form soaps. They dissolve all the fat oils and the resins. Oils are supposed to be compounds of an organic acid with gly- cerine as a base When an alkali is added to the fixed oils, at a boiling temperature, the glycerine is displaced and the new compound formed is soap. Oils contain a large quantity of hydrogen. Fixed oils are bland and mild to the taste, volatile oils acrid and nauseous. Opalotypes. Pictures on opal or porcelain glass have lately come into considerable repute, and deservedly so, for when well- executed they are extremely beautiful. The American photographers are most noted for this class of photograph. There are several methods of preparing them, the best of which we need only here indicate, because they all depend on well-known processes. The smoothest and flattest pieces of opal glass should be selected for the purpose, and cleaned just the same as plates for negatives. The pictures may be taken from negatives either in the camera, by which means the image can be enlarged or diminished, or they may be taken by placing the dry sensitive plates in contact with the negative. The finest opalotypes we have seen have been taken by Mr. Wenderoth, of Philadelphia, on albumen (see " Albumen Process"), and afterwards toned in gold. Collodion may be used either wet or dry, and gives excellent results ; so also does the collodio-chloride of silver. Optical Centre. Every single lens has a certain point called its optical centre ; no such point, however, exists in the case of an achromatic lens, or combination of lenses. This should be distinctly understood, because ignorant persons frequently commit the blunder of speaking of the optical centre of a combination of lenses. Confining our remarks, therefore, to the case of the single lens. If a ray of light, incident at any degree of obliquity upon a single lens, strikes it at such a spot as that the direction of the refracted ray within the glass, produced if necessary, passes through a certain point in the axis of the lens called the optical centre, the direction of the ray after emergence will be parallel to that at incidence. This effect is brought about by the following circumstance : — A ray of light after refraction through a plate proceeds in a direction parallel to that which it had before. Now if the course of the ray within the glass when produced passes through the point called the optical centre, and we draw a tangent to the anterior sur- face of the lens at the point of incidence of the ray, and another ORG 231 tangent to the posterior surface of the lens at the point of emergence of the ray, we shall find that these two tangents are parallel, so that the lens for that particular ray may be considered as a plate, and the ray does not suffer deviation by being refracted through the lens, but merely displacement. The position of the optical centre is constant, and independent of the obliquity of the incident ray ; so that in any whole pencil, no matter what its obliquity may be, which is incident upon the front surface of a lens, there is, provided that surface be large enough, a particular ray, and only one, the direction of which, after refraction, passes through the optical centre. The optical centre of a single lens is found thus : — If r be the radius of the front surface of a lens, s the radius of the back surface, and t the thickness of the lens, then the distance of the optical centre, measured along the axis of the lens from the centre r t of the face of the front surface, is equal to 8 — r The optical centre of a double convex lens is within the glass ; — of a plano-convex lens it is at the centre of the face of the back surface; — and of a meniscus lens it is without the glass and behind it. By giving to r and * the proper algebraical sign, and a given magnitude, the position of the optical centre of any single lens may be readily found. The use of the optical centre will be understood by referring to the figure on page 1. The focus, or circle of least confusion, of the pencil QAJB is somewhere in the neighbourhood of c. Now the optical centre of the lens being within the glass, the ray QCc passes through it, and may be considered as very approximately a straight line. If then we draw this line, and set off Cc equal to the focal length of the lens, we find the point c very approximately, and without going through the laborious investigation of the bent pencil QRF. If, in this figure, AB were an achromatic lens, the point c would be found approximately by considering the lens as single, or homo- geneous, and of the same external form. Organic Matter. This term is used in Photography somewhat vaguely, to denote all animal and vegetable substances which in- fluence the action of the nitrate of silver solutions, or the developer. All organic bodies do not possess this power ; only those that are readily oxidisable. The general effect of organic matter in combination with the 232 OKP OXY reduced silver of the photographic image is to increase the density of the image when viewed by transmitted light, to redden the colour, to add surface vigour to positive prints, to diminish the chances of permanence of the photograph, and sometimes, but not always, to reduce the sensitiveness of the excited plate or paper to light. Orpiment. As. S 3 . Yellow sulphide of arsenic. Ammoniacal solution of orpiment is sometimes used as a dye. Orpiment is the basis of the pigment called " King's Yellow." Orthographic Projection. This is a mode of representing an object in perspective, when the eye is supposed to be at an infinite distance, so that the visual rays from the different points of the ob- ject are parallel instead of converging to a point at a finite distance, as in common perspective. See " Perspective." Orthoscopic Lens. See " Lens." Oxygen. 0=8. An elementary gas, the name of which is derived from its property of producing acids. The atmosphere contains about one-fifth part, by volume, of oxygen gas, in a state of mixture ; and water eight-ninths, by weight, in combination with hydrogen. Oxygen is the great supporter of life and combustion. Animals, by breathing, withdraw it from the air and return carbonic acid in exchange. Vegetables, during the action of light in the daytime, absorb carbonic acid and return oxygen; hence the balance is main- tained. Oxygen is rather heavier than air, and when pure is tasteless, colourless, and inodorous. Its chief use to the photographer is for affording a brilliant light for exhibiting photographic magic lantern slides, or when a powerfully actinic artificial light is required in any of his operations. (See the following article.) The readiest mode of obtaining oxygen for this purpose is as follows : — A copper retort, containing a mixture of about 3 parts of chlorate of potash and 1 part of black oxide of manganese (thoroughly well mixed together), is placed upon a common fire. In a short time oxygen gas is evolved. This is conveyed by an india-rubber tube attached to the nozzle of the retort into a bottle of water, called the purifier, which may stand upon the ground. After passing through the water, which cools and purifies it, the gas passes along another india-rubber tube into a large wedge-shaped india-rubber bag, which is capable of containing sufficient gas for an evening's enter- tainment. This bag may be filled in about half an hour, from half a pound, or less, of the mixed manganese and chlorate. When re- quired for use, the gas bag, filled with oxygen, is placed upon the OVA OZO 233 ground between two boards hinged together, and upon the upper one weights are laid sufficient to force the gas out at the required rate, along a tube, either to the spirit lamp of the oxy calcium light apparatus, or to the point where it unites with the hydrogen jet of the Bude light. Perfectly pure oxygen is obtained from chlorate of potash alone, and collected over mercury after having been passed through a tube containing fused chloride of calcium. Ovalbumen. This term is used to denote the albumen derived from eggs in contradistinction to seralbumen, or the albumen of the blood. Oxalic Acid. C 2 0 3 , HO + 2 HO. This poisonous substance is found in several varieties of plants in combination Avith potash or lime. The method of preparing it commercially need not be de- tailed here, because it is of no practical value in Photography. It is often formed in small quantities spontaneously in decomposed collodion, and is very probably one of the substances which render such collodions very insensitive. Oxalate of silver is feebly sensitive to light. Oxidation means the combination of bodies with oxygen. Spon- taneous oxidation occurs with many photographic chemicals when they are exposed to the air. Hence it is important to keep them as much as possible from its influence in well closed bottles. Oxgall. To photographic colourists this substance is of consider- able value. It can be obtained at the artists' colourmen. Albumenised prints often refuse to take the colour applied to them. In that case a little oxgall, brushed over the print, remedies the evil at once. Oxymel, or syrup of honey and acetic acid. This substance is best prepared for photographic use by boiling, for a very short time, one pound of honey with a pint of distilled water. Allow the solution to cool, then filter and add one ounce of acetic acid. It was at one time much used for preserving photographic plates. They were excited and washed as usual. The oxymel was then poured on and off the sensitive surface several times, and the plates set aside to dry. This process, although good in some respects, is exceedingly slow, and has now been entirely superseded by better processes. Ozone. Is supposed to be an allotropic form of oxygen in which it exhibits new properties. For instance, oxygen, as it generally exists in the air, has no chemical action on iodide of potassium, &c, but when converted into ozone, it decomposes them. Ether is^ also readily 234 OXY PAN ozonised, becoming acid, and redistillation from potash does not seem to have much effect in again purifying it. Ozone is formed when electricity is discharged into the air. It is, however, best prepared by placing a stick of phosphorus in a large bottle, and half covering it with water. Ozone will soon be generated, and may be detected by its peculiar smell. The best method of detecting it in the atmosphere or elsewhere is by means of " ozone papers," which consist of slips of thin paper dipped in starch, and then in iodide of potassium. If ozone exists in the air these papers will turn blue by a decomposition of the iodide — the liberated iodine going to the starch and producing the blue colouration. Oxycalcium Light. This brilliant white light is produced by passing a jet of oxygen into the flame of a spirit lamp, and direct- ing the flame upon a small ball or cylinder of lime. Panoramic Camera. This is a form of camera invented by Mr. Sutton, in which pictures may be taken upon one flat plate, including an angle of 90°, or more if required, without introducing the defects due to oblique pencils, such as distortion, indistinct- ness, &c. The following figure will, it is hoped, be intelligible with a few words of explanation. PAN 235* The lens is mounted in a long narrow tube or box, the same height as the camera. This . revolves about an axis placed immediately over the stop. Inside the camera are placed two hoops, very strong and stout, one at the top, the other at the bottom of it. The dark slide always presses against these hoops. They are circular arcs, the centres of which are in the axis of rotation of the tube. The ends of the dark slide are furnished with wheels, which, as the slide is moved round, travel in grooves at the bottom of the camera, shown by the dotted lines. These dotted curves are evolutes of the lower circular hoop. The top of the lens-tube is continued till it passes over the top of the upper hoop, and the bottom of the lens tube is continued till it passes under the bottom of the lower hoop ; the dark slide is then placed between these projecting ends of the lens-tube. A piece of wood lined with velvet is screwed to these ends, and, by pressing against the back of the dark slide, keeps it in its place against the hoops. It is evident now, that as the lens-tube is turned about its axis, and directed in succession towards the different objects within the field of view, the dark slide moves with it, and is always pressed against the hoops, thus bringing the proper part of the sensitive plate opposite to the lens — the velvet-lined piece of wood sliding at the same time against the back of the slide, and the wheels travelling along the evolutes. The sides of the lens-tube are furnished with folding doors, so as to diminish, at pleasure, the width of the vertical band of picture exposed at any moment ; and its rotary motion may be regulated by means of a rack and pinion on the top of the camera. The shutter of the dark slide may be withdrawn, and inserted again, either through a slit in the camera at A or B. The mode of doing this may be left to the ingenuity of the reader. The accuracy with which this instrument will do its work will depend upon the accuracy of its construction. The optical prin- ciples involve no difficulty, and the theory of the instrument is quite correct. All parts of the picture will be equally sharp, and vertical lines will not be bent out of the perpendicular. The perspective of the picture will, however, be "panoramic," and not "plane," so that the horizontal line of objects will vanish in curved and not in straight lines. If this be thought an objection, the picture may either be mounted upon a bent cardboard, or bent round a glass cylinder, and viewed in a diaphanoscope, with the eye in the centre of the cylinder. Oiled paper prints, viewed in this way, would probably be finer than anything that has yet been seen in Photo- graphy. 236 PAN The panoramic camera would no doubt be found a valuable instru- ment for taking skies. Panoramic Lens. This is a new and singular lens, invented by- Mr. Sutton, and patented by him in the year 1858. It is intended for taking views which include ninety degrees or more of angular extent, upon spherical bowls, or cylindrical glasses, at one operation, and without any moveable mechanism such as that described in the former article. The lens is composed of a thick spherical shell of light flint glass, having its inner cavity filled with water, and a diaphragm placed at the centre. The principle on which it is rendered achromatic is based upon the fact, that if a lens be made with concentric spherical surfaces, so as to form a portion of a spherical shell, it will act as a concave or diminishing lens. Hence, in the compound in question, we have a concave lens made of glass, which has the higher refractive and dispersive power ; and a convex lens composed of the water which fills the inner cavity, and which has the lower refractive and dispersive power. It follows, therefore, that by assigning a suitable radius to the inner surface of the shell the compound may be rendered achromatic. The calculation will be given presently, after exhibiting the general construction of the lens, which will be understood at once from the following figures : — PAN 237 238 PAN that if a single round hole were used instead of this contrivance, the centre of the picture would be about twice as strongly lighted as the sides. The way in which this peculiar diaphragm acts is shown in the third figure, from which the reader will easily perceive that the most oblique pencils have the same size as the central one. It now remains to show how to calculate the inner radius of the glass shell, so that the compound may be achromatic. The object is to find the radius of the inner sphere so that two given lines of the spectrum may be united in the principal focus of the lens. Let us agree to call X, Y, the lines which are to be united, so as to render the lens achromatic. Let unity be the outer radius of the spherical shell, and r the radius of the inner sphere ; then r is the unknown quantity. Let m = refractive index of line X from air into glass. m'= refractive index of line X from air into water. n = refractive index of line Y from air into glass. n'= refractive index of line Y from air into water. IT =* principal focal length of lens for lines X and Y, when united. Let us first calculate ~F for an axial pencil of homogeneous light, corresponding to the line X. To do this let v lt v 2 , v s , be the geometrical focal lengths of the pencil, measured from the centre of the sphere, after refraction at the 1st, 2nd, and 3rd surfaces respectively. Then, at the 1st surface we get 1 — =— o— l) (i) surface, m' 1 m' — m mv l v 2 mr surface, m 1 wl — m m'v^ v 3 m'r surface, (2) (3) m — 1 (4) m PAN 239 Eliminating v 2 between equations (2) and (3) gives 1 1 m' — m = 2 . . . . (5) v s m'r And eliminating v 1 and v 3 between equations (1), (4), and (5) gives 1 f m' — m m — I ~) -=-a< — — + — y F L mra r m J Proceeding in the same way with the axial pencil corresponding to the line Y, we get 1 f n' — n n — 1"1 - = -2<^ + \. f L nnr n J Equating these two values of F gives m' — m m — 1 n' — n n — 1 + = + , mm'r m nn'r n which is a simple equation for determining r, so that the lines X and Y may be united, and the lens thereby achromatized. In order to show the practical application of the above formula and the results to which it leads, let us discuss the following case which actually occurs in practice. Let light flint glass be used, and let us suppose X to be that part of the spectrum in which the visual rays have their maximum intensity, and Y that part in which the actinic rays have their maximum intensity. And let m = 1-57. ft ss re. m' = 1-33. ri = 1-34. these quantities being sufficiently near the truth for the purpose of illustration. Substituting them in the formula above stated gives r — -53016 F=3'42. Hence it follows, that in the case of a panoramic lens made of light flint glass and water, the inner radius of the shell is about one half the outer radius ; and the focal length of the lens about 3 } times the outer radius. These dimensions involve no practical difficulty either in making or in using the lens. With respect to spherical aberration, that of course is reduced within practicable limits by means of the small stop, as is done in all other view lenses. If the shell of glass were much thinner, the PAN aberration would be positive ; and if much thicker, the focal length would be greatly increased, and excessive negative aberration would be produced. Fortunately it so happens that the aberration, in passing from positive to negative, as the thickness of the shell is increased, passes through a point at which it vanishes altogether, while that point is very nearly such as gives freedom from chromatic aberra- tion. In point of fact there is some negative aberration when the lens is made of light flint ; but there would cease to be any if it were made of dense flint, while it would be greatly increased by making the lens of crown glass. In the latter case the focal length would be increased to six times the radius, and the negative aberra- tion would become so excessive as to render the lens useless. On the other hand, dense flint stops so many of the actinic rays as to coun- teract any good effect that would result from the use of a larger stop. The camera and printing frame must, of course, be made to suit the use of spherical bowls or cylindrical glasses, instead of flat plates. This form of surface can be easily coated with collodion, and the picture developed by the alkaline method ; neither does the printing from cylindrical glasses involve any practical difficulty. Printing from spherical bowls might perhaps be effected upon silk, pressed into contact with the inner surface of the bowl ; but the picture, when mounted flat, would exhibit distortion. Strictly speaking, the printing ought to be effected by means of a smaller panoramic lens placed at the centre of the bowl, and the positive should be taken upon another bowl, and viewed in a diaphanoscope. The cylin- drical pictures, if mounted upon a flat cardboard, will exhibit dis- tortion ; and in order to view them correctly, the cardboard should be bent into the same curve as the negative, and the eye placed at the centre. The Panoramic Lens is particularly suitable for taking paper negatives ; because they can be easily bent to the required curve in the camera, and flattened out afterwards. A lens 4-ins. diameter and 14-ins. focus will take a picture 30-ins. long and 15-ins. wide; with the exposure required for a common flat picture 9x7, and with unsurpassed definition. It only remains to say a word or two about fluid lenses. They have been used for the object-glasses of telescopes, and perform remarkably well. The advantage of a fluid in the interior of a combination of lenses is that it destroys internal reflexions, and prevents flare. When a fluid is used no Canada balsam is required to cement the lenses together. The mounting of the Panoramic Lens is perfectly water-tight, and the water can easily be changed, and the lens taken to pieces as often as is required. PAN PAP 241 The patent for the Panoramic Lens has been purchased by- Mr. Thomas Ross, and he is the only maker of the instrument in Great Britain. Pantoscopic Camera. This is an instrument for taking pano- ramic views, including any angular extent up to 360° upon a flat plate, and with a common view lens, by means of mechanism and clockwork. It was invented by Mr. J. R. Johnson, of Red Lion- square, and is a most ingenious affair. Very successful views of {Swiss scenery have been taken with the instrument, by Mr Braun, and the definition is unsurpassed. The principle of construction is not exactly the same as that described in the article on the " Pano- ramic Camera ;" neither can it be well explained within the narrow limits of such a work as the present. The reader who is curious on the subject must be referred to the inventor himself, who will always be happy to exhibit and explain the construction of an instru- ment which is now manufactured by him for sale. The testimony of the writer in favour of Mr. Johnson's invention will perhaps have more weight when it is remembered that he is himself the inventor of a camera devised to answer the same purpose, and also the patentee of the Panoramic Lens. He has no hesitation in pro- nouncing the Pantascopic Camera a most ingenious invention, and a complete success. Paper-Making. Paper may be made of any fibrous material which can be torn and beaten into pulp. The substances commonly employed are linen and cotton rags, and for coarser kinds of paper, grass and straw. We cannot in this work go at any length into the process of paper-making, but will briefly describe the mode of making hand-made paper, from which the general principles of the manufacture will be understood. The rags are mostly imported from Germany and the different ports of the Mediterranean. They are sorted by women ; torn up into shreds, and cut into pieces, then dusted in a machine, and washed, first in water, then in a strong alkaline ley, and afterwards in water again ; they are then ready to be made into pulp. To effect this, they are placed with water in a machine called the " engine," where, by the rapid rotation of a complicated piece of apparatus, they ate torn into the finest imaginable tatters and shreds. This is called " stuff." When the stuff is about half made, it is called " half stuff," and is then " bleached," generally with chlorine, or chloride of lime, one or two pounds of chloride of lime being sufficient for a cwt. of fine rags. This is added to the 242 PAP " half stuff " in the engine, and the mixture is beaten together for an hour or so, then the bleaching liquid run off, fresh water con- tinually added, and the half stuff reduced, by the continued action of the engine, to "fine stuff." Sometimes the sizing mixture and colouring matter are now added, but in general the sizing is an after operation performed upon the finished sheets of paper in its bibulous state. The stuff is now run off into the " stuff chest " or vat. Hand-made paper is made as follows : — Two men, one called the " vat-man," the other the M coucher," stand on opposite sides of the vat, which is covered with a board or " bridge" between them. The vat is about five feet square, and four feet deep, with slanting sides, and made of wood or stone. The st uff is then diluted with warm water, and continually stirred. The vat-man takes in his hands a mould, which is the size of the sheet of paper to be made. This mould is a frame of wood, having wooden bars running across, about an inch and a half apart, and across them is laid a wire frame, the wires being from fifteen to twenty per inch ; or sometimes a wove mould is used, covered with fine wire cloth. On the mould a " deckel," or moveable edge frame, is used to prevent the edges of the paper from being rough. The vat-man puts the deckel upon the mould, and dips it in a vertical position to about half its depth in the stuff, then turns it into a horizontal position so as to cover it with the stuff, and gives it a peculiar kind of shake, which distributes the stuff evenly upon the mould. The mould must be held perfectly level during this operation, or the sheet will be thicker at one end than the other. The mould is then pushed across the bridge to the " coucher," who, after draining off the water, lays the surface of the pulp upon a piece of felt, to which it adheres, and then returns the mould to the vat-man. This piece of felt, with what we may now call the sheet of paper adhering to it is laid with other similar pieces in a pile, which, therefore, consists of alternate layers of paper and felt. The pile, consisting of six or eight quires, is put, and remains for some time, under a pressure of about 100 tons, to squeeze out all the water, and the sheets, of paper are then removed, pressed again without the felts, and hung across a hair line to dry, in the drying- room. In this state the paper is bibulous, or blotting-paper, and the next operation is to size it. English papers are in general sized with a mixture of gelatine and alum, to which sometimes flour, resin, and yellow soap are PAP 243 added. French papers are sized with a le3s soluble size, consisting for the most part of starch, with a little potash.* The sheets of bibulous paper are placed, one at a time, in a vertical position in the tub of size, and pressed into close contact. After a time the papers are taken out, scraped, and pressed to remove the superfluous size, then parted, and pressed again, and afterwards hung up to dry, a* process which occupies two or three days, and must not be done too quickly. The paper is then pressed again. The paper has now to be glazed or hot-pressed. This is done by placing a sheet of paper between two glazed pasteboards, alternately in a pile, and between every fifty pasteboards a hot iron plate, then subjecting the pile to the press. Or a pile of sheets of paper placed between pasteboards may be rolled backwards and forwards upon a plate between cold iron cylinders. This communicates a glaze to the surface of the paper. The paper is now finished, and has merely to be trimmed, and the sheets counted and sorted, and tied up in reams, each containing 480 sheets. The above operations of making paper by hand may be success- fully imitated by machinery ; the paper is jh&a said to be " machine- made." It may be made in sheets of indefinite length. Paper frequently contains metallic spots, consisting of particles of iron, brass, or zinc, detached from the machinery, or introduced through carelessness in sorting the rags. The roughness of the * With respect to the sizing of French papers. The following recipe was given some years ago by the Socicte d' Kucourageruent of Paris : — 100 kilogrammes of dry paper stuff, 12 „ starch, 1 „ resin, previously dissolved in 500 kilogrammes of carbonate of soda. 18 pails of water. This size evidently renders the paper alkaline. The following formula is given by M. Braconnot, in the " Annales de Chimie," Vol. 23 :— "To 100 parts of dry stuff, properly diffused through water, add a boiling uniform solution of 8 parts Hour, with as much caustic potash as will render the liquid clear. Add to it 1 part of white soap previously dissolved in hot water. At the same time, heat half a part of resin with the requisite quantity of weak potash lev for dissolving the resin, mix both solutions together, and pour into them 1 part of alum dissolved in a little water. This size also renders the paper alkaline. Alum has an acid reaction, and therefore English paper sized with alum and gelatine is acid and not alkaline. K 2 244 PAP PEN felts between which the paper is pressed also occasions inequalities of texture. It is highly desirable to remove these imperfections from paper which is to be used in Photography. The practice employed by the French paper-makers of colouring their paper with artificial ultramarine (sulphide of sodium,) is also highly objection- able for photographic purposes, both as regards the appearance of the paper, and from the probability that the introduction of an alkaline sulphide into it might assist the process of fading of positive proofs. Should the process of printing positives in carbon ever come into general use (which it is tolerably certain that it will), the imperfections now existing in paper for positives will be of less moment. Paper, India or China. India paper, or Chinese paper, is much used by engravers for printing the first proofs from the plate. It is thin and silky, of a beautiful buff colour, and made of the fibres of the young bamboo, which are triturated, ground, and boiled to a paste. This is set to fermemt in a heap covered with mats, and the paper is then made from it. The length of the fibres renders it better adapted for receiving copperplate proofs than the best European paper. This paper is smooth on one side and bears on the other the marks of a brush which is used in the finishing process. It is made in sheets four feet long and two feet broad. When proofs are printed upon Tndia paper no cement is used to attach the thin sheets of India paper to the thick plate paper, the mere pressure exerted by the cylinders in the operation of drawing the proof being sufficient to cause perfect adhesion between the two surfaces. Parchment Paper. See " Artificial Parchment." Paste. Mix well together flour and a little cold water, then add as much more cold water as is necessary to make the paste of the consistence required ; then boil the mixture, stirring it well all the time ; lastly, strain it through muslin. Starch is better than paste for mounting photographs. Sometimes powdered resin is boiled with paste to give it body, and corrosive sublimate added to make it keep. In the latter case, it is highly poisonous, and should not be left about carelessly. Pendulum. A pendulum suspended in the portrait room may be used with advantage for measuring time by seconds. The pen- dulum which beats seconds, that is, which oscillates in a second of time (approximately), whatever be the angle of oscillation, is about forty inches long. If, then, a 40 -inch pendulum, hanging against PER 245 the wall of the portrait room be set swinging before removing the cap from the lens, the operator, by counting its vibrations, may regulate the time of exposure as accurately as if he consulted a watch. A clock having a pendulum of that length would be a use- ful addition to the portrait room. Perspective. There are two kinds of perspective delineation with which the photographer is concerned, viz., " Plane," and " Panoramic Perspective." In plane perspective, objects are represented upon a vertical plane placed between them and the spectator. Straight lines, called " visual rays," are supposed to be drawn from the various angular points A, B, C, &c, of the objects to the eye, and where these lines perforate the vertical plane, or "plane of the picture," as it is called, are corresponding points a, b, c, &c, through which, if the figure be completed, it is the plane perspective representation of the objects as seen from the point occupied by the eye (not eyes) of the specta- tor. According to this definition a plane perspective view is nothing more than a plane section of the system of pyramids of which the visual rays are the edges and the eye the common vertex ; the eye being considered a mathematical point. The rules of perspective, therefore, merely relate to the cutting of pyramids by a plane, and are purely geometrical, not referring in any way to the structure of the eye, or the image formed upon the retina, or the rules of optics. Perspective is nothing more than a very simple problem in solid geometry, and it is marvellous to find that so little is accurately known of it by artists, and that so many elaborate and expensive works should have been written about it, when in fact the whole thing lies in a nut-shell, as we shall now show; not, however, without calling on the reader for his patient attention, and careful study of our remarks. Let us first suppose the object to be represented to be an infinite straight line, making an angle, with the plane of the picture, and to meet it in points B, C, D, &c. It is evident that the perspective views of all these straight lines would be terminated in a common point X, and would consist of lines AX, BX, CX, &c, radiating from X ; this point X is therefore called the " vanishing point " of that particular system of parallel straight lines. Hence we arrive at the following general rule : — The vanishing point of any system of parallel straight lines is the point where a line drawn through the eye parallel to that system cuts the plane of the picture. If a horizontal plane be drawn through the eye, the line in which it intersects the plane of the picture is called the " horizontal line ;" and if a line be drawn from the eye perpendicular to the horizontal line, the point in which it cuts it is called the "point of sight." Hence it follows that 1st. The vanishing point of a system of parallel horizontal lines is somewhere upon the " horizontal line " of the picture ; the point being found by drawing through the eye a line parallel to any one of the system of lines. 2nd. The vanishing point of a system of parallel horizontal lines at right angles to the plane of the picture is the " point of sight." We next come to the case of the vanishing point of a system of parallel lines which are parallel to the plane of the picture ; that is, to the case in which the angle vanishes. These lines have no vanishing point, because the line drawn through the eye parallel to them never meets the picture. They are consequently represented by parallel lines in the picture. Observe the practical conclusions : — 1st. All vertical straight lines in nature are represented by vertical straight lines in the picture. They do not vanish towards a point in the zenith, as is erroneously supposed. In fact, in strict accuracy, vertical lines would vanish downwards towards the centre of gravity of the earth. 2nd. The horizontal lines of a building which are parallel to the plane of the picture are horizontal lines in the picture. Should the reasoning by which these conclusions are established PER 247 be thought somewhat metaphysical, then we may return to the case of the section of a pyramid. Place a square board vertically behind the plane of the picture, and parallel to it. Then, since the section of a pyramid by a plane parallel to its base is a figure similar to the base, the perspective view of the square is also a square ; that is, neither the vertical nor the horizontal lines have any vanishing point. We have now discussed the whole theory and mystery of plane perspective. If the reader has carefully followed our reasoning he will not require to spend his money in treatises on perspective, (which are generally full of gross blunders), but may trust to his own good sense to apply the rules which we have established. The following remarks should be borne in mind : — In views of marine scenery, the horizontal line is always higher than the sea line, because of the dip of the visible horizon ; and the sea line is a curve convex to the horizontal line, and most nearly touching it in the point of sight. The perspective view of a sphere is an ellipse in every case, except that in which the line joining the eye and the centre of the sphere is perpendicular to the plane of the picture, so that the centre of the sphere is on the point of sight. For let a visual ray travel round a sphere, it sweeps out a cone with a circular base, and the oblique section of such a cone is an ellipse. If the plane of the picture be inclined to the vertical, vertical lines have a vanishing point either above or below the horizontal line. The reflections of vertical objects in water are vertical, and have no vanishing point, because the image of a vertical line is in the vertical produced beneath the surface of the water, and not in a hori- zontal line lying upon the surface of the water, as it appears to be. When the reflection of a vertical line is not represented as a continua- tion of that line, but as making an angle with it, the perspective is incorrect, no matter in what part of the picture the vertical line may be, or how situated with respect to the point of sight. The reflec- tion of an object which is out of the perpendicular is not necessarily in the same straight line with it, but in general makes an angle with it. The reflection of the sun or moon is always vertically under it, no matter where the point of sight may be. Also, the bar of light pro- duced by the reflection of the sun or moon in rippling water is always vertical, and does not appear to approach the spectator, as it is incorrectly represented to do in many pictures. Some of the above remarks may be received with surprise and incredulity by some readers, but a little consideration will show that they are strictly correct. 248 PHO Panoramic Perspective is when the picture is represented upon a sphere, or vertical cylinder, of which the eye is in the centre. In this kind of perspective the rules are somewhat more complicated, and need not be stated in this work ; it will be sufficient to observe that, in a panoramic picture flattened out, straight lines vanish in curves, not in straight lines. When the image is formed upon the focussing screen of a camera having a small pin-hole in front instead of a lens, it is in perfectly true perspective ; for if we consider the pin-hole as the vertex, of the system of pyramids formed by lines drawn from it to the objects, and that these lines are produced through the hole so as to form another system, equal and similar to the former, but inverted, it is evident that the section of this second system made by the focussing screen is equal and similar to a section made by a screen placed symmetri- cally with it on the opposite side of the pin-hole, and therefore equal and similar to a perspective view obtained in the ordinary way, but inverted. Phosphori. Bodies which emit light in the dark, after having been exposed to light, are called " solar phosphori." When we consider that bodies which have been exposed to heat continue to radiate heat for some time after being removed from the source of heat, and that heat and light are most probably undulations in the same ether, differing only in the length of the wave, the existence of solar phosphori cannot be considered a remarkable phenomenon ; on the contrary, the wonder is that so few bodies should exhibit the property possessed by them. The first solar phosphorus on record was discovered about the year 1630 by Cascariolo, a shoemaker of Bologna, who found that calcined sulphate of baryta was luminous in the dark after having been exposed to sunshine, and that it continued luminous for some hours. The matter speedily assumed importance, and considerable quantities of calcined Bolognian spar (the native sulphate employed) were sold as an article of curiosity. " Canton's phosphorus" is another example of the property. It is made by first calcining oyster shells in the open fire for half an hour ; then selecting the largest and whitest pieces, mixing them with about one-third their weight of flowers of sulphur, pressing the whole into a crucible with a closely luted cover, and keeping it at a red heat for an hour. The contents when cold may be turned out and the best pieces selected. They will be found to shine in the dark after having been exposed to sunshine. Nitrate of lime fused at a dull red heat is also a solar phosphorus. PHO 249 The blue and violet rays appear to be most effective in producing the phosphorescence. It is not found that solar phosphori generally emit light of the same colour as that to which they have been ex- posed. There are phosphori from heat as well as from light. The native phosphate of lime found near Estremadura, in Spain, and also some varieties of fiuor spar, particularly one called " chloro- phane" are the most remarkable instances. These become luminous when slightly heated, or by friction. Some animal substances are spontaneously phosphorescent. The flesh of the tench, carp, herring, and sole is luminous before putre- faction commences. The property is rarely possessed by the flesh of quadrupeds, and has never been observed in that of birds. The phosphorescence of the sea is a beautiful phenomenon frequently observed, but one which has not yet been accounted for. It is probably due to the presence of phosphorescent animal matter. The glow-worm and fire-fly are familiar instances of phosphores- cence possessed by living animals. Decayed wood and certain mosses have been known to exhibit phosphorescence, but the property is rare in the vegetable kingdom. Some salts, (sulphate of potash for instance,) emit light during crystallization. Phosphoric Acid. P0 5 =71. This acid is used in a diluted form in Willis's aniline process. It may be prepared by pouring 4 fluid ounces of nitric acid, and 8 ounces distilled water on 6 drachms of phosphorus in a retort, and applying the heat of a sandbath. The dis- tillation should proceed till the residue in the retort is of a syrupy consistence. The syrup is then poured into a platinum vessel and heated to a dull red. It fuses and concretes on cooling into a transparent mass. This is glacial phosphoric acid, which may be diluted to any required extent. Phosphorus. P=31. This remarkable elementary body is contained in the bones and fluids of animals, and also in the vegetable and mineral kingdoms. In bones it exists as phosphate of lime, and is obtained from them by acting on calcined bones with sulphuric acid, and distilling the superphosphate of lime thus pro- duced with the addition of charcoal. The phosphorus, which is volatile, passes over, and its vapour is condensed and drops into water. It is at first a soft translucent yellowish white substance, but becomes red by exposure to light, which is supposed to afford an instance of allotropy. It is insoluble in water, but soluble in oils and in ether. Phosphorus is highly combustible, and burns slowly and spon- 250 PHO taneously in the air, but magnificently in oxygen. It should be kept, and cut, under water. Its chief use is for making lucifer matches. Photo-Galvanography. This is a process for producing copper plates ready for the printer, by the joint action of light and electricity. A company was formed at Hollo way, in 1856, for carrying out the process, under a patent obtained by Herr Pretsch, who superintended operations. We paid a visit to the establishment in November of that year, and Herr Pretsch was kind enough to explain the various stages of the process. We subsequently wrote the following article in " Pho- tographic Notes," No. 15, describing what we had seen and heard. " A positive photographic print is first taken, — generally on paper. In order to produce from this a copper plate the following operations are employed : — "First; — A sheet of glass is coated with gelatine containing bichromate of potash and other chemicals. When dry, the positive is laid upon it, face upwards, and it is exposed to light in a pres- sure frame for a few hours. The time of exposure of course depends on the intensity of the light. Sunshine is preferred, but is not necessary. The picture upon the gelatine is developed in raised and sunk parts by immersion in a fluid, the principal constituent of which is water. Where the light has not acted, the gelatine swells and forms a ridge, or a series of minute granulations. Where the light has acted, the gelatine is hardened and does not swell. The picture upon the gelatine is very curious, and resembles a positive by reflected light, the shadows and dark parts being rough and the lights smooth and polished. " Second; — A mould of the picture upon the gelatine is taken in gutta percha. This mould is an intaglio picture, precisely resem- bling the finished copper plate. The gutta percha mould is about half an inch thick. " Third ;— A copper plate is made from the gutta percha mould, by means of the electrotype process. This part of the process is very slow, occupying perhaps a week or two. The copper plate thus obtained is called the matrix. It precisely resembles the original gelatine picture. " Fourth ; — The copper plate from which the proof is to be printed is now obtained by the electrotype process from the matrix. This is a slower process than the last, because the copper is much thicker. It occupies about three or four weeks. " The entire process therefore occupies about six weeks. From PHO 251 the final plate four or five hundred good impressions may be struck off in the ordinary way ; — a considerable number of plates may be ob- tained from the matrix ; — a considerable number of matrices may be taken from the gutta percha mould ; and a considerable number of gutta percha moulds from the gelatine picture. Here then are the means of almost indefinite multiplication. Some idea of the number of proofs which might be obtained from the original gela- tine picture would perhaps be got by multiplying 500 by itself four times. This gives more than sixty thousand million impressions ; that is, sufficient proofs for six times the present population of the earth. The most elaborate subjects may be engraved by this process in as short a time as the simplest, the amount of detail in a photo- graph or photo-galvanograph making no difference, for light, che- mistry, and electricity do the work. The time at present required for any subject is a few weeks ; the time frequently spent on en- gravings is two or three years." The company has ceased operations, probably from the pro- cess not being in a sufficiently perfect state to render it inde- pendent of the help of the engraver for retouching the plates. This is much to be regretted, because the process is perfectly suit- able for a class of bold artistic subjects which, taken from nature by Photography and multiplied in printers' ink, would be of great ser- vice as studies for artists, or copies for drawing masters to lay before their pupils. Photogen, Moule's. A lantern, fitted with a chimney and re- flector, and adapted for burning Bengal Fire, or other suitable chemical compound, in the portrait room at night, so as to enable the photographer to take portraits by artificial light. It has been patented by Mr. Moule. Photography is a term used to denote all sorts of pictures which are taken by the action of light on chemical compounds. Its literal meaning is " the art of writing by light." The alchemists discovered the fact that " Horn silver," i.e., chloride of silver, was blackened by exposure to light, but their observations led to nothing practical. Scheele, in the year 1777, was the first who examined the decom- posing effects of light on silver compounds ; but it was left for Wedgwood and Davy, about the year 1802, to apply the now known fact to artistic purposes. They threw the shadow of the object which they wished to copy on a piece of paper impregnated with nitrate of silver, thus obtaining a white image on a black ground. They found leather the most sensitive medium. Now, it is very 252 PHO remarkable that these distinguished philosophers did not at once discover the great fact which Mr. Reade and Fox Talbot afterwards found out, which is the basis of almost all modern Photography. Leather contains tannin or gallic acid — this was the secret of the greater sensitiveness of leather, but they failed to comprehend its full significance. The sensitive surfaces prepared by Wedgwood and Davy were not sufficiently sensitive to receive an impression in the camera, although it would appear the attempt was made. Davy, however, succeeded in obtaining a faint image in the solar microscope. Till the year 1814 we find no record of further progress in Photography, when Joseph Nicephore Niepce perfected a camera process which he called " Heliography." In this process he dis- carded the use of salts of silver, and used as his sensitive medium a resinous substance called "Bitumen of Judaea." The resin was spread over the surface of a metallic plate, and exposed to the actinic rays, either under a picture or in the camera, The same process has now been applied by Mr. Pouncy, of Dorchester, to carbon-printing from negatives. By exposure to light, the resin, when acted on by the actinic rays, changes its properties and be- comes insoluble in turpentine and other solvents which usually act upon it. After some hours' exposure in the camera, the resin-coated plate was removed and steeped in any of the solvents of asphaltum or bitumen of Judsea, which dissolved away the resin where the light had not acted. The shadows were represented, by the surface of the plate where the resin had been dissolved, and the lights by the unaltered resin. There are several specimens of Niepce' s original process in the British Museum, some of which also show his attempts at etching by the same process. Some ten years later, Niepce and Daguerre, the latter of whom had been working in another direction, became acquainted, and mutually experimented in the photographic art. Daguerre had previously been directing his attention to the salts of silver as the sensitive medium ; but it was not till the year 1839 — after the death of Niepce — that the daguerreotype process was published. In it the sensitive surface was iodide [of silver on a silver plate, and the developer vapour of mercury, which is much the same process that is now employed, although, by means of bromide and other appliances, its sensitiveness was subsequently much , enhanced. Daguerre also succeeded in fixing his proofs by means of hyposulphite of soda, a salt which was discovered several years previously by Sir John Herschel, and its pro- perties described by him in the " Edinburgh Philosophical Journal.'* Its importance in photographic operations is now thoroughly appre- PHO 253 ciated, and no salt equal to it in value for fixing purposes — although several others have been suggested — has yet been discovered. In the same year — viz., 1839 — that Daguerre published the details of the daguerreotype process. Mr. Fox Talbot, in the month of January, communicated to the Royal Society a memoir relating to the preparation of a more sensitive paper than had hitherto been employed. He first dipped the paper in a weak solution of chloride of sodium, and afterwards in a stronger one of nitrate of silver. By this method a sensitive medium was obtained, on which leaves of plants, engravings, and other unequally translucent subjects could be copied in reverse, by pressing them against the sensitive surface, and allowing the light to pass through the leaf, &c. These reversed copies, or negatives, could be used again to pro- duce, by the same means, natural representations of the objects vcopied. Mr. Talbot fixed his pictures with a concentrated solution of common salt : which shows most clearly that, at that time, he had borrowed nothing of his discovery from Daguerre, nor from the researches of Herschel on the hyposulphites, published in 1821. Two years later — in 1841 — Mr. Talbot patented a process, called the " Calotype," by which an invisible image impressed in the camera by a very short exposure to light, could be developed into a negative, from which any number of positives, or naturally-lighted pictures, could be printed on the silver-chlorized paper. This pro- cess, which is the foundation of all modern Photography — excepting Mr. Willis's " Aniline Process " — consists in imbuing a sheet of paper with iodide of silver, which, when exposed for a very short time to light, although seemingly unchanged, is energetically acted on by deoxydising agents, such as gallic and pyrogallic acids, proto-sulphate of iron, &c. These developing agents decompose the nitrate of silver with which they are conjoined, and deposit the metallic element only on those parts which have been affected by the light ; and the density of deposit is proportional to the actinic impression. To equalise the energy of the developer, it was soon found necessary, even in those early days of Photo- graphy, to restrain it by means of a weak acid, such as acetic, &c, otherwise the silver would be reduced on those parts of the iodised paper which had not been actinically impressed. Mr. Talbot's process (see "Calotype"), has not been much improved upon since he first published it. It is doubtful whether Mr. Talbot or the Rev. T. B. Readc is the real discoverer of the modern photographic negative process. In 1839, two years before Mr. Fox Talbot patented his invention, Mi*. Bray ley, Librarian of the London Institution, described in a 254 PHO lecture delivered to the members, a similar process communicated to him by the Bev. T. B. Eeade. The truth seems to be, that Mr. Talbot got the hint of gallo-nitrate of silver from the late Andrew Boss, the celebrated optician, to whom Mr. Eeade had mentioned its powerful action on iodide of silver exposed to light. The only point on which Mr. Talbot seems to have improved on Mr. Eeade's suggestion is in the employment of acetic acid in the gallic de- veloper. The next stage in the history of Photography is the employment of glass plates to receive the medium on which the negatives are impressed. Sir John Herschel was here first in the field, but his sug- gestions were not practically carried out — at least to any great extent — till the year 1847, when Mepce de St. Victor, the nephew of Joseph NicephoreNiepce, who was associated withDaguerre in the daguerreo- type process, proposed the use of iodised albumen films on glass, and succeeded in obtaining good negatives thereby. Immediately after this process was published, photographers in Britain and elsewhere set to work and produced some of the finest photographs, which, with our modern appliances, have not to this day been surpassed. The Messrs. Eoss and Thompson, of Edinburgh, and Mr. Eobertson, of Constantinople, in particular, have signalised themselves by their large and beautifully artistic photographs taken by this process. We come now to the " collodion process," which is the founda- tion of what may be called the truly commercial phase of Photo- graphy. M. Le Gray, of Paris, in a little work on the waxed paper process, first suggested the probability of its being useful as a vehicle for the sensitive iodide of silver, but it was reserved for the late Mr. Archer, of London, to carry out this idea practically. No one now knows whether M. Le Gray's suggestion ever reached Mr. Archer's ears ; most probably it did not, because in that same little publication of Le Gray's all sorts of things were mentioned as vehicles for the iodide. Mr. Archer's communication appeared in the " Chemist " in the autumn of 1851. In the same paper he also proposed the use of pyrogallic instead of gallic acid, as a developer. Since that time collodion has been almost universally adopted as the medium for photographic negatives, the only improvements on the original formula being modifications of manufacture and of develop- ment, whereby much greater sensitivenes has been obtained. Parallel with the above photographic discoveries were others in a different direction, to some of the most important of which we can but just allude. Mr. Mungo Ponton, in the year 1839, announced to the " Eoyal Scottish Society of Arts " that bichromate of potash might be used to sensitise paper. The parts exposed to light PHO 255 became of a dark orange tint, which was insoluble in water, while the yellow colour, not acted on by light, could be removed by washing. The full significance of this discovery was not appreciated till Mr. Fox Talbot subsequently showed that it was only in combina- tion with organic matter that this salt was sensitive to light at all. In the year 1852 he took out a patent for the use of bichromate of potash and gelatine for a new process of engraving on steel. This is the foundation of almost all the photo -engraving and photo- lithographic processes of the present day. In 1855, M. Poitevin patented a carbon printing process, founded on the same principle. He dabbed over a sheet of paper a mixture of bichromate of potash, gum-arabic, and finely-divided charcoal. When dry, it was exposed to light under a negative and placed in water. The parts unacted on by light were washed away, leaving the white paper exposed ; the rest remained unchanged. This, again, is the foundation of all the carbon printing processes of the present day, except Mr. Pouncy's, which depends on the sensitiveness of bitumen of Judyea to actinic radiations. Considerable discussion has taken place as to who was the first that ever could show a presentably good photograph in veritable carbon. There can be little doubt but that our countryman, Mr. Pouncy, was the first who could do so. Upon the same bichromate salt, combined with gelatine, depend the different photo-lithographic and photo-zincographic processes. The most important modifications of the former appear to have been invented by M. Asser, of Amsterdam, and Mr. Osborne of Australia, independently. Mr. Osborne's process was patented in 1859. In the winter of the same year, Col. James, of the " Ordnance Surrey Office," Southampton, transferred to zinc a chromo-carbon print, which was the first photo-zincograph ever executed. Herr Pretsch, of Vienna, many years ago applied a bichromate salt with gelatine to a process which he called " Photo-galvano- graphy " (q. v.). Many pictures printed by this process were issued in this country. The photo-relief processes of Mr. Swan, and of Mr. Woodbury, are only modifications of this. There are many other photographic applications of various salts which are of some importance and interest, which will be found described under their respective headings. Photo-Lithography. M. Poitevin seems to have been the first who made attempts in this direction as early as 1856 ; but his process, although the key to all others, possessed many disadvan- 256 PHO tages. Mr. Osborne, late of Melbourne, has brought it to a high state of practical efficiency, and published in a paper read at the "Franklin Institute," U. S., the working details, which are as follow : — " My process is designed for the reproduction of line drawings and engravings of every description, and the substance upon which I work and from which 1 print is lithographic stone. The problem I had to solve may be stated as follows: — From a given original existing as a black and white delineation, to produce by the chemical properties of light a facsimile upon stone, identical in character with an ordinary lithographic drawing, which has been by the necessary preparation fitted for the printer. " Before proceeding to describe the way in which this is accom- plished, it may be well to state, for the better elucidation of my subject, that lithographic stone consists of a certain variety of car- bonate of lime, which is found in the celebrated Solnhofer quarries, in Bavaria, and is exported to this and other countries in the form of slabs a few inches thick, and of various sizes. For use, one sur- face of such a stone is ground smooth and level, and sometimes polished, and upon this the artist executes his work. If a line be drawn upon such a surface with a pen dipped in a peculiar ink, the essential characteristic of which is that it contains fatty matter in solution ; or with a crayon made with a mixture of fat, wax, and resinous substances coloured with lampblack ; or if we bring in any other way a greasy line upon the stone, and having done so subject the whole surface to the action of a slightly acidulated gum water, the face of the stone so prepared will be found to have undergone a change, and to have acquired certain remarkable properties which it did not possess before ; in fact we have produced a true lithographic drawing, and from the line above mentioned we can now print im- pressions in the press. The changes which have taken place by the action of the fatty matter, acid, and gum, upon the carbonate of lime are rather complex : they are for the most part little understood and indifferently explained. The subject is indeed a difficult one, and an exhaustive examination of the phenomena which present themselves in lithography would form an important and extensive treatise. On the present occasion I must content myself with stating in general terms that the printing capabilities of a lithographic drawing, after "preparation " or " etching," as it is called, depend upon the adhesive attraction which those parts of the surface con- stituting the drawing manifest for greasy matter brought in contact with them, and the repulsion of the same for water. They depend still further upon the converse of this state of things over the rest PHO 257 of the face of the stone, such portions having a strong affinity for water, and when wet as strong- a repulsion for grease. Thus a sort of antagonism is set up, and the surface of the stone is, if I may be permitted to use the expression, polarised, as far as its adhesive affinity for grease and water is concerned. This change in physical properties is due to the chemical combinations which have taken place ; these, as before stated, are difficult to account for thoroughly, but it will suit our present purpose to bear in mind the leading fact, which is this : — that a portion of the greasy matter contained in the lithographic ink enters into combination with the lime of the stone, forming a new substance with it, the boundaries of which are sharply circumscribed. This substance is an insoluble lime soap, and it is neither raised above nor depressed below the face of the stone, and yet constitutes the printing surface, the production of which is the aimof both the lithographic and photo-lithographic artist. " To obtain a proof on paper from a stone in this condition the following method is employed : — The stone is first evenly damped with a sponge or cloth. The printers' roller charged with ink — a compound of prepared oil and lampblack — is then passed backwards and forwards over the work, whereby it deposits its ink only on the design or drawing, for the moisture on the other parts of the surface hinders its adhesion to them, and causes them to remain perfectly clean. A sheet of paper is next laid upon the stone, which is then drawn through the press ; this sheet is then lifted oil", and it will be found that the pressure has caused the accumulated ink upon the stone to attach itself to the paper, and produce there the desired impression. These operations are repeated for the next sheet, and so on until the requisite number is printed. " The foregoing very imperfect sketch of l&rographic general prin- ciples has served at least to define what the end and aim of the photo-lithographer should be. What I have now to describe is the means whereby the hand of man may be dispensed with, and its place supplied by photographic manipulations. " Let us suppose that a map has been compiled and drawn with great care, and that it is desired to multiply copies of this original in the lithographic press. The first step in the process is to obtain a negative, for which purpose the map is placed upright upon a plan-board, and the camera opposite to it at such a distance as to give the desired ratio between original and copy. A negative is now taken on glass coated with collodion quite in the usual way, save that the greatest care is observed to avoid distortion of all kinds, and to produce a negative of the highest excellence, success s 258 PHO in which depends entirely upon the knowledge, judgment, and ex- perience of the operator. " A sheet of plain positive photographic paper is now coated on one side with a mixture consisting of gelatine, softened and dissolved in water, to which a quantity of bichromate of potash and albumen has been added. The paper, evenly covered with this fluid, is dried in the dark, when it will be found possessed of a smooth glassy surface, and a brilliant yellow colour. This surface is still further improved by passing it through the press in contact with a polished plate. "A suitable piece of positive photo-lithographic paper thus manu- factured is now to be exposed to the action of the light under the negative of the map already described. This is accomplished in an ordinary pressure-frame, the time required varying from ten to fifteen seconds to several minutes, according to the brightness of the weather ; but it is always short compared with that necessary for the production of a picture on paper prepared with chloride of silver. The positive thus obtained presents itself to the eye as a brown drawing upon the clear yellow of the sheet. If the prepared surface of the paper were now moistened with water, and the attempt made to apply printing ink to it, we would find a strong tendency in the albumino-gelatinous surface to behave towards greasy and watery substances in a manner quite analogous to that already stated as peculiar to a lithographic stone while printing. We would also find that the solvent action of water at any temperature is quite in- capable of removing the picture which the sun has imprinted upon it. The light, in fact, has so acted upon the chemical substances brought together upon the surfaces of the paper that the organic matter is no longer soluble. These are the characteristics of the change due to exposure which we have to remember. "But the exposed photographic copy of the original is not moistened, or subjected to any solvent action at this stage of the proceedings ; it is, on the contrary, covered all over, while dry, with a peculiar lithographic ink known as transfer ink, which is accom- plished by running it through the press with its face in contact with a stone which has already received a coating of such ink. After it is separated from the blackened stone it will be found to have brought away with it an evenly distributed film of inky matter forced by the pressure into intimate contact with the unexposed as well as the .exposed portions of the surface. This operation is known as " blacking " the positive print ; that now to be described is called <£ coagulation" its object being to effect a change of that nature upon the albumen contained in the coating of organic matter. PHO 259 For this purpose moisture and heat are necessary, and both are applied very simply by letting the blackened photographic copy swim upon the surface of boiling water with its inky side upwards, for it is important not to wet that with hot water. After the lapse of a certain period, determined by the experience of the operator, he proceeds to the next step in the process, that of " washing off** For this purpose the print is laid upon a smootli surface, such as a plate of glass or porcelain, and friction with a wet sponge or other suitable material is applied to the black inky coating, under which the photographic image still exists, and to develop which is now the object in view. The operator soon becomes aware that the moisture which percolated through the paper from the back has exerted a softening or gelatinising influence upon the gelatine in the sensitive coating ; it lias caused it to swell, and to let go its hold upon the ink. But this change does not extend to those parts of the coating which were acted on by light ,• in other words, to those places which were unprotected by the opacity of the negative they remain intact, uninfluenced by the solvent or moistening effect of the water. Accordingly, the operator finds a facsimile of the original map gradually develop under his hand as he continues the friction. This process is proceeded with until all traces of ink are removed save those required to form the picture, which must be clear and distinct in all its details. Abundance of hot water is then poured over it, so as to remove every particle of soluble matter, and it is then finally dried, which completes its preparation. AV e are now possessed of a photogiaph in lithographic ink, identical in every respect with the original, not simply upon paper, but upon albumenised paper — a matter of much importance, as will be pre- sently explained. The presence of the albuminous layer under the picture is the result of the coagulation which took place while the print was swimming on the hot water ; alter that change no amount of washing could remove it, although the gelatine was not proof against such treatment. " A stone to which a fine smooth surface has been imparted i^ now slightly warmed and put in the lithographic press. Upon this is placed inverted the positive print, after it has been damped by lying between moist paper, and the whole is then passed repeat) illy through the press. On examination, the paper will now be found to have attached itself firmly to the stone, so that some force is re- quired to separate the two. When the former is removed it brings with it the albuminous coating, which gives to it its while damp a parchment-like appearance. But the ink is gone ; it has left the paper for the stone, and on the latter we find a reversed drawing of the s 2 260 PHO map, one which, after it has been properly " prepared," will print as well as if it had been drawn by hand. The rationale of this method of transfer is easily understood : the greasy ink having a great affinity for the substance of the stone, combines with it to form a lithographic drawing in the strictest sense of the word, and while this is taking place the damp albumen upon the paper holds the sheet in its proper place, so as to prevent a shift of any kind, and enable the pressure to be applied as often as the operator wishes. " I have thus overcome the difficulties of the problem I under- took to solve ; I have succeeded in making light do the work of the lithographic artist as far as copying the original is concerned. The printer can now multiply impressions from such a stone with as much facility as if the drawing upon it was of ordinary and not of photo- graphic origin." Photomicrography. This term is used to denote the art of taking enlarged photographic pictures of microscopic objects. Many microscopists, both in this country and in America, have been very successful in this branch of Photography. Dr. Maddox's pictorial enlargements, in particular, are wonderfully fine, and leave nothing to be desired. It is needless for any one who is not well acquainted with the manipulations of Photography, and well versed in the method of using the microscope with the different kinds of illumination, to attempt this branch of the art. A first-rate instru- ment is also a sine qua non. Prom a paper read by Dr. Maddox at a meeting of the North London Photographic Association, we extract the following details of his modus operandi in producing the splendid results which he has obtained. " The instrument is supported at some distance from its centre on three double, strong legs. The mirror is removed, and one of Abraham's achromatic prisms substituted. The stage slides along the support of the microscope, and carries a revolving disc or diaphragm with a tube screwed to one of the apertures. In this tube slides another, carrying a Coddington lens. The slide of the stage has a traversing motion by tangential screws. The arm carries adapters to receive different objectives. A fine screw, with graduated milled head, surrounded by a spiral spring, passing through the axis of the body or pillar, and acting on the arm, forms the slow motion, while the coarse adjustment is made by altering the position of the stage. One-half of the microscope tube, which un- screws in the middle, projects from the near surface of the arm : this is at once received into a wide, stout card- board tube, lined with dull PHO 261 black paper, through one end, closed with a centrally-pierced cap of leather. This tube is divided into two parts, having a telescope motion, connected together by a sliding joint made to fit on one, and carrying from its edge a thick circular flap or ring of black cloth, which passes over the end of the other tune, and effectually closes the telescope part against the admission of light. At this juncture a diaphragm can, if needed, be introduced. The other or near end of the card tube is fixed on the brass tubing of a half-plate portrait combination, the lenses being removed : this fits to the front of a camera, and between the front and back part of which is a collaps- ing portion. A long screw unites the sliding body to the other for the facility of ordinary use in focussing ; and the parts of the camera are kept central with the microscope by wooden guides at the sides. The front portion or body of the camera can be fixed beneath by the screw that belongs to the ordinary triangle. A thick velvet collar slides on the microscope tube, up to and against the leather cap, to shut off the entrance of any light. " Let us suppose an appropriate object selected. It is first, if not zcell known, carefully examined with the compound microscope and the object-glass to be employed ; the corrections for the thickness of the glass cover made, if needed ; and some part in view selected as that furnishing the best general idea of the object. It and the objective are then removed to the micro-camera, centred, and focussed. Now becomes apparent the value of the prism, which is so turned (using direct sunlight) as to obtain, when looking on its surface, an image of the object-glasses carried back by the Codding* ton lens. When these are included in the sun's image, and the illumination in the glass screen is perfectly equal, a very minute alteration of the prism sends the rays obliquely to the Coddington lens and to the object. The screen is again examined, and if the character of the image shows the shadows to be too great, the prism is again moved until it furnishes a good general appearance. This slight movement often brings suddenly into view a fine line or some markings not previously noticed. The image is now viewed with a moderate magnifying power fixed in a tube, so that its focus corre- sponds to the far side of the greyed glass. A difficulty sometimes meets us here that is very troublesome when the weather is coM. We often require to go fairly over the entire image with the eye, and if this occupy any time the breath is apt to condense on the near side of the screen, our view is rendered imperfect from the refraction caused by the vapour, and the lines appear confused. This especially happens when the covering cloth is drawn close to exclude light. So troublesome did I find this at one time that I entirely set 262 PHO aside the aid of the magnifying lens, and trusted to the eye suddenly catching some fine marks or lines. The Coddington lens I have employed, not for any optical value it possesses over an achromatic lens, but from its construction as part of a sphere I find it gives a very equal illumination, and is less likely to be injured by the heat rays in the focus of the prism. It naturally lessens the effective optical angular aperture of the high powers ; but I do not find this a disadvantage. The prism is placed at such a distance, when used with the Coddington lens, that its focus would just pass the surface of the object were it used alone. In this way it fills the condenser ; but if used without the latter, which is occasionally the case, I prefer the rays to have crossed before reaching the surface of the object, as there is less chance of their injuring the objective by loosening the medium used in cementing the lenses." Collodion is the best medium to use. The common bromo-iodised collodion, now generally adopted and developed in the ordinary way, is all that is wanted. Then as to the proper time of exposure, that must be determined experimentally ; experimentally also must the allowance be made for the difference between the visual and chemical foci in microscopic objectives, which are always over-corrected. They project the violet or chemical rays beyond the others. Hence it will be always necessary, when the visual focus has been sharply obtained, to move the object glass, by means of the fine adjustment screw, slightly away from the object, before a sharply defined photo- graphic image can be obtained. The requisite distance depends on the power of the objective used and, strange as it may appear, on the mode of illumination. The distance is, of course, greatest with the low powers. For instance, Mr. Shadbolt found that an inch and a half objective required to be moved one-fiftieth of an inch ; a two-third objective, one two-hundredth of an inch, and so on for higher powers. The best modes of illumination are either sunlight or bright dif- fused day-light, but Dr. Maddox has shown that the combustion of magnesium wire offers a tolerably good source of illumination when the solar rays are not available. Under such circumstances a con- denser is necessary. Photo-relief Printing (Woodbury's process). This name is given by its inventor to a process for obtaining pictures from an intaglio in relief by pressure. Mr. Swan, of Newcastle, also in- vented contemporaneously with Mr. Woodbury, a similar process. The principles upon which both processes are based will be described briefly, because the practical results aimed at have PHO 203 not as yet fulfilled the sanguine expectations of the in- ventors. A raised photograph on a surface of bichromated gelatine on .talc is obtained from a negative in the usual way. From this pic- ture in relief an electrotype cast is made by means well known to electrotypists ; or another method is adopted of laying the raised gelatine picture face upwards, and still attached to the talc, on a perfectly flat bed of iron, and placing over it a plate of type or orher soft metal. This is now subjected to heavy pressure in a hydraulic press, which forces the dry gelatine into the type metal, thus form- ing an exact transcript in reverse of the relievo picture. To print from the intaglio mould obtained by either of these two processes, a small quantity of a warm solution of gelatine containing any de- sired colouring matter is placed on the mould, the paper or glass is laid on it, and the whole subjected to pressure for a minute or two till the gelatine sets. The paper is then stripped from the matrix, when the image will be found imprinted on its surface. The principle of this method of printing is quite different from all others. The depressed portions of the mould represent the dif- ferent degrees of shadows, the level surface the high lights. When the pressure is applied, the flat portions squeeze out the fluid ink, leaving the surface of the paper white, while the hollows in the mould retain the pigment in quantity proportional to their depth. Photo-Papyrography. This is an ingenious contrivance of Colonel Sir Henry James, the inventor of Photo-Zincography. In cases where the number of reproductions of maps, drawings, &c, is very small, it saves much time and labour if the negative be taken in a reversed position in the camera — that is, with the glass instead of the collodion nearest the lens ; or a reversing mirror might be used for the same purpose. A print in greasy ink is then obtained as described under the article " Photo-Zincography." When dry it is laid, picture side downwards, on a piece of paper, and passed through a lithographic press. By such means, it is said, excellent impressions can be obtained. We have not seen any specimens of these photo-papyrographs, but from Sir Henry James's description, the process seems to be a very promising one. Photo-zincography. This process in principle is the same as photo-lithography, but some of the details are different. It was invented by Colonel Sir Henry James, and is now extensively cm- ployed in the Ordance Survey Department at Southampton for copy- ing maps, &c. 264 PHO A suitable paper is floated for two or three minutes on a warm solution (about 100°) of the following substances : — Bichromate of potash, 2f ounces, dissolved in 10 ounces of hot water, to which are added 3 ounces of the purest gelatine previously dissolved in 40 ounces of hot water. The paper, after becoming dry, should be again floated on the same solution, and hung up to dry at the opposite corner to that by which it was first suspended, in order to distribute the sensitising solution uniformly. This must be done in the dark room. This paper will not keep long in a serviceable state even in the dark room, because the bichromate gradually oxidises gelatine without the action of light. Two days are about the limits of its keeping qualities. The sensitive paper is exposed to the solar rays under a negative in the pressure-frame as usual. One minute in bright sunlight is often sufficient. The general indications, to judge of sufficient exposure, are the appearance of the parts where the light has acted most strongly. They should be of a deep tawny colour, tinged with green, and the shadows yellow. Now comes that part of the process where the lithographer steps in to complete the work of the photographer. The print is removed from the pressure-frame, and inked by the following method : — In an iron pot put 2 ounces of Burgundy pitch, 1 ounce of palm oil, and 1 ounce of bleached bees' wax ; place the pot over a fire, and as soon as they begin to melt, keep stirring the mass till they are thoroughly incorporated, which will not take place till the ingre- dients have nearly reached the point of ignition. Then remove the pot from the fire, and intimately mix with the contents 1 pound of chalk lithographic ink, and half a pint of what is called in the trade middle linseed oil varnish, both of which must have been previously thoroughly incorporated by pounding in a mortar. When required for use, a portion of the ink is melted with suf- ficient turpentine to make it of the consistence of honey. A little is then placed on a printing roller, and a flat zinc plate inked with it in the usual manner. The print is then laid face downwards on the zinc, and the whole passed through a press, by which means it receives an even coating. The print is then removed from the zinc plate, and laid back downwards on water, at the temperature of about 100° Fahrenheit, for a few minutes. It is next placed on a level slab, and all the superfluous ink removed with a soft sponge dipped in gum water. It is afterwards treated with repeated baths of warm water till the ground is quite clear. When dry, it is ready for transferring to zinc or stone. PHO 265 Mode of Transfer, Sfc. Colonel Sir Henry James's instructions on this part of the process are so very lucid and precise that we cannot do better than quote them : — The Transference of the Print to Zinc, and Preparation for Printing, When the zinc plates are received from the manufacturer, the surface has to be p re pared to receive transfers. They are first planed with a razor blade, the back of which is set in a wooden handle ; the ordinary edge is ground down flat, so that there are two edges to scrape with in turn, like the edges of a skate. The plate is thus cut down till all surface scratches, blisters, and other defects are obliterated. It is then ground down to a flat surface with pumice stone, and smoothed with snake stone, to take out any scratches made by the pumice stone. Finally, a grained structure is given to it by rubbing with fine sand and water, and a zinc muller. The muller is simply a disc of zinc, about half an inch thick and four inches in diameter, fixed to a wooden handle. It is grasped by the handle with the thumb uppermost, and rubbed over the surface of the plate with a circular movement. The sand is brought to the requisite degree of fineness by sifting it through a wire sieve of from 80 to 120 holes to the square inch, according to the kind of grain required for the plate. The time required for two men to grain a zinc plate 3 feet long by 2 broad has been found to be about an hour. As soon as this process is completed, the plate is thoroughly washed with water and well dried. It should be kept from contact with any substance likely to communicate greasiness to it ; and tlie sooner it is used for trans- ferring the better, as the action of the atmosphere will tend to diminish the affinity of the surface lor t he greasy ink. When it is desired to clean and prepare for receiving transfers a plate which has been used, the ink of the old transfer is cleared oil with turpentine ; the plate is then washed with strong alkali, and cleaned with water; an acid is then poured over it. This is prepared by taking equal parts qf sulphuric and hydrochloric acid, and to 1 part of the mixture adding 12 parts of water. The acid is allowed to remain for two or three minutes on the plate ; it is then washed off with plenty of water, and the plate is regrained in the manner already described. The photographic print is laid between sheets of damp paper for a few minutes, placed face downwards on the zinc plate, with two or three sheets of paper over it, and passed through the press. • 266 PHO If the transfer print is not more than three or four days old, it will be sufficient to pass it through once ; but an old print, on which the ink has had time to harden, will require to pass through the press two or three times. The sheets of paper covering the transfer are then removed, and it is damped with a wet sponge for two or three minutes ; this causes the gelatine in the lines to swell, and makes the ink leave them more readily. The- print is then pulled carefully off the plate, and nearly the whole of the ink should remain on the zinc. The transfer is now etched; the etching liquid consists of a decoction of galls and a little phosphoric acid, mixed with a thick solution of gum and water. It is prepared as follows : — Four ounces of Aleppo galls are bruised and steeped in 3 quarts of cold water for twenty-four hours ; the water and galls are then placed in a vessel over the fire, and allowed to boil up. This decoction is then strained. The gum water should be about the consistence of cream. One quart of the decoction of galls is added to 3 quarts of the gum water, and to the mixture is added about 3 ounces of the solu- tion of phosphoric acid, which is prepared by placing sticks of phos- phorus in a pint bottle of water ; this is stopped with a cork, in which is cut a small hole ; the bottle is three quarters filled with water, and the ends of the sticks of phosphorus rise above the sur- face, and become oxidised by the air admitted into the bottle. The phosphoric acid, as fast as it is formed, is dissolved by water. In a few days the solution is strong enough to use. The etching liquid is poured on the plate, and wiped over the surface with a sponge or camel-hair brush ; it is allowed to remain on for a short time, varying with the strength of the design : with fine work, twenty seconds would be sufficient ; strong lines should bear the action a minute without injury. As soon as the solution has acted sufficiently, it is wiped with a soft cloth dipped in water — care being taken to remove all trace of it, if there are fine lines. The transfer ink is next cleared from the zinc plate with turpen- tine, or, if the design is weak, with turpentine mixed with olive oil and gum water. It is then rolled up with printing ink, the roller being very thinly and evenly coated. Impressions can then be printed in the usual manner ; 1500 is not an unusual number for the plate to stand without sensible deterioration. The photographic print can be transferred to a lithographic stone in a similar manner. PHO PTG 267 When the subject admits of it, paper, enamelled with zinc white, should be used, as the impressions produced are most perfect. It is prepared in the following- manner : — Pour ounces of Russian glue are soaked in 3 quarts of water for some hours, and then heated till dissolved; 1£ pound of zinc white is ground with water on a slab, and then mixed gradually with the solution of glue and passed through a hair sieve. A coating is brushed on the paper with a pound brush, and the streaks are obliterated by going lightly over the surface with a flat camel-hair brush. A second coating is applied in a similar manner, and hung up to dry : when dry it is ready for use. For further particulars and details we refer the reader to Colonel Sir Henry James's work on " Photozincography." Photometer, or Actinometer. An instrument for measuring the intensity of the actinic rays. Several methods have been de- vised for effecting this object, but they are all so liable to objections that it i3 hardly worth while describing any of them in this work. Phototype. A secret process of printing in carbon, or pigments, invented by M. Joubert, of Porchester Terrace, Payswater. Pigments. Positive prints may now be obtained in various pig- ments by mixing them with an organic substance and bichromate of potash — applying the mixture evenly to the entire surface of a sheet of paper, drying it, and exposing it under a negative — then wash- ing it in water, or a suitable solvent, which removes the pigment from those parts of the paper which have not been acted on by light, and leaves it firmly cemented to the paper in the parts which have been so acted on. The process of printing in pigments has not yet re- ceived much attention, and the results are at present more or less imperfect as compared with those by the old processes ; but, since prints, by the methods in common use, are extremely liable, if not certain, to fade, it is of the utmost importance that the methods of printing in carbon and permanent pigments should not only yield proofs artistically equal to silver prints, which they already do, but to be carried out commercially on an economic scale. The following is a brief account of some of the common pig- ments : — Black. Ivory black is made by calcining ivory dust in a close crucible. Lamp black is the soot produced by the combustion of oils, resins, and other vegetable substances. Umber. A brown mineral found in the island of Cyprus ; it is composed of silica, alumina, and oxide of iron and manganese. 268 PIG When calcined for half an hour at a red heat, the pigment called burnt umber is produced. Asphaltum. A fine rich brown pigment. See " Asphaltum." Sienna. An argillaceous mineral found in Italy, and also near Wycomb. By calcination it becomes burnt sienna. Smalt blue. A glass coloured with oxide of cobalt, and pul- verised. Cobalt. Hydrate of alumina mixed with hydrated oxide of cobalt, dried and calcined. Sulphate of indigo. CJiemic blue, Saxony blue. Indigo dis- solved in about six times its weight of sulphuric acid, then diluted with water, and neutralised with potash. Prussian blue. A compound of cyanogen and iron. It is not considered a permanent pigment. Stone blue. Finely -powdered indigo mixed with starch paste, and made into lumps. Copper blue. A mixture of carbonate of copper and chalk, ex- posed to the air until it assumes the proper colour. Ultramarine. A pigment composed chiefly of a costly mineral called lapis lazuli, brought from China and Persia. Artificial ultramarine. A pigment containing sulphide of sodium, obtained by fusing together, in a crucible, porcelain clay, sulphur, and carbonate of soda. French photographic papers are tinted with this villanous alkaline sulphide, which is enough of itself to cause the fading of any photograph. Blue verditer. Nitrate of copper mixed with chalk. Copper green. Native sub-carbonate of copper. Brunswick green. Carbonate of copper mixed with calcareous matters. Vienna green. A mixture of arsenious acid and verdigris. Green verditer. An accidental variety of blue verditer. Sap green. The juice of the berries of buckthorn, black alder, or evergreen privet, mixed with lime water and gum arabic, and evaporated until quite thick. Iris green. The juice of the petals of the iris added to quick- lime. Carmine. An extract from the cochineal insect. Lake. The colouring matter of raw shellac. Brazil-wood lake. A mixture of a decoction of logwood, alum, and chloride of tin, to which carbonate of soda is added to form a precipitate. Madder. A colouring matter obtained from the root of the Rubia tinctorwm, which grows in the South of Europe. PIG PLA 269 Brown pink. To a decoction of French berries and fustic, boiled with potash in a tinned vessel, alum is added. The precipitate is " brown pink." Dutch pink. Turmeric is substituted for fustic, and whiting for alum, in the preceding formula. Orange red. Sandix. White lead calcined. Red lead. Minium. . Litharge (oxide of lead) roasted in a reverberatory furnace. Indian red. Peroxide of iron. Red chalk. Clay iron-ore. Venetian red. Oxide of iron. Alum white. A calcined mixture of honey and alum. White lead. Basic carbonate of lead. Permanent white. Carbonate of baryta* Zinc white. Oxide of zinc. Chrome yellow. Chromate of lead. Indian yellow. A concretion formed in the intestines of the camel. King's yellow. Sulphide of arsenic. Naples yellow. A calcined mixture of lead, antimony, alum, and salt. Patent yellow. Chloride of lead. Queen's yellow. Turpith mineral, or sub-sulphate of mercury. Yellow lake. French berries boiled with potash, and precipitated with alum. Ochres. Native oxides of iron mixed with argillaceous and cal- careous earths. Verdigris. Acetate of copper. Indigo. A product obtained from the indigo plant. Sejna. The black liquid contained in the cuttle-fish. Tt consists of carbon, along with albumen, gelatine, and phosphate of lime. Vermilion. Cinnabar. Protosulphide of mercury. Terra verte. Silicate and phosphate of protoxide of iron. Fins. Pins are used in Photography for hanging up papers by the corners to dry, or for pinning the corners to a board. Silver pins, or black coated with enamel, are the best. Pipe Clay. A clay analogous to kaolin, and found in the Isle of Purbec and Dorsetshire. It contains a large proportion of alumina, and is sometimes used for decolorising old nitrate baths. Plaster of Paris ; Gypsum. This useful substance is made by roasting sulphate of lime at a temperature of about 500°, by which the water of crystallisation is expelled. When plaster of Paris is made into a paste with water, it soon solidifies, and this property 270 PLA constitutes its value for taking casts or moulds. Stucco and sca- gliola are made by mixing plaster of Paris, coloured in various ways, with size and water, and polishing the surface. Gypsum, or native sulphate of lime, is frequently used as a manure, particularly for clover crops. Sulphate of lime is soluble in 500 parts of water, and it is this salt which principally renders water " hard." Plate. In optics a transparent medium bounded by parallel plane surfaces is called a " Plate." When a ray of light is refracted through a plate, its direction at emergence is parallel to that at inci- dence, and it does not suffer deviation, but only displacement ; the amount of displacement depending on the thickness of the plate. The same thing happens when a ray is refracted through any num- ber of plates of different materials in contact ; it merely suffers dis- placement and not deviation ; — the medium external to the plates being supposed to be the same. Plate Glass. This is made of the same materials as crown glass, and does not contain lead. Vast quantities of it are now used in Photography. Plate glass is made by pouring a quantity of the fused " metal " upon a table or cuvette of cast iron, and then passing a roller over the surface. The plate is then annealed, or allowed to cool slowly in an oven, or carquaise, along with others. When cold, the plate is removed, and carried in an upright position to a part of the manufactory where it is to be roughened down and polished. This is accomplished by fixing one side of the plate with plaster of Paris to a horizontal stone table, and another plate to a piece of apparatus above it. The apparatus is then put in motion, and the surface of the upper plate rubbed upon that of the under one, with wet sand between them, by a circum-rotatory motion, at the same time that a peculiar lateral motion is given to the table which supports the lower plate. When the plates are in this way sufficiently worked on one face, the process is repeated on the other. The plates are next smoothed in the same way by substituting moist emery for moist sand, and the polishing is effected by colcothar (oxide of iron) applied by rubbers of felt. The final polishing is given by women, who rub two plates together with a little moistened putty of tin between them. Manufacturers of plate glass should be particular not to pack it up, when intended for Photography, with printed papers between the sheets; for it has been found that permanent impressions are thus communicated to the glass, which are reproduced upon the photo- graph. Tt has been affirmed that these impressions sometimes cannot be removed by the strongest nitric acid. PLA POT 271 Plate Paper. The thick bibulous paper upon which engravings are printed. Platinum, Bichloride of. Pt Cl 2 =169-6. This, the perchloride of platinum is formed by dissolving the metal in nitro-hydrochloric acid, and evaporating to dryness. The dark brown residue forms a deep yellow solution in water, which, when quite neutral, and added to lime water, gives a copious white precipitate in the sun's rays. It has been proposed to determine by the weight of this the actinic power of the light at the time. The aqueous solution has been tried as an etching liquid in engraving photographs on steel plates ; and as a toning agent for paper positives instead of chloride of gold. Plumbago, Black Lead, Graphite. This substance is com- posed of carbon and iron, and contains about 8 per cent, of iron. The finer kind is used for black-lead pencils, and the coarser kinds for polishing grates, diminishing friction in machinery, &c. It is almost exclusively obtained in a pure form from the mine of Borrowdale, in Cumberland ; it is infusible, very difficult of com- bustion, and sometimes occurs crystallised in hexangular plates. In an impure form it is not an uncommon mineral, and is found in detached masses, generally among primitive rocks. Plumber's Solder. Equal parts of lead and tin. Polarised Light. See " Light." Portrait Room. See " Glass House." Positives. This is the name given to photographs which have the lights and shades rendered as they are seen in nature; and it is used to distinguish such pictures from negatives, which have the lights reversed. Potash. KO, 110 = 56. "Caustic potash," or hydrate of potash, is obtained by boiling together in an iron vessel slaked lime and a solution of carbonate of potash. The carbonic acid leaves the potash and goes to the lime, forming an insoluble pre- cipitate of carbonate of lime, and the potash remains in solution. When a little of the liquid taken out ceases to effervesce on the addition of an acid, the decomposition is complete. The clear liquid is then drawn off into an iron or silver vessel, evaporated to dryness, fused in its own basic water, and run into moulds. The sticks thus formed still contain a little carbonate ; this is re- moved by dissolving thein in absolute alcohol, when the carbonate of potash is precipitated as insoluble, but the plan is open to objection since the alcohol is liable to be decomposed. 272 POT Caustic potash is soluble in half its weight of cold water. It is highly alkaline and caustic, acting energetically upon most organic substances, and dissolving sulphur, alumina, silica, and several sulphides ; its aqueous solution, also, dissolves the oxides of some of the metals, as manganese, zinc, lead, tin, antimony, cobalt, nickel, &c, and acts upon glass, particularly when at a boiling heat. It is freely soluble in alcohol, and fuses at a red heat. Potash, Bichromate of. KO, 2 Cr0 3 =147'5. This salt is obtained by acting on the chromate of potash with nitric or other acid. The acid abstracts half the potash, and deep orange or red crystals are deposited, which are the bichromate. Bichromate of potash, in contact with gelatine or" other organic matter, is decomposed by light, and gives up half of its oxygen to the organic body, which, if previously soluble in water, is now in some instances — gelatine, for example — rendered insoluble. This impor- tant property of bichromate of potash is the foundation of several photographic processes, and the chemical action is strictly analogous to that of the use of nitrate of silver alone on paper. A visible image is impressed on the paper, on which reduced silver and other metals (iron, &c.) may be afterwards precipitated by suitable developing solutions. When protosulphate of iron and gallic acid are employed, the picture, or its chemical composition, resembles writing ink. The iron and other metallic solutions are sometimes presented, with the bichromate in the paper, to the action of light, and the variety of modifications appears to be infinite. The insoluble compounds of chromium, formed by light, and the action of the light itself, have both been used in the art of dyeing, the chromium forming a mordant on the textile fabric, in parts exposed to light through a perforated pattern, on which mordant the colours are subsequently applied ; or the colours may be put on with the bichromate, and subsequently washed out from the parts not acted on by the light, just as in a similar photographic process on paper. Papers prepared with bichromate of potash and nitrate of silver have also been found to give images varying in colour from red to green, and blue. Potash, Carbonate of. KO, C0 2 =69. The pearl-ash of com- merce contains a variety of impurities, such as chloride of potassium and sulphate of potash, and is obtained by the combustion of veget- ables and lixiviating the ashes. The solution thus obtained is drawn off, evaporated to dryness, and then calcined to destroy the organic matter which it contains. A pure salt is obtained by exposing pure cream of tartar to a red heat and separating the carbonate by means of distilled water. POT 273 Carbonate of potash is extremely deliquescent and soluble in less than its own weight of water. It is quite insoluble in alcohol. For this reason it is sometimes shaken up with weak alcohol to deprive it of its water. None but the pure salt should be used for this purpose. Potassium. K=40. This singular metal is obtained by ex- pelling the oxygen from potash, either by exposing it to intense heat in contact with charcoal, or by voltaic electricity. It is a bluish white metal of great lustre, which fuses at 150°, boils at a red heat, floats upon water, and takes fire by coming into contact with water in consequence of its great affinity for oxygen. It must be pre- served under naphtha. Potassium, Bromide of. K Br=119. When bromine is added to a solution of caustic potash, bromide of potassium and bromate of potash are formed. The mixture is evaporated and heated to redness, by which means the oxygen is expelled from the bromate, and bromide only remains. It crystallises in cubes, which are very soluble in water, but only soluble in alcohol accord- ing to the proportion of water which it may contain. In this respect it exactly resembles the iodide of the same metal. Bromide of potassium was extensively used in the paper — es- pecially the waxed-paper — negative processes. It is also occasionally used in collodion, but is rapidly being displaced by other bromides more soluble in strong ethereo-alcoholic solvents of pyroxyline. Potassium, Chloride of. K CI. = 74-5. This dissolves in 3 parts of water at 60°, but is insoluble in alcohol. This salt is a residue in several chemical processes, and is often present as an im- purity in the iodides and bromides of the metals, and in nitre as occurring in commerce. In preparing papers it is not much used, though the nitrate of potash formed in sensitising would not be so likely to damp the paper and spoil it when kept, as nitrate of soda. Potassium, Cyanide of. K, C 2 N, or K Cy.=65. The prin- cipal employment of this salt in Photography is in forming a small proportion of cyanide of silver with the iodide, and in fixing. The former of these uses is not to be commended. As met with in commerce, it often contains a large proportion of carbonate of potash, which is detected by the effervescence produced when an acid is added to its solution : from the usual mode of its prepara- tion, it often also contains cyanate of potash, but not in very large quantity, and this is not injurious. It is deliquescent, and especially T 274 POT so when contaminated with carbonate, but it is not soluble in cold alcohol. By dissolving it in boiling alcohol the impurities are removed, and on cooling, pure cyanide of potassium is deposited. "When dissolved in water it may be crystallised in cubes. It is as poisonous as prussic acid (hydrocyanic acid) itself : the best antidote, and one very convenient to the photographer, is protosulphate of iron. It is used from 2 to 20 grains to the ounce of water in fixing photographs, according to the purity of the article. Its proper- ties for this purpose differ from those of hyposulphite of soda, and it is more energetic. Like " hypo,'" it forms double salts with the chloride and other compounds of silver, which salts contain one equi- valent of cyanide of silver and one of cyanide of potassium : they are not decomposed by water, as the double salts produced by other fixing agents are, so that no apprehension need be entertained of precipitating cyanide of silver in the washing: and.it dissolves much more silver on this account than weak "hypo" will do. It also acts more quickly ; being weak, it is soon saturated, and is more quickly and completely removed by washing, so that comparatively very little washing is required. It is never found to crystallise in the film, and but very few instances have occurred in which the washing has been so careless as to leave enough in the film to react on the picture. Cyanide of potassium has also an affinity for oxygen, and has therefore considerable reducing powers, so that the oxides of copper and other metals thrown into it in the fused state are presently brought to the state of pure metal, and the cyanide becomes cyanate. This reducing power is also seen in fixing collo- dion pictures, which have a more metallic lustre when fixed with it than when the "hypo" salt is used. All these are important advan- tages, but it is not adapted for fixing pictures on albumen and paper, from the power it has of dissolving the organic basis, and reduced silver in combination with it, as well as the unaltered chlorides and other salts. From its highly poisonous properties, this otherwise very useful salt has been banished from many photographic laboratories. Some persons are so susceptible to its influence, that, by absorbing it through the skin, or a cut in the hand, while fixing a plate, serious inconvenience has been experienced. It should also be borne in mind by the operator who uses it for fixing a negative that an acid decomposes it, and causes it to give off fumes of that highly poisonous substance, hydrocyanic or prussic acid, the first symp- toms of which are a sense of constriction of the throat and giddi- ness. It must, therefore, be used, under all circumstances, with the utmost caution. POT 275 Potassium, Fluoride of. K F=58. Fluoride of potassium was at one time used in Photography in the iodising solutions for waxed papers. It is doubtful whether any advantage was thereby gained, for, although fluoride of silver is sensitive to light, and will yield a developable image, it is apparently much less sensitive than the iodide and bromide. It may be prepared by saturating hydrofluoric acid with potash, and evaporating to dryness in a platinum vessel, not in a glass one, because it acts on it like hydrofluoric acid. Potassium, Ferro-cyanide of, or Yellow Prussiate of Potash. K 2 Cfy + 3HO, or K 2 C 6 N 3 Fe + 3HO = 211-4. This salt is soluble in 4 parts of cold and 2 parts of hot water ; it is in- soluble in alcohol, which throws it down in yellow flakes from its aqueous solution. The crystals are four-sided tables derived from a primary octahedron. The salt is not poisonous. When in crystals, the oxygen and hydrogen of the water of crystallisation are exactly in such proportions as are required to convert the metals into protoxides and the cyanogen into hydrocyanic acid (prussic acid). Ferro-cyanide of potassium is prepared as an article of commerce by putting chips of hoofs, animal horns, woollen rags, greaves, &c, into an iron pot, and burning them at a very high heal with potash, so as to form what is called " prussiate cake." This, when cold, is lixiviated with water, and evaporated. The resulting crystals are an impure ferro-cyanide of potassium. These are purified by bein" redissolved and recrystallised. The vessels and stirrers used in the operation should be of iron, as they then supply the iron contained in the salt. If not in sufficient quantity, iron filings should be added. Potassium, Iodide of. KI=166. Iodine and potassium unite energetically, giving out heat and light; and potassium burns in the vapour of iodine. The result of this combination is the while anhydrous salt — iodide of potassium. The mode of producing this salt commercially is to add iodine to a solution of potash, until it assumes a brown colour ; then evaporate to dryness, add a little charcoal, and fuse the residue at a red heat, which decomposes any iodate of potash that may be formed ; dissolve, filter, and recrystallis'e the salt. Sometimes iodide of potassium is made by passing sulphuretted hydrogen through a brown mixture of liquor potassa; and iodine, till it becomes colourless; expelling any excess of sulphuretted hydrogen by heat, filtering, neutralising with potash, and crystallis- T 2 276 POT ing. Another way is, to decompose either iodide of zinc or iodide of iron by carbonate of potash. Iodide of potassium crystallises both in cubes and prisms. The crystals are anhydrous, and very slightly deliquescent in damp air. They are extremely soluble in water, 100 parts of which dissolve at 65° 143 parts of the salt, with production of cold. It is soluble in alcohol only in proportion to the quantity of water which the alcohol may contain. The impurities contained in commercial iodide of potassium are iodide, carbonate, and sulphate of potash, chloride of potassium, and sometimes of zinc and iron. Carbonate and sulphate of potash are detected by adding chloride of barium to the solution, which throws down white insoluble car- bonate, or sulphate of baryta. Another test for carbonate of potash is to expose the alcoholic solution of the iodide to sunshine ; if it soon becomes slightly discoloured, it is pure, if not, it contains car- bonate. The presence of much carbonate of potash renders iodide of potassium highly deliquescent. Aqueous solution of iodide of potassium is immediately discoloured by the addition of chlorine water, or nitric acid, but not by the weaker acids, in the dark ; in the light the discoloration proceeds more rapidly. This salt is much used, and is the best for iodising papers for the calotype and waxed-paper processes. It is not, however, the best for collodion, unless the purest materials are employed to dissolve the pyroxyline, and the developer be pyrogallic acid ; its proneness to decompose when acid or active oxygen — ozone — is present in the collodion renders it inferior to other iodides, which are not so liable to decomposition, and possess qualities of another kind equally good, if not superior, to itself. Besides, when highly rectified, ether and alcohol are used to dissolve the pyroxyline, as they should always be : a sufficiency of a potassium iodiser cannot be dissolved in the collodion. Potassium, Sulphide of. See " Liver of Sulphur." Potassium, Sulpho-cyanide of. K, S 2 Cy=97. If 184 parts of dry ferro-cyanide of potassium, 128 of sulphur, and 69 of dry carbo- nate of potash are pounded up together, thoroughly mixed, and fused for a considerable time in a covered iron crucible, sulpho-cyanide of potassium and sulphuret of iron are formed. The fused mass, when it has become quite cold, must be stirred up with water and filtered, then the solution on evaporation will yield prismatic crystals of this salt, pure enough for photographic purposes. PRE PHI 277 The sulpho-cyanides of ammonium and potassium have been re- commended for fixing photographic positive prints, in lieu of hypo- sulphite of soda, which is so prone to form deleterious compounds, which act on the images and promote fading. It is very doubtful whether the sulpho-cyanides are less hurtful. This remains to be proved. Precipitate. Any substance which separates in a solid or pulveru- lent form from a solution. The substance which produces the pre- cipitate is called the precipitant, and the act by which it is produced is called precipitation. Preservative. In Dry-Plate Photography it is customary to apply to the sensitive film some organic substance, in order to give density and transparency to the negative, and to cause the film to adhere. It is found that unless organic matter of some kind be introduced in this way the negative is thin and veiled. Albumen and tannin are the best preservatives at present known, the former being used in the Taupenot and Fothergill, and the latter in the Tannin processes. The term " preservative " is a misnomer, and has arisen from a mistaken notion of the real use of the application of organic matter. Its real office will be easily understood by com- paring two pictures treated in every way alike, except, that one has, and the other has not, a preservative applied to it. Print Varnish. A compound of benzole and almond oil. When applied to a dead-looking silver print upon plain paper, it gives it freshness and vigour, and at the same time imparts to the paper the agreeable colour of India paper. If the print has not been properly fixed, the application of the varnish renders this fault instantly apparent, by showing the sulphide that exists within the pores of the paper. Print varnish does not give the slightest glaze to the picture. It should always be used with sun-prints upon plain paper ; but not with developed prints. Printing Ink. This may be applied as a backing to collodion positives. It does not crack. To apply it, first rub the back of the plate over w r ith it, then smear a piece of paper with it, press the two blackened surfaces together, and put the plate at once into the passe-partout. It may also be used for blackening the skies of collo- dion negatives. Printers' ink is made by grinding lamp black in printers' varnish, which is a mixture of linseed oil, resin, and yellow soap. By add- ing more varnish it may be thinned to any extent. 278 PRI Printing-ink Process— Pouncy's. The nature of this new and very remarkable process will be best understood from the specifica- tion of it by the patentee. It is as follows, and bears date July 28th, 1863: — " My said invention relates to certain improvements in obtaining, transferring, and printing from photographic and any other pictures or images, and also in preparing the materials for such processes, the main or principal feature of my invention consisting in the em- ployment of a sensitive or sensitised ink, or composition, on which pictures or images may be produced by the agency of light, and which may be transferred or printed from in the manner hereinafter mentioned. The surfaces employed for the reception of the pictures or images may be paper, silk, linen, cotton or mixed fabrics, leather, wood, ivory, glass, porcelain, or stone, or surfaces of metal, or of metallic alloys, may be used, or any other surface. The surface proposed to be employed is coated with an ink, or composition, consisting of carbonaceous or other colouring matter (according to the colour or tint of the picture desired to be produced), fat, tallow or oil, bichromate of potash, or bitumen of Judaea, or both of such last-mentioned substances, and benzole, turpentine, or other hydro- carbon or analogous spirit. The respective proportions of the several ingredients hereinbefore mentioned will vary with the circumstances attending the operation. The mode of compounding such ingre- dients and the necessary proportions thereof, will be well under- stood, by persons conversant with the preparation of similar or analogous inks or compositions, with the addition of the two last- mentioned substances- dissolved in either of the spirits before named. I may mention, however, that if the photographic picture or image is to be transferred on to stone, or any other surface suit- able for being printed from, a larger quantity of oil or fatty matter is employed in preparing the ink, or sensitive composition, than when the picture or image is simply taken on paper or any other surface, and is to be used merely in that form. The ink or compo- sition must be prepared and applied to the surface of the material employed in the dark, or in a place from which the actinic rays of light have been excluded, or by an artificial light, which does not act photographically. The coated surface is to be dried and ex- cluded from the light until it is about to be used, when a photo- graphic picture or image may be produced thereon, by any of the usual and well-known methods applicable to the purpose. When the picture or image is to be produced by means of a negative pic- ture or image applied to the sensitised surface, prepared as aforesaid, and the substance or material is sufficiently transparent to allow the PBI 279 rays of light to act through the same, the negative picture or image may be placed on the uncoated surface, and the light applied so as to act on the coating through the transparent substance or material. " The rays of light having hardened the parts required, and pro- duced the desired effect on the sensitised coating, the parts not acted on thereby, remaining soluble, are then washed off with benzole, turpentine, naphtha, or other hydrocarbon solvent. The picture or image will thus be left upon the surface employed, in printing ink, or in a composition in the nature of or analogous to printing ink, and from which impressions can be obtained, as hereinafter mentioned. " The pictures or images, obtained in the manner hereinbefore mentioned, are applicable to a great variety of purposes, both useful and ornamental. They may be preserved simply as works of art, or they may be transferred (if a suitable material for receiving the sensitive coating has been employed) to the surface of porcelain or other ceramic manufactures, and ' burnt in' or permanently im- parted thereto, according to the well-known methods of performing such operations. " When the sensitive ink or composition is applied to a lithographic stone, for the purpose of printing therefrom, the surface of the stone must be 1 grained' before the application of the ink or composition, and the surface of the coating should be also grained, after it has been laid on the surface of the stone. The process of graining is well understood, and therefore I need not describe the same. Be- fore transferring the picture to the stone, the surface thereof should be moistened with water, and it should be us^ed in the press in a cold state, and not heated, as is frequently the case in ordinary litho- graphic printing." Further particulars will be found in a shilling pamphlet, by Mr. Sutton, entitled " Photography in Printing Ink," and published by Messrs. Sampson Low, Son, and Marston, Ludgate Hill. Printing Process. By " printing" is meant the reproducing a positive, in which the lights and shades are true to nature, from a negative in which they are reversed. The operation, not being at- tended with the destruction of or injury to the negative, may be repeated indefinitely, and therefore any number of prints may be taken from the same negative. There are two methods of printing ; one consists in copying the negative by means of a lens, the other by pressing it upon a sensi- tive tablet in a pressure frame, and exposing it to direct light. In both cases the light which produces the priut is transmitted through 280 PRI the transparent parts of the negative, and stopped by its opaque parts. The particulars of the former method are described in the article on " Copying" (q.v.) It only remains, therefore, to describe the latter mode of proceeding ; viz., printing by superposition. There are two methods of printing by superposition of the nega- tive upon a sheet of sensitive paper. One is called Sun-printing, the other Development-printing. In sun-printing the paper is said to be either " plain" or "albu- menised." The process of sun-printing upon plain paper is as follows : — Use the best Papier Saxe, or Hive, both of which are manufactured on the Continent, and have a finer surface than English papers. Float the face of the paper for a minute upon a bath composed of— Filtered rain water . . . 1 oz. Gelatine 3 grains. Chloride of sodium . . .6 grains. The ingredients are to be boiled together, strained, and used when cold. Hang the papers up to dry by a pin at one corner. Excite the paper by brushing it over with, or floating it for a minute or two, on the following solution of ammonio-nitrate of silver: — Distilled water . . . . 1 oz. Nitrate of silver . . . .50 grains. "When dissolved, add ammonia, a drop or two at a time, until the brown turbidity at first formed is exactly redissolved, and the solu- tion becomes again clear. Hang up to dry in a warm dark room. The papers should, if possible, be used the same day on which they were excited, because they spontaneously turn brown by keep- ing. Expose under the negative in the pressure frame until the picture is somewhat over -printed, and then proceed to tone, fix, &c, as described below for albumenised paper. To Print on Albumenised Paper. For the method of preparing this paper, see " Albumenised Paper." To sensitise it, make a solu- tion in the following proportions : — Nitrate of silver . . . . 80 grains. Distilled water .... 1 ounce. This strength of solution is suited for paper coated with albumen containing from ten to fifteen grains of salt. If less than ten grains of salt be used, then fifty or sixty grains of nitrate of silver to the ounce of water is quite sufficient to fully sensitise it. The strength PM 281 of the nitrate bath should always be regulated by the amount of salt in the paper. The thickness of the paper also exercises con- siderable influence. The thicker it is the stronger should the solu- tion be. The above proportions are well suited for thick papers. They may be diminished when using thin papers. Pour a sufficiency of the solution into a flat-bottomed porcelain or glass tray, to cover the bottom to at least the depth of one-eighth of an inch. Lay down on the surface of the liquid, the albumenised side of the paper carefully, so as to avoid air bubbles. A very little practice makes this operation easy. Let the paper lie on the sen- sitising fluid for two or three minutes, then hang up by the corner to dry in the dark room. Like the sensitised plain paper, it will not keep long without turning brown. It should therefore be used as soon after it becomes dry, as convenient. Expose in the pressure frame as before, until the picture is some- what overprinted. Toning Operation. "When the proofs have been removed from the pressure frame, keep them in a dark cupboard till the printing for the day is over ; then wash them thoroughly in several changes of common water, to remove all the free nitrate of silver, which would otherwise interfere with the subsequent operations. The first washing water should be preserved in a large stock jar, and the nitrate precipitated with chloride of sodium. In this way more than half the silver used can be recovered. See " Wastes." The prints are now ready for the gold toning bath, for which * many formula? have been recommended. The two most generally adopted are as under : — (1.) Chloride of gold . . . 1 grain. Water . . . . .10 ounces, and carbonate of soda just enough to neutralise the acidity of the gold or make it very slightly alkaline. (2.) Chloride of gold . . . 1 grain. Water 10 ounces. Acetate of Soda . . . 50 grains Bath No. 1 will be in good working order after being mixed one hour. Bath No. 2, should be mixed at least twelve hours before use, otherwise the prints will be mealy and altogether unsatisfac- torily toned. There is little difference in the tone produced by both, but the acetate of soda bath may be used over and over again for many days by simply replenishing it with gold as it gets exhausted. The carbonate bath soon becomes unserviceable and deposits the gold which it has in solution. The above proportions of gold ought fully to tone one whole sheet of prints (22 x 18 inches). When 282 PRI more proofs are placed in the bath the quantity of gold must be increased proportionally. The prints should be taken from the washing water and at once immersed one by one into the toning bath. It is necessary to move them about without intermission, otherwise they will certainly be unevenly toned. Of course those first immersed will have assumed the requisite depth of colour before the others. For this reason it is advisable to have a large dish of plain water near at hand to throw the prints into as they get toned. It will not do to throw them into the fixing bath at once, unless an assistant is ready to thrust them at once under the fixing solution, for if a single drop of hyposulphite is allowed to enter the toning bath, both it and pro- bably many of the prints will be irretrievably injured. The fixing solution : — Hyposulphite of soda . . .6 ounces. Water 1 pint. The object of this bath is to dissolve out all the chloride and other salts of silver which have not been reduced by light. The picture could have no permanence unless they were removed. An immersion in a bath of the above strength, for from ten to twenty minutes, fixes the prints. The bath should not be usedoftener than two or three times, nor should many prints be immersed in it at the same time. After fixing, the prints have again to be washed in abundance of water, so thoroughly, that no traces of hyposulphite shall be left I adhering to them. It takes several hours and many changes of water to effect this completely. When washed, they may be spread out to dry on sheets of clean blotting paper, or suspended by clips. Printing "by Development. Use Hollingworth's thin photo- graphic paper ; (the thick sort is useless.) Immerse it in the fol- lowing bath : — Filtered rain water . . 1 ounce Salt 6 grains Lemon juice . . .1 drop. The time of immersion may lie between one minute and 24 hours without producing any marked difference in the result. Excite the paper by floating it upon a nitrate bath made thus : — Distilled water . . .1 ounce Nitrate of silver . . . 30 grains Lemon juice . . . 6 or 8 drops. Hang it up to dry, and use it as soon as possible. Expose it in the pressure frame until a faint trace of the picture is visible. PRI 283 Develop it thus : — Turn up the edges of the paper all round so as to make it into a tray. Lay it, with a sheet of blotting paper underneath, upon a horizontal sheet of glass, and pour upon the darkest part of the picture a little saturated solution of gallic acid, which spread with a bent glass rod. The development immediately commences, and is completed in a few minutes. Do not stop it at too early a stage, before the blacks have acquired the proper intensity. "When the picture has been well washed, to remove the gallic acid and excess of nitrate of silver, it may be toned in alkaline gold if deemed desirable ; but this is seldom necessary, because the blacks are generally very fine without this complication. Of course the pictures must be fixed in hyposulphite of soda, and thoroughly washed. Many modifications of the printing of positives by development will suggest themselves, by means of different haloid salts of silver, but the above is perhaps as good as any. Prints, Colouring. Before colouring an engraving or photo- graph upon bibulous paper it must be sized, by applying to it the following mixture : — Dissolve four ounces of glue and four ounces of white soap in three pints of hot water; add two ounces of powdered alum; stir well together, and it is ready for use. It is to be applied cold, either with a sponge or flat camel-hair brush. Prism. In solid geometry a prism is a solid described by the motion of a straight line which in passing round the boundary of a plane rectilineal figure always preserves its parallelism, the solid being terminated at the other extremity by a plain figure parallel to the first. In optics, however, the term prism is confined to the case of a prism with a triangular base, and its sides rectangles perpen- dicular to the base. In optical experiments with the prism, the edge of the prism is in general very sharp, the two adjacent planes which form it being in- clined at a very small angle, called the " refracting angle of the prism." When the prism is so placed with respect to a ray of light refracted through it as that the emergent and incident rays make equal angles with the sides of the prism, the deviation of the re- fracted ray is a minimum. Suppose a prism to be placed in its position of minimum devia- tion with respect to a ray refracted through it near its edge, and let D be the deviation of the refracted ray, jx the index of refraction of the material of which the prism is made, and a the refracting angle 284 PRO PRU of the prism ; then, if the angle made by the incident ray be small, D = 0 — I) a. A ray of light refracted through a prism is decomposed into rays of different refrangibility and colour, because the deviation of a ray depends upon the refractive index of the prism for that ray, and since white light is not homogeneous but composed of light of different degrees of refrangibility, the refractive index will vary with the dif- ferent rays of which white light is composed, being greatest for the violet and least for the red rays, therefore the deviation will be different for rays of different colours. If a second prism, precisely similar to the first, be placed against it in such a way as that its edge is next to the base of trie first prism, the two prisms will form a plate, and a ray refracted through them will neither suffer deviation nor decomposition ; that is to say, the effects produced upon it by refraction through the first prism will be exactly counteracted by refraction through the second, so that the second prism will re-compose into white light the rays which were dispersed by the first. The second prism therefore achromatises the first ; but the refracted ray does not suffer deviation, there is, therefore, no optical utility in such an arrangement. But if the second prism be made of a different material from the first, having different refractive and dispersive powers, and a suitable refracting angle be given to it, the first prism will be achromatised by the second, and the ray will suffer deviation. This important result depends on the fact that the dispersive power of a medium is not proportional to the deviation produced by it. This being the case, opticians are fortunately able to achromatise lenses and prisms. To be very exact, however, it must be remem- bered that in consequence of the irrationality of dispersion two prisms in contact can only unite two of the coloured rays or lines of the spectrum. Proof Spirit. Alcohol, sp. gr. -92 at 62° Fahrenheit. See "Alcohol." Prussian Blue. A peculiar compound of cyanogen and iron, the exact formula of which has not been determined, but may be repre- sented approximately by 6 K O -f 4 Fe. 4 Ofy. 3 . This substance is much used both as a dye and pigment. It is made by precipitating solutions of peroxide of iron with ferro-cyanide of potassium (yellow prussiate of potash.) It is insipid, inodorous, insoluble both in water and alcohol, and not poisonous. The alka- lies decompose it, and it does not therefore, as a dye, resist the action of soap. According to Chevreul, it becomes white by exposure to PUM PYR 285 sunshine, but recovers its colour in the dark. It has a strong attrac- tion for water. Pumice Stone. A grey porous stone found in the neighbour- hood of active and extinct volcanoes, and supposed to have been thrown up by them. It is used by painters for smoothing surfaces intended to be painted ; and, when pounded, by other artificers for polishing glass, metals, &c. To photographers it is useful in assist- ing to remove silver stains from the hands. Purple of Cassius. This is a fine purple pigment used in enamel painting, and staining glass of a red colour. It is composed of the mixed oxides of gold and tin, and is precipitated by immersing a piece of tinfoil in a solution of chloride of gold. Its composition is Au. O + 3 Sn. 0 2 + 4 HO. Putty Powder. Polisher's Putty. Peroxide of tin. Pyro-Gallic Acid. C 12 H 6 0 6 = 126. This substance is ex- tensively used by photographers as a developer in the negative collodion process. It may be made by exposing gallic acid to a temperature of about 420°, when it sublimes, and may be collected in the form of white shining scales ; but is apt to be contaminated with empyreumatic oils. A better plan is to treat finely -powdered galls with successive por- tions of cold water until exhausted, then to collect all the infusions and evaporate them to dryness. The spongy deliquescent mass thus produced must then be pounded and spread upon the bottom of an iron vessel 3 or 4 inches deep and 1 foot in diameter, the top of it being covered with a piece of blotting-paper pierced with pin holes, and surmounted by a paper cap 12 or 18 inches high. The pan is then cautiously and uniformly heated for some hours at a temperature of 400°, but not over 450°. The crystals of pyrogallic acid collect in the cap, and the other products are absorbed by the blotting-paper. Pyrogallic acid is not a real acid, and does not redden litmus paper. It is white, crystalline, inodorous, and bitter ; and very soluble in water, alcohol, and ether. The aqueous solution blackens by long exposure to air, and deposits a brown powder. It gives a deep indigo colour to a solution of jorofosulphate of iron, if pure and t ree from persulphate, to which it gives an orange colour. Pyrogallic acid is blackened by chlorine, but iodine has no effect upon it. It is a powerful deoxidiser, and reduces the oxides of the noble 286 PYK metals ; hence its use as a developer in Photography. It combines with oxide of lead, and forms a white powder. Pyro-Ligneous Acid. A crude vinegar obtained by the de- structive distillation of wood. When purified, it is used as a sub- stitute for vinegar in many processes of the arts, and also in making- pickles, sauces, &c. Pyro-Ligneous Spirit. Pyroxylic Spirit. See " Wood- Alcohol." Pyroxyline. General formula C 36 H( 30 ) - oc (N0 4 )# O 30 . This name is applied to a remarkable series of compounds obtained by acting on vegetable fibres with nitric and sulphuric acids. They are all of that class denominated in chemistry " substitution com- pounds," that is, they are vegetable fibre in which hyponitric acid or peroxide of nitrogen (N0 4 ) is substituted for an equal number of atoms of hydrogen which the fibre may contain. They are all ex- plosive in a greater or less degree. The late Mr. Hadow, of King's College, has analysed and described several varieties made in cold acids, which he thus tabulates : — A. C 36 H 21 (N0 4 ) 9 O 30 B. C 36 H 22 (N0 4 ) 8 03„ C. C 36 H 23 (N0 4 ) 7 O,,, D. _ C 36 H 24 (N0 4 ) B O,,, A is highly explosive and insoluble in ether and alcohol ; the other varieties as they descend in the scale are less explosive but more soluble to C in ether and alcohol. But pyroxyline for photographic purposes must not be made in cold acids, because such pyroxyline forms a collodion viscid and clotty, refusing to flow over the plate evenly, and being otherwise deficient in good photographic properties. Innumerable formulas have been published for the preparation of pyroxyline suited for Photography, some of which are good and others utterly unreliable for any purpose. The three following for- mulas are condensed from articles in the " British Journal of Photo- graphy," written by one of the editors of this dictionary, who has had extensive experience in the manufacture of this compound. 1st Formula. Nitrate of Potash and Sulphuric Acid. Pulverised and dried nitrate of potash . 8 ounces by weight. Sulphuric acid, sp. gr. 1840 . . 14 „ by measure. Water ....... 1 „ „ Best dried cotton wool . . .165 grains. PYR 287 The nitrate of potash should first be reduced to a fine powder in a mortar, thoroughly dried in an oven or near the fire, and then weighed. The sulphuric acid of commerce (unless purposely diluted) varies in strength from 1836 to 1845. We have taken 1840 as a stand- dard on which to found the proportions of the above formula. Should the specific gravity be as low as 1836, the only change in the above proportions which will be necessary is the omission of the water altogether, and should it be as high as 1845 the water must then be increased to two ounces. The cotton may be of that kind known at the chemist's as medi- cated cotton wool of long fibre : the short fibred varieties are useless. It must be well dried by artificial heat before use, on account of its strong hygroscopic properties. The vessel in which the pyroxyline is to be made should be one composed of some thick non-conducting material. A Wedgwood mortar answers well, but if that be not conveniently at hand, one of the common red earthenware glazed pipkins will be found an efficient substitute. Provide also beforehand two strong glass rods, or, better still, two sharp-pointed pieces of dry wood, for the purpose of pressing down the cotton in the solution, and keeping it from matting and being unequally acted on after immersion. When all these materials have been got together and placed in a convenient place for commencing operations, the pipkin or mortar is to be warmed. This is best effected by pouring into it hot water, and allowing it to remain there till the outside feels warm to the touch. Empty out the water ; quickly wipe the inside dry with a cloth, and place the vessel on the " hob," or under a flue, which will carry off the pungent and hurtful fumes formed during the operation. Without loss of time put in first the powdered nitrate of potash, then the water, and stir for three or four seconds with one of the glass rods, or pointed sticks. Add the sulphuric acid, and stir for three minutes, as before, until the mixture is complete. When the three minutes have expired, an assistant should be at hand with the cotton, which he is to throw into the vessel in small, well-pulled-out tufts, one at a time, while the principal operator pushes them under with the rods, and keeps moving them about. The time occupied in immersing the cotton ought not to extend over two minutes. When all is immersed it should be teased and pulled about under the acid for five minutes, at the end of which time the process may be considered complete. A longer soaking is often recommended, but there is no necessity for it, and by so doing a 288 PYK risk is run of allowing the temperature to fall too low, and thereby injuring the quality of the pyroxyline. The moment " time" has been called by the assistant, dash the contents of the pipkin into a large tub of water, and with the rod or with the hands — if they are not too delicate for such work — instantly pull out the cotton, and move it backwards and forwards under the water. This is important, for were it not at once intimately mixed with a large body of water, part of the cotton would be dissolved by the sudden rise of temperature consequent on a partial dilution of the acids in immediate contact with the fibre. The washing is best effected by soaking the pyroxyline in many changes of water, and squeezing it as dry as possible in the hand between each change. But greater care than ordinary is needed in the washing of cotton prepared from nitrate of potash and sulphuric acid ; for a secondary compound, generally supposed to be bisul- phate of potash, is formed in the course of the operation, adhering, with almost obstinate tenacity, to the fibre of the pyroxyline, and it is with difficulty removed. As bisulphate of potash is very soluble in water, we are inclined to attribute to this adhering salt a more complex composition than that usually given to it. At all events, it is effectually got rid of by steeping, for half an hour, the well- washed pyroxyline in a little distilled water, to which a few drops of ammonia have been added. The ammonia is easily removed by three or four subsequent washings, and its application seems to have no injurious action whatever on the cotton. Pick out and dry the matted fibres, on blotting-paper, in a warm room, but not near the fire, nor by the aid of steam. When dry, the weight will be found to have increased to from 210 to 220 grains — sufficient for two pints of moderately thick collodion. It is extremely soluble, leaving scarcely a " wrack behind," and in our opinion is superior to anything we have been able to manufacture from any proportions of the mixed acids. Why it is so we cannot explain : we only mention the fact. We have not in the above remarks said anything about tempe- rature — that most important modifying agent of the photographic properties of pyroxyline. The formula has been purposely con- structed so as to do away with that troublesome complication, and to avoid the necessity for precautions with respect to it. The quan- tities also are given with the same view. But, if any one is curious on the point, by carefully attending to every detail of the above pro- cess, he will find that a thermometer immersed in the mixture along with the first tuft of cotton will indicate, invariably, a temperature ranging from 146° to 150° Fahrenheit; and, further, it will not be PYR 289 found to fall more than one degree during the five minutes' soaking of the cotton. The above temperature is considered the best for a good normal pyroxyline, suited for both the wet and the dry pro- cess. But if the cotton has not been well dried, the temperature will be found to rise after its immersion, and, in consequence, the resulting pyroxyline will be somewhat disintegrated in texture, and deficient in weight after drying. 2nd Formula. With Strong Acids. Sulphuric acid, sp. gr. 184-5 . .12 fluid ounces. Nitric acid, sp. gr. 1450 * . . 4 „ "Water . . . . . .17 drachms. Carded and dried cotton wool. . . 270 grains. 3rd Formula. With Weaker Acids. Sulphuric acid, sp. gr. 18 i0 . . 12fluid ounces. Nitric acid, sp. gr. 1400 . . . 6 ,, Water ...... 6 drachms. Carded and dried cotton wool . .270 grains. If the acids be somewhat weaker than the above omit the water altogether, and slightly increase the quantity of nitric acid. We have never found that extreme purity of the acids is a point of much importance. Provided they be of the right specific gravity, the ordinary commercial kinds answer every purpose exceedingly well, and there is really no occasion to add to the expense of pyroxyline by insisting on the absolute absence of the usual im- purities which accompany them. The nitric acid of commerce, for instance, has generally a yellowish appearance from the presence of chlorine, which has some action on the cotton fibre, but not of a prejudicial kind. Neither does a slight evolution of red fumes in the stock bottle seem to have an appreciable eftect on the pyroxy- line. It only weakens the acid ; and therefore it is a useful pre- caution, as before stated, to keep the bottle in a cool, dry, and dark cupboard for its better preservation. The manipulations are nearly the same as those connected with the nitrate of potash process described above ; but in this case more precautions are necessary to guard against failure. In the first place, the specific gravity of the acids must be accurately determined, in order to regulate the quantity of water to be added. The tem- perature, too, generated by an admixture of acids of varying con- centration with the water, differs very mnch. Hence it will be u 290 PIE, necessary to rely only on the indications of the thermometer as to the proper time for the immersion of the coiton. If the acids are strong the temperature will rise spontaneously higher than the re- quired degree ; if weak, artificial heat will be needed. A good plan of procedure in every case, when preparing small quantities of pyimy line, whether the acids be strong or not, is to fill the glazed earthenware pipkin or thick porcelain vessel with boiling water, and allow it to remainfor a short time immediately before mixing the acids, in the same way as in the nitrate potash process described above. The thick non- conducting sides of the vessel will retain the heat for a considerable time, and enable the operator to work with the utmost deliberation. When everything is ready, pour in the nitric acid and water, then the sulphuric acid. Intimately mix them with the thermometer bulb, and note the temperature. When it falls to 150° Fah., im- merse the cotton, tuft by tuft, according to previous instructions, and in order that the pyroxyline may be of uniform solubility in ether and alcohol, move it about and pull it out under the acids for six or seven minutes after the immersion of the last tuft. Wash as before, excepting that in this case the treatment with ammonia is unnecessary, and may be omitted. Pyroxyline prepared by the above formulae should, when perfectly dry, show an increase of from twenty-five to thirty per cent, over the original 270 grains. If the action of the acids be continued for ten minutes or more, twenty per cent, of increased weight will be nearer the mark ; but we have never found any advantage arising from a more prolonged immersion than that recommended, especially if the completion of the process be facilitated by moving about the cotton when under the acids. It is of importance to notice, that the operation of soaking the col.tou should always be conducted in a dry atmosphere, for we have found the percentage of increase in weight to vary considerably, according to the dryness or damp- ness of the air in the operating-room. From a moist atmosphere the exposed acid mixture greedily absorbs water, and becomes weakened to such a degree as often materially to lessen the weight of the pyroxyline, and also alter its photographic properties. For the same reason the plan often recommended of placing the vessel con- taining the acids in a pan of warm water is open to objection. If the pyroxyline be not required for immediate use, a few obser- vations respecting the best method of storing it will not be out of place, because, under many circumstances, it is peculiarly liable to spontaneous decomposition ; more so, indeed, than the less soluble and more explosive compounds, in which the peroxide of nitrogen seems to be more strongly combined. It will keep perfectly for at QUI 291 least two years, if the following precautions be adopted : — 1st. It must be thoroughly washed and dried. 2nd. It should be stored in a well-ventilated vessel — for instance, it is dangerous to keep it closely pressed together in a stoppered bottle ; a thin cotton bag, in which it is loosely stowed away, we have found to answer the best. 3rd. Light, heat, and moisture, facilitate decomposition; stow it away, therefore, in a dark, dry, and cool place. The first symptoms of deterioration are generally manifested by the evolution of red fumes, which consist of the oxides of nitrogen ; but not always so, for we have seen instances of entire decomposition unaccompanied by this appearance. In one case a thoroughly washed and imperfectly-dried sample of very soluble cotton was ex- posed, in a hermetically sealed tube, to the light. After a few months it had changed its character completely, and was no more suited for photographic purposes. The change was marked by a strong acid reaction, and a partial disintegration of the fibre. In another case a tuft of similar pyroxyline, treated in a test tube w ith boiling water, exploded with considerable violence, while oilier samples, equally soluble in ether and alcohol, were gradually decom- posed with evolution of red fumes. The instance where explosion occurred was certainly an exceptional one ; but it shows how un- certain our knowledge of the composition of this compound still is, after all the research which has been brought to bear on the subject ; and it further shows the necessity of great caution in its preparation and stowage for future use. Quickness of Lenses. The comparative "quickness of lenses," (as it is termed,) depends partly on the colour of the glass, the number of glasses in the combination, the number of reflecting sur- faces, &c. ; but mainly on the aperture of the lens, and its local length. It is evident that, other things being equal, the intensity of light in the image depends first on the quantity of light admitted, and secondly on the area over which it is distributed. It varies, there- fore, directly as the aperture, and inversely as the size of the picture. But the size of the picture given by a lens varies directly as the square of its equivalent focal length; and the area of the aperture, or diaphragm, varies as the square of its diameter. Therefore the time of exposure varies directly as the square of the equivalent focal length, and inversely as the square of the diameter of the aper- ture or stop. In the same lens the time of exposure varies inversely as the square of the diaphragm used. For instance, with a diaphragm of u 2 292 RAI REA half an inch diameter the time of exposure must be four times as great as with a diaphragm of one inch. Raisin Process. This is one of the multitudinous modifications of the dry processes which are ever cropping up. It was originally described by Dr. Schnauss. It, however, possesses considerable advantage over the other saccharine preservatives which have been from time to time suggested, inasmuch as raisins, besides the sugar, contain a substance analogous to tannin. The method of preparing the preservative solution is this. Boil one ounce of raisins for about five minutes in ten ounces of distilled water. Set aside to cool ; then filter, and apply to the thoroughly washed collodion film as in the tannin process. The development, as usual, is either by the alkaline or acid pyrogallic method. Ramsden's Eye-Piece. This telescopic eye-piece is used as a focussing magnifier to magnify the image formed on the focussing screen of the camera. It is composed of two plano-convex lenses, equal in all respects, and mounted in a tube with their plain sides outwards, at a distance apart equal to two-thirds of the focal length of either. When using this magnifier, the image on the ground - glass should be nearly in its principal focus. It is used in tele- scopes when spider-lines are placed in the focus of the object glass. It is sometimes called the Positive Eye-piece, and is not achromatic. Rapid Dry Processes. These are dry processes in which no longer exposure is required for a dry than for a wet plate. The tannin process, either with a bromised, or a bromo-iodised collodion, and alkaline development is a rapid dry one, and so is the new Fothergill process. When the usual acid developer is used, and a preservative of gum arabic, very short exposures may be given, when the collo- dion is bromo-iodised ; and Dr. Hill Norris's rapid dry plates appear to be prepared on this principle. But in all cases, the presence of bromide of silver appears to be imperatively necessary in a rapid dry plate, however developed. Iodide of silver is extremely insensitive when the free nitrate has been removed from the film. For further information, consult a pamphlet by Mr. Sutton, entitled " Instan- taneous Dry Collodion Processes," and published by Messrs. Sampson Low, Son, and Marston. Realgar. As S 2 . Red sulphide of arsenic. This substance is used in making "White Indian Fire" ( be the angle of incidence <£' „ refraction the law of refraction is expressed by the equation. sine =/x sine

' be such that 0=90°, sine

. We shall have occasion to refer to this statement again presently. Having now explained the theory of copying a transparent nega- tive with the sky behind, we pass on to a description of Woodward's solar camera. SOL This instrument is shown in the next figure. Instead of having t he sky behind the negative, there is a large condenser 1) E, and 6 U sunshine is reflected through it by means of a plane mirror J) G. The copying lens is placed at F, the focus of the condenser. Now see what happens. A round image of the sun, about -J -in. diameter, is formed at F, and exactly covers the central part of the copying lens, the outside of which is useless and may be covered with a metal diaphragm to cut off stray rays of light. We shall therefore consider the lens as reduced to |-in. diameter ; and rays of sunshine will then pass from all parts of the condenser to all parts of the lens. So that if P be a bright point as before, a full pencil of light will pass from P to cover the entire |-in. disc, or lens, at F ; and this pencil will have its conjugate focus at p. The same thing will be true of all other bright points of the negative, and thus an enlarged and very brilliant image of it will be formed upon the focusing screen ah. Observe particularly the following features of this arrangement : — In the first place the condenser is not achromatic ; and in the next place its focus is brought exactly upon the copying lens. Unless the latter condition be attended to, the enlarged image will be more or less imperfect in brilliancy and definition ; and with respect to the condenser not being achromatic, that is of no consequence what- ever, because we have before shown that the pencil F 1' (i mav consist of rays of all colours, and that these will be brought to a locus at p if the copying lens is achromatic. If the condenser were achro- inatised it would be thicker, and light would be lost, while no good whatever would be done as a set off to these disadvantages. The best form of the condenser would be a " crossed" lens, with the most convex side towards the mirror; but it is generally made plano- convex, which is simpler, and answers nearly as well. The form of the condenser should be such as to reduce spherical aberration to a Y 2 324 SOL SPE minimum, and make the image of the sun as sharp as possible, in order that the bases of the pencils PPG may be as small as pos- sible, and the enlarged image as sharp as can be got. When a portrait lens is used for copying, it must be placed with its back lens next to the negative, and the focal spark of the con- denser should be brought upon the centre of the front lens. The distortion produced in this way is scarcely perceptible, but its effect is to bend the marginal lines outwards at their extremities. An achromatic plano-convex lens may be used for copying, and this will give no distortion. It should be only |-in. aperture, and placed with its flat side towards the negative. Solarisation. An effect produced by over-exposing to light a sensitive film of iodide of silver. In the daguerreotype process it is seen in the blueness of the over-exposed parts of the picture. A similar effect is exhibited in the over-exposed parts of a collodion positive ; and in over-exposed negatives, or collodio-iodised films, de- veloped by pyrogallic acid, it appears as a pale redness instead of a black opacity. The cause of this peculiar behaviour of iodide of silver is at present unknown. Specific Gravity. The specific gravity of any substance is the weight of a unit of volume of that substance, at a temperature of 60° Fahrenheit. The unit of volume, in the common table of specific gravities, is the volume of that quantity of pure distilled water which at 60° Fahrenheit weighs 1000 grains; and in the same table the unit of specific gravity is 1000 grains. The specific gravity of water is therefore 1. If a vessel containing the unit of volume be filled with absolute alcohol, its increase in weight will be 794 grains, therefore the sp. gr. of absolute alcohol is '794. If the same vessel be filled with pure concentrated sulphuric acid, its increase in weight will be 1845 grains, therefore the sp. gr. of sulphuric acid is T845 ; and so on. Specific Gravity Bottle. This is a small glass bottle shaped like a decanter, and furnished with a stopper which is drilled with a hole, also with a counterpoise, or brass box filled with shot, which exactly balances it when empty. The specific gravity bottle holds exactly 1000 grains of pure distilled water at 60° Fahrenheit. Its capacity is therefore the unit of volume of the specific gravity table. SPE 325 To use it, fill it quite full with the fluid to be tested, and put in the stopper. The superfluous fluid overflows through the hole in the stopper. Wipe the bottle quite dry, and weigh it, together with its contents, in a delicate balance, the counterpoise of the empty bottle being placed in the opposite scale. The number of grains required to be added to the scale which contains the counterpoise, and which consequently represent the weight of the fluid, divided by 1000, is its specific gravity. Specific Heat. By the specific heat of a body is meant the time it takes to cool from a certain given temperature to another given temperature, when placed in vacuo in a polished silver vessel. By some writers the specific heat of a body is supposed to be its " capacity for heat," as if heat, which is the undulation of an ethereal medium, could be stowed away among the particles of a body. Surely the notion of " capacity for heat " is absurd. That different bodies should require different times to pass from one temperature to another involves no difficulty of comprehension, and specific heat is simply the measure of the time required. Dr. Graham says, " Of all liquid or solid bodies water has much the greatest capacity for heat ; hence the sea, which covers so large a proportion of the globe, is a great magazine of heat, and has a beneficial influence in equalising atmospheric temperature. Mercury has a small specific heat, so that it is quickly heated or cooled ; another property which recommends it as a liquid for the ther- mometer, imparting as it does great sensibility to the instru- ment." The reader may amuse himself by trying to state in a different form, and on the assumption of the undulatory theory of heat, the facts implied in the above sentence. The time which a body takes either to become colder or hotter depends in great measure on the condition of its surface. If the surface be smooth, polished, and white, the time is increased ; if rough and black, the time is diminished. When an ethereal undu- lation strikes the surface of a body, it depends upon the condition of the surface whether the undulation be continued among the particles of ether within the body, or whether it be reflected among those which are external to the body ; and conversely, when a body is contracting, and therefore radiating heat, it depends upon the con- dition of its surface, whether the undulations of the ether within the body be communicated to the ether without it, or returned by internal reflexion among the ether within it. A smooth polished surface is favourable to the reflexion of undulations, and unfavourable 326 SPE to their direct propagation. Colour is no doubt owing to a pecu- liarity of surface. See ''Latent Heat." Spectrum. Suppose a small hole of any shape made in the window shutter of a darkened room and that sunshine be admitted through it, and the light received upon a white screen, placed per- pendicularly to the line joining it and the hole ; then a round image of the sun will be formed upon the screen, the diameter of the image depending upon the distance of the screen from the hole. Observe that whatever may be the shape of the hole, the image of the sun will be round. A triangular hole would not give a triangular image, nor a square hole a square image ; the sun being round, its image is round ; for the image is not produced by a ray of sunshine which enters through the hole and falls upon the screen, producing a spot of light the same shape and size as the hole, but by pencils of light which diverge from every part of the sun, and after crossing each other in passing through the hole, proceed till they reach the screen, where they form a round image of the sun, the size of which increases as the distance of the screen from the hole increases. Thus, the sun being about half a degree in angular diameter, if the screen be placed 10 feet from the hole, the sun's image will be about 1 inch in diameter ; if 20 feet from the hole, 2 inches in diameter, and so on- This being understood, let a glass prism be placed with its edge immediately behind the hole. Then, since white light is not homo- geneous, the pencils will all be decomposed by refraction through the prism into pencils of the different coloured lights of which white light is composed ; so that the screen, instead of receiving a single round image of the sun in white light, will receive upon a different pari of it as many different coloured round images of the sun as there are different kinds of light in white light separable by refraction. These images will partly overlap one upon the other, and produce a long image of the sun, having belts across it of different colours, arranged in the following order (if the refracting angle of the prism be suitably taken), viz., red, orange, yellow, green, blue, indigo, violet ; which colours are called the " prismatic colours," and the entire coloured image the " prismatic spectrum." Should, however, the refracting angle of the prism be too small, there will be a space of white light in the centre of the spectrum, produced by the coincidence of a portion of each of the coloured images. The reader will perhaps find this account of the way in which the prismatic spectrum is generally produced somewhat different SPE 327 from the accounts given of it in popular treatises on Optics ; these popular explanations generally proceeding on the assumption that the light which is admitted through the hole is a single beam of light, — which is manifestly incorrect. Every photographer knows that an image is formed of external objects by light admitted through a small hole in the front of a dark box, and received upon a focusing screen; and that this is true, however small the hole may be and whatever its shape. We have, therefore, to deal with an image of the sun, and not with a single ray of white light. It appears, then, that when the spectrum is formed by admitting the light through a hole, however small, the bands of different colours contain an admixture of lights of different refrangibilities. In order to obviate this evil, M. Fraunhofer admitted the light through a long and extremely fine slit, instead of a hole, and placed the prism with its edge parallel to the slit, and at a considerable distance from it. But even this arrangement is not sufficiently exact, for the additional precaution must be taken of covering the prism with an opaque diaphragm, having an extremely fine slit parallel to the edge of the prism, and therefore parallel to the other slit, so that the light from the first slit may also pass through the second slit. In this way the spectrum is rendered nearly pure, and the different parts of it free from the admixture of other colours. And here it is important to observe that in the experiments of Sir David Brewster, in which he detected white light in every part of the prism in a state of admixture with his supposed simple colours, red, blue, and yellow, allowance had not been made by bun for the impurity of his spectrum, so that his conclusions that Newton's theory was wrong, and that the seven colours of the spectrum may be reduced to three, was founded on exueriments conducted in ignorance apparently of an elementary principle in geometrical optics. When a pure spectrum is examined by a telescope, it is discovered to be intersected by a great number of dark lines, ^\s shown in the following figure, and which are called " Fraunholer's Lines," he having first discovered them. These lines are produced by the absence of light of particular degrees of refrangibility. In the spectra produced by light from different sources the lines do not occur in the same order ; neither do they occur at the same relative distances when light from the same source is refracted through prisms made of different materials. In the light from the sun and planets the spectral lines occur in the same order ; but in the spectra formed by light from the fixed 328 SPE Eed. Orange. Yellow. Green. Blue. stars, or by the electric light, or by light produced by the com- bustion of different substances, the spectral lines do not occur in the same order, nor are they equally numerous. With respect to the calorific, luminous, and actinic properties of different parts of the solar spectrum ; — It has been shown in the article on light, that light, heat, and actinism are most probably undulations in the same ethereal medium, differing only in the length of the wave ; and it is found by experiment, that both heat, light, and actinism, are capable of producing chemical changes in bodies ; so that the term " ac- tinism " merely means that certain sub- stances are chemically affected by certain rays (called the actinic), residing at a cer- tain part of the spectrum. It is therefore somewhat unscientific to call certain rays "actinic" because they produce chemical changes in certain bodies, and then to say generally that the chemical rays chiefly lie at the violet part of the spectrum, when we know that every part of the spectrum is capable of producing important chemical changes in some sub- stance or other. To say that the heat rays reside mostly at the red end of the spec- trum, luminous rays mostly in the yellow part, and chemical rays mostly in the violet part, is so far unscientific that it is not the statement of a universal law ; for the effects exhibited by the different parts of the spectrum upon a substance placed in it, depend upon the nature of that sub- stance, and are different with different substances. This being the case, we have not included in the foregoing figure of the spectrum the three wave lines of light, heat, and actinism, with which it is gene- rally adorned in popular works on Pho- tography. In the following table the lengths of the waves corresponding to the principal lines of the solar spectrum are expressed in millimetres: — Indigo. Violet. Extreme red . A B . C D E F . G H , 1 Extreme violet SPE •00075 millimetres. •00074 •0006879 •0006559 •0005888 •0005265 •0004856 •0004296 •0003963 •00037 •00036 Spectrum Analysis. One of the most important discoveries of modern science, and due to the joint labours of Bunsen and Kirchoff. The solar spectrum is, as is well known, composed of bands of the different colours, red, orange, yellow, green, blue, indigo, violet, intersected by a vast number of dark lines, which were first dis- covered by Fraunhofer, and which are due to the absence of light of particular degrees of refrangibility. The solar spectrum invariably exhibits these dark lines in the same order and in the same places. The spectra from the fixed stars exhibit different systems of dark lines. The spectrum from the Drummond light exhibits no dark lines. And lastly, the spectra from different coloured flames do not exhibit dark lines but bright lines, depending in colour and posi- tion upon the nature of the substance which gives colour to the flame. For instance, if a salt of sodium is added to the flame the spectrum consists only of a strong double yellow line, at the part 1) oi* the solar spectrum ; and the salts of the other alkaline metals added to the flame produce bright lines of different colours in different parts of the spectrum ; so that you can ascertain with absolute certainty whether any chemical compound contains any of the above metals by simply examining the spectrum produced by adding some of that compound to the flame which produces the spectrum. Moreover, other metals besides those which form the bases of the alkalies and earths possess similar properties of giving peculiar bright lines in different parts of the spectrum ; and the application of these facts in chemical analysis is a new and important branch of that art, and is called Spectrum Analysis. Now follows an important part of this subject. It happens that the bright lines given in the spectra from various coloured flames, and which depend in colour and position upon the nature of the 330 SPE substance added to the flame, are one and all identical in position with the various dark lines of the solar spectrum. For instance, the yellow bright line, due to a salt of sodium in the flame is identical in position with the dark line D of the solar spectrum ; and so on, in other cases. There is consequently a connection between the dark lines in the solar spectrum, and the bright lines in the spectra from various coloured flames. The nature of that connection has at length been ascertained, and the conclusion is one of immense scientitic importance. It is this, — that the sun's atmosphere con- tains the various substances volatilised, and in a state of incan- descence, which are present in the terrestrial flames, and give the bright lines which correspond to the dark ones in the solar spec- trum. This conclusion depends, however, upon further facts which remain to be stated. We have said that the Drummond light gives a spectrum which is entirely free from lines. But if a coloured flame containing sodium, or potassium, or any of the other alkaline metals, be interposed be- tween the Drummond light and the prism, the spectrum, instead of showing the bright lines due to the coloured flame, now shows dark lines in the same position. This arrangement causes the lines to become dark instead of bright, — or negative instead of positive. The legitimate inference therefore is that the sun consists of a cen- tral incandescent solid, the light from which is homogeneous, and would not of itself yield dark lines in the spectrum ; but by passing through an incandescent gaseous atmosphere, containing sodium, potassium, iron, nickel, &c, &c, the various dark lines are pro- duced. Speculum Metal. This is generally a compound of about 6 parts copper, 2 parts tin, 1 part arsenic. It is used for metal reflectors. The great reflector of Lord Eosse's telescope is made of 126*4 parts copper and 58*9 parts tin, without the addition of any arsenic. The word "Brass" was omitted in the letter B. We may observe in this place, that brass is a compound of copper and zinc, with the addition sometimes of a little lead, tin, and iron. The latter metal should not be introduced in the brass used for philo- sophical apparatus. The proportions of the metals in brass for turning are as follows : — Copper . . .61*6 parts. Zinc .... 35-3 „ Lead . . . .29,, Tin . ... 0-2 „ SPE STA 331 Spermaceti. This substance is sometimes used instead of wax in Photography. It is a soft white crystalline substance, precipitated from the oil of the spermaceti whale on cooling after the death of the animal. It is purified by pressure and boiling in a weak solu- tion of caustic alkali, after which it is washed, melted in boiling water and cast into blocks or cakes. It is soluble in about 50 parts of alcohol sp. gr. *820. Pure spermaceti is certainly better than im- pure wax in Photography ; and, from its being softer and more easily melted, it penetrates the pores of paper more readily. Stains. Black stains on the hands, occasioned by nitrate of silver, may be removed by a strong solution of cyanide of potassium and plenty of rubbing ; but as this substance is extremely poisonous and apt to act injuriously on the system by absorption through the skin, and more particularly through abrasions thereof, we do not recommend its use. A solution of iodine in iodide of potassium brushed over the stains will convert them into iodide of silver, which is soluble in hyposulphite of soda. Moist hypochlorite of lime rubbed over the stains with pumice stone will also effectu- ally remove them. The most energetic agent of all is a solution of 150 grains of cyanide of potassium, and 12 grains of iodine in one ounce of water. Stains on linen or cotton may be removed by the second method mentioned above. Standard Gold. A sovereign weighs 5 dwts. 3 27 grain-, and is composed of 1 L parts gold to 1 part copper. The standard gold of France contains 9 parts gold to 1 part copper. Standard Silver. Standard silver consists of 11*10 parts of silver and *90 parts of copper. A shilling weighs 3 dwts. 15 27 grains. Standard Spirit. A mixture of alcohol and water, having the sp. gr. "92 at 62° Fahrenheit. It contains very nearly equal parts of absolute alcohol and water. Starch. C 12 H 10 0 10 = 12 atoms of carbon and 10 of water. This substance occurs abundantly in vegetables, and is gene rally obtained by steeping the powdered grain or seed, or the raspings of the root, bulb, or stem in cold water, which becomes white and turbid, and, after being strained, deposits the starch in the form of a white granular substance which is then dried at a gentle heat. Common starch is manufactured from flour. Arrowroot, tapioca, 332 STA STE and sago are different forms of starch. Starch is frequently made from potatoes. When starch is heated to a certain temperature, it becomes con- verted into a gum called " Dextrine " (q. v.). It forms a blue com- pound with iodine, called iodide of starch, an aqueous solution of which is bleached by light ; it combines also with sulphuric acid, forming sulphate of starch, and with lime and baryta; also with tannin. Starch becomes converted into sugar by the action of an azotised principle, called " Diastase," and also by the action of dilute acids. Starch is insoluble in cold water, alcohol, and ether. When boiling water is poured upon it, clots are formed which cannot afterwards be diffused through water. Solution of starch is best made by pounding the starch, and mixing it thoroughly with cold water ; then adding hot water, or boiling it, stirring it uniformly until a gelatinous mixture is obtained. A solution of starch is sup- posed to consist of the granules considerably distended and diffused through the water. When, however, the indurated envelope of the starch granule bursts, the contents are distributed through the water, and form a transparent gelatinous liquid which, on cooling, throws down an opalescent deposit. The substance held in solution in the clear liquid has been called " Amidine." Starch, Iodide of. Iodine combines with starch, and forms a purple compound. Starch is generally used as the test for free iodine. When paper containing starch is immersed in a solution of iodide of potassium it speedily assumes a purple tint, the strength of which depends upon the quantity of starch present in the paper. Also, wherever nuclei of starch exist deep purple spots are produced. Papers which have been bleached with chlorine are also reddened by a solution of iodide of potassium. Stearine. (Gr. arEap, suet.) The principal constituent of solid fats. It is composed of stearic acid in combination with glycerine. Stereomonoscope. This is an instrument invented by M. Clau- det for exhibiting upon a screen of ground glass a single picture having the true effect of solidity. This result, which may at first sight appear paradoxical, is accomplished thus : — A pair of stereoscopic pictures is first taken in the usual way. Magnified images of them are then thrown, by means of a pair of lenses (one for each picture) upon the same part of a large upright- screen made of coarsely ground glass, the axes of the lenses con- verging at a suitable angle. The spectator then stands at a distance STE 333 of a few feet on the opposite side of the screen, and looks with both eyes at the image formed upon it. The image formed by the right hand lens is seen by the left eye, and that formed by tiie left hand lens by the right eye, in consequence of the roughened state of the glass, which is filled with minute transparent spots. A true stereo- scopic effect is said to be produced in this way by the combination of the images. Stereoscope. (Gr. orepeog, solid ; oKOTTEit)> I see.) This is an in- strument for exhibiting two perspective views of an object, taken from different stations, as one having the appearance of solidity. It is the invention of Professor Wheatstone, and was first made public and the theory of it explained by him in the year 1839. At that time the pictures were exhibited by means of reflectors, but the in- ventor suggested that lenses might be used instead. Some years after this, an instrument was brought out by Sir David Brewster, in which small photographic pictures are placed in a dark box, and viewed through half-lenses mounted in tubes ; and, soon after the introduction of that very imperfect instrument, Messrs. Knight, of Foster Lane, patented an improved form of stereoscope, in which large semi-lenses are used, mounted in the front of the instrument, without tubes ; and this, in the course of time, became so generally preferred to the other, as to supersede it to a great extent. But it was soon found that the semi-\enscs, whether large or small, pro- duced an unbearable amount of distortion in the solid picture, making straight lines look as if they were concave to the spectator ; so, in order to obviate this evil, whole lenses were tried, and these were found to answer in certain cases. But as the theory of the lenticular stereoscope is even now but little understood by opticians or the public at large, it was not perceived that, in order to render that form of instrument perfect, it was necessary not only to use whole lenses to get rid of the distortion, but to take the pictures in a camera suitably constructed, and mount them properly. We shall endeavour in the present article to explain the theory of the stereoscope in a clear and intelligible manner. But the reader must first consult and study the article on " Binocular Vision," for it is here assumed that the principles of binocular vision are clearly understood. Theory of the Stereoscope. The theory of the stereoscope consists in showing, first, how the instrument may be employed to represent things truthfully as we see them in nature ; secondly, how it may be employed to represent them falsely as we should see them if our eyes were wider apart; 334 STE and, thirdly, to explain how it happens that distortion is introduced by using an instrument of improper construction — straight lines being represented by curves, and so on. Let L, E be the eyes of the spectator ; A, C, B lamp posts of different heights having lights, or luminous points, A, B, C, at the top. Draw the visual rays L A, L C, L B, E A, E C, E B. Cut them by a vertical plane P Q, parallel to the line which joins L and E. Then the points a,c,b, a,c,b, where the visual rays pass through this plane will be the images of A, B, C, as seen from the stations L and E, P Q, being supposed to be the plane of a perspective picture. {See " Perspective.") If the plane P Q be placed as shown in the figure, so that the perspective view as seen from L may be completely exterior to that seen from E, the pictures will be as represented on a card beneath L and E. {See the Lower Part of the Figure.) Now if the perpendicular distance between L and the plane P Q be such that the points a,b,c, can be distinctly seen by an eve at L, that is to say, if this perpendicular distance be not too short for distinct vision, and if we place the card P Q before the eyes, as in the figure, the right picture being cut off from the left eye and the, left picture from the right eye by a partition, as shown in the figure, then instead of seeing two pictures a be, a be, only one image will be seen, and that one image will appear to be the lights A,B,C in their natural position, and at their true distance. For when the left eye is directed to a along the line La, the right eye is directed to a along the line E«, and the optic axes La, E#, being produced meet at A, so that the spectator instead of seeing two images a,a, upon a plane P Q sees one image A at the true distance, and in the true position of the light A. Similarly with respect to the other images b,c,b,c, which combine and produce single images at B and C. It is evident that the perpendicular distance of P Q from L orE (which we will call F), will depend upon the size of the angle ALB, or AEB ; for the smaller that angle is the further the plane P Q may be placed from L and E, and therefore the more easy it will become for the images a,b,c, to be seen distinctly by persons of ordinary vision. Most persons can see a thing distinctly at a dis- tance of eight inches. If then F = 8, the angle ALB may be about 16°. Now we come to the principles of the stereoscope. Suppose we place at each of the stations L,E, a photographic camera, the lens of which is eight inches focus, and adjusting these cameras with their axes strictly parallel, take the pictures of A,B,C, 336 STE (including an angle of 16°). Then when these pictures are properly mounted they will be identical with the perspective views of A,B,C, upon the plane P,Q; and if placed in a box having a partition in the middle, and holes to look through at P and Q, at a distance of eight inches from the pictures, the spectator on looking through the holes, will perceive a single image of the points A.B,C, at their true distance, and their true relative positions. A box so constructed may be called a " Simple Stereoscope," -because it does not involve the use either of lenses or reflectors, and the pictures properly taken and viewed in it have the inexpressible charm of truthfulness. Of what use, then, it may be asked, are lenses and reflectors ? To this question we will now endeavour to reply. In the first place the simple stereoscope, when adapted to persons of ordinary sight, does not include an angular field of more than 16°; which is in general too small. A photographic picture should include at least an angular field of from 25° to 30°. This can only be effected by bringing the pictures as near to the eyes as five inches, at which distance most persons find a difficulty in seeing them dis- tinctly; but this difficulty is overcome by placing a whole lens of five inches focus in each of the holes L,R, and viewing the pictures (which must be taken with a lens of five inches focus), through them. This instrument is called the " Lenticular Stereoscope." In the following description of it we shall recapitulate what has been said, and discuss the matter more fully. The Lenticular Stereoscope. This is an instrument for exhibiting a pair of stereoscopic pictures, which include an angular field of about 30°, and have been taken in a stereoscopic camera provided with lenses of five inches equivalent focal length, having their axes parallel, and mounted at a distance L U (=2i inches) from centre to centre. A very good form of Lenticular Stereoscope for exhibiting paper pictures is shown in the following figure, which scarcely needs ex- planation. The whole lenses fixed in the, front of the box are placed 2| inches from centre to centre, and should not be less than 1 inch in diameter. They should be of 5 inches focus, and achromatized meniscus lenses with the hollow side outwards, in fact the same lenses as those used for taking the pictures, which may be unscrewed from the camera, and screwed to the stereoscope. The back of the solid box should have holes in it as represented by the dotted lines, and also a parti- tion in the middle. The holes should be 2\ inches from centre to STE 337 centre, and their diameter determined by trial. If the front box be 2.V inches deep, the back diaphragms should be a trifle more than 1 inch in diameter. A set of diaphragms of different sizes to lit in the back of the front box would be a useful addition to the instrument. The inside of the front box should be properly blackened. The pictures may be circular, mounted 2^- inches from centre to centre, and about 2£ inches in diameter. By the centre of each picture is meant the point where the axis of the lens cuts it. In mounting the pictures this is the point to be considered, and no reference what- ever is to be made to any of the objects in the pictures. These are always nearer together than 21- inches, when mounted. The mounted pictures are fastened by springs to a slider which moves in a slit in the bottom of the stereoscope, so that the distance of the pictures from the lenses may be varied to suit different sights, or for trying experiments ; the proper distance, however, being 5 inches, which is the focal length of the lenses. The pictures should be mounted upon black cardboard. In order to make the objects appeal more distant than the cardboard, that is, in order to make them appear as if viewed through a circular hole nearer to the spectator, the following rule in mounting them should be attended to. Just before trimming the edges, mount them temporarily 2\ inches from centre to centre, upon a piece of cardboard, with pins at the corners; then measure the distance between the nearest object in each pic- ture (this will be less than 2.V inches, probably not much more than 2^ inches), and take a dimension a little less than this for the dia- meter of the pictures. With half. this diameter as radius, and the z 338 STE centre of the picture as centre of the circle, strike a circle on each picture, which will be the margin required. In conclusion, we would observe that the object of taking the fenses as large as an inch in diameter is not that the margins of the lenses may be looked through, but simply because when they are too small their circumferences are seen by the eyes, and form two inter- secting circles- upon the solid picture. Were it not for this circum- stance the lenses need not exceed half an inch in diameter. The stereoscope for exhibiting transparent pictures is of the same general form as Knight's Cosmorama stereoscope, the other points being the same as that of the Stereoscope described above. It now only remains to explain the exact effect of the lenses upon the pictures. Turning to the figure at the commencement of this article; if a lens of focus ~F were placed at L, and the eye pressed close to it, a pencil diverging from b would after refraction through the centre of the lens be converted into a pencil of parallel rays, without suffering deviation, so that the eye would see the point b in the same direction as if no lens were interposed, that is along the line L#B r but would have less difficulty in bringing to a focus upon the retina a parallel pencil than one diverging from a point so near as b. When, therefore, the eye is placed so close to the lens as to see things exactly through its centre there is no magnifi- cation. If, however, the eye be placed at a little distance from the lens it does not look at the side objects exactly through the centre, and therefore the lateral pencils suffer a slight deviation, which in- creases the apparent angle ALB, and produces magnification, which may, however, be counteracted by putting the pictures a little further from the lens than its focal length. If, then, we leave out of consideration the unavoidable defects of all lenticular optical instruments, it appears that the form of stereo- scope that has been described exhibits pictures, when properly taken, in such a way as to represent objects precisely as they would appear to unassisted vision, so that natural truth is perfectly realized. And the reader will particularly observe that in this form of stereoscope the images are not displaced or caused to overlap by any optical contrivance. How then, it may be asked, is the effect produced ? Let us examine carefully the pictures upon the card PQ. In the first place the points a,a, — b,b, — c,c, are upon the same horizontal lines, which is simply because the cameras had their axes parallel, and not converging to a point. In the next place, if we measure the distances aa, bb } cc, we find a a the greatest because A is the most distant object ; b b the next because B is the next object in point of distance ; and c c the least STE 339 because C is the nearest object. But all these distances are less than LR. If, however, in addition to the three lamps, a fixed star D were introduced in any part of the picture, the visual rays L D, RD would be parallel, and the points d,d, where they cut the pic- tures would be at the same distance apart as L and R. If then we join L,R, with points c,c, nearer together than L and R, (and in the same plane with them) the lines Lc Re meet at a finite distance C ; but if we join L,R with points d,d, at the same distance apart as L and R the lines hd, Rd are parallel, or only meet at an infinite distance. Hence it follows that in mounted stereoscopic pictures the furthest objects are the widest apart, and the nearest objects the nearest together ; objects at an infinite distance only being as far apart as the distance between the eyes. These considerations will explain at once why it is that there is no necessity for displacing the images of the pictures by half lenses ; the objects being suffi- ciently displaced by the perspective in the pictures themselves, and any further displacement being wrong in principle. The Brewsteriau stereoscope, therefore, involves an error of principle. We have now done with the Lenticular stereoscope. Its defects are those which are inseparable from all optical instruments in which lenses are used, and the pictures are so small that it is impossible to include in them the same amount of detail as in larger pictures. But at the same time for many purposes, the smallness of the pic- tures, involving but a small expenditure of material, and permitting the use of a light and portable form of apparatus in which both pictures may be taken simultaneously, is a real advantage. We now proceed to an account of the reflecting stereoscope. The Reflecting Stereoscope. Referring to the figure at the commencement of this article. If the visual rays are cut by a plane pq instead of PQ the pictures will be larger than before, and instead of being entirely exterior one to the other, will overlap, and be mixed together, so to speak. But if we take the pictures in cameras placed at L and II, with lenses of focal length Lm or R«, having their axes parallel, and then, by means of reflectors throw the virtual images of the pictures so taken into their proper positions on the plane pq, and view these images by eyes at L and R, a truthful solid image will be produced, as in the former case ; because the left eye will not then see the picture from the right station, nor the right eye that from the left station. The arrangement is exhibited in the following figure : — After what has been said on the subject of the lenticular stereo- z 2 340 STE scope the following figure will only require a few words of explana- tion. A \ w The reflectors are placed at right angles to each other, and the pictures at right angles to the dotted line, or base, passing through the angle formed by the reflectors, the distance from that angular point being equal to Ijm or Hn, and therefore to the focal length of the lens with which the pictures are taken. The distance of the middle points of the pictures, (that is the point where the axis of the lens cuts them), viz., m, from the dotted base is half the distance LE. The pictures are taken in a non-reversing slide, so that their images on the plane pq as seen in the reflectors, are not reversed ; this is an important point to attend to. STE 341 The image of the left hand picture isjpmb; that of the right hand picture qua, the lines pb, qa, being separated for the sake of distinctness, but in point of fact the images lie on the same plane. The left eye cannot of course see the image of the right picture, and vice versa, so that the images overlapping produce no confusion. The image of each picture and the picture itself are symmetrically situated with respect to the reflector by which it is viewed. All this being understood, we come to the mode in which the instrument acts. A pencil from b in the left picture, after reflexion at the left mirror enters the left eye as if it had come from the point b on the line/?& ; the point b is therefore seen by the left eye in the direction Lb. Similarly, a pencil from b in the right picture, after reflexion at the right mirror enters the right eye as if it had come from the point b on the line qb ; the point b is therefore seen by the right eye in the direction 116. These two lines R6, L5, are the instantaneous direc- tions of the optic axes, and being produced they meet at B, which is the true position of the object B. Similarly with respect to the other objects A, C. Therefore by means of the reflecting stereoscope a true representation is afforded in natural relief, and actual distance, of the objects in the picture. The reflecting stereoscope is not open to any theoretical objection. As an optical instrument it is absolutely perfect, being subject to no defects of distortion or aberration. For any scientific purpose, therefore, the reflecting stereoscope should always be preferred to the other. The reflectors may be made of polished speculum metal if objection be raised to glass mirrors, and the pictures may be taken simultaneously in a camera with double lenses 2£ inches from centre to centre, constructed as shown in the following figure, which explains itself. In this double-reflecting camera the non-reversing slide need not be used. It now only remains to add a few remarks on taking stereoscopic pictures. Tn the first place, the effect of taking the stations wider apart than 2i inches (the parallelism of the cameras being still preserved) is to bring the near objects in the solid image nearer to the spec- tator than they were before, and nearer than they are in nature, while the most distant objects remain in their true position. This may be in some cases allowable, because the stereoscope is intended to serve certain educational purposes ; and it may happen sometimes that, by giving bolder relief to objects than they really have, the thing to be explained may be rendered more intelligible. 342 STE In the next place, the effect of directing the axes of the lenses to the same point at a finite distance introduces distortion in the solid image, unless the pictures are placed in the stereoscope at the same angle of inclination to one another as the focussing screens of the cameras. When, however, the stations are taken very wide apart (several feet, for instance), if the parallelism of the cameras be pre- served, the pictures are partly thrown outside the focussing screen, besides being taken with very oblique pencils ; so that, when cor- rect principles of operation are once departed from, for any purpose, other changes in the arrangement of things become necessary. We cannot, however, within the compass of the present work, do more than explain completely the theory of the correct form of the instru- ment ; it would require a separate treatise to follow up the subject through all the modifications which it may assume. Enough has been said to enable any intelligent reader to think out the remainder for himself. Tn printing stereoscopic pictures from a negative, taken in a double-lens camera, by contact in the pressure frame, it must be borne in mind that the print requires to be cut in half and the pic- tures transposed, in order to bring the picture that was taken from the left station before the left eye in the stereoscope, and vice versa. If this be not attended to, a pseudoscopic effect is produced. In printing stereoscopic transparencies by means of a lens, the following plan may be adopted : — A copying camera, rather more than double the length of the STE 343 stereoscopic camera, is provided, and the lenses of the stereoscope are fixed in the middle of it; there must also be a partition dividing the camera in half Lengthways. In this way the left lens copies the left picture at the same time that the right lens copies the right pic- ture. The negative must be placed with its back next to the lenses at one end of the box, and the sensitive positive plate in a common slide at the other end. The camera is then directed towards the sky, and the wet collodion process employed. An exposure of a few seconds is sufficient. The positive need not be divided and the pictures transposed, for, when placed in the stereoscope with its plane side next to the lenses, and a ground glass laid against the film, the pictures are in their right position to be viewed. By put- ting the lenses midway between the negative and positive, the positive becomes of the same size as the negative. The best lenses to em- ploy are doublets, with a small stop between the back and front lenses in each. In the stereoscopes and stereoscopic pictures commonly sold, there are the following serious defects : — 1st. The pictures are frequently taken in converging cameras, and then mounted upon the same flat surface. 2nd. The pictures are generally mounted so wide apart that the most distant objects in each are wider apart than the distance between the centres of the eyes. 3rd. An attempt is made to obviate the evil produced by the above practice, by using semi lenses in the stereoscope, which dis- place the images. This of necessity produces distortion, because straight lines are always represented by curves when the outside part of a lens is used to view objects through, instead of the centre. 4th. The displacement of the images is in general so great as to cause the optic axes to converge to points situated within two or three feet from the nose, instead of the true distance of the objects. The effect of this is to make the solid picture look like a small model of the object, which the spectator could, if he chose, lay his hand upon, or touch with a yard measure. Lastly. The focal length of the lenses of the stereoscope is in general six inches, while that of the lenses in the camera is only live inches. This makes objects appear much smaller than they do in nature. To sum up. The common stereoscope and pictures make objects look very near, very small, and distorted. The stereoscope described and recommended in the present article makes them look of their true size, at their true distance, and without perceptible distortion. In this article the term " solid image" has been several times used. 344 STO SUL The employment of this term may perhaps be thought objectionable ; we do not, however, know of a better, and if the thing meant has been clearly understood, the end has been answered. Stone Blue. A mixture of indigo and starch, moistened with water, made into cakes, and dried. Sometimes Prussian blue is used instead of indigo. Sulphides. Sulphur combines in various proportions with most of the metallic and non-metallic elements, and forms compounds called " sulphides," or " sulphurets." Sulpho-cyanides are salts, whose composition only differs from the cyanides in that they contain two equivalents of sulphur. When the alkaline cyanides are fused with sulphur, they readily take to themselves another atom of sulphur, and are converted into sulpho- cyanides. They possess the properties of dissolving certain salts of silver, and for this reason they have been proposed as fixing agents, but their superiority over hypo-sulphite of soda is very doubtful. Sulphur. S = 1 6 . A yellow, crystallised elementary body, found chiefly in the neighbourhood of volcanoes. About 20,000 tons are consumed annually in England, and are imported chiefly from Sicily. It exists abundantly in combination with lead, copper, and iron. Sulphur is highly combustible, and burns with a blue flame, pro- ducing sulphurous acid. It is insoluble in water, and permanent in the air. It fuses at 232°. When heated to over 430° and under 480°, it becomes viscid and of a brown colour ; if it be then poured into warm water it becomes soft like wax, and may be used for taking impressions ; it becomes hard when cold. Sulphur is totally insoluble in water and alcohol. Some of the essential and fat oils dissolve it, but the best solvent of all is bisulphide of carbon. Sulphur Toning. When an argentine photograph having the reddish tint produced by the combination of suboxide of silver with organic matter is placed in water containing a small quantity of sul- phide of ammonium, the tint gradually changes from red to purple, and thence to green-yellow. This is called sulphur toning, the yellow substance being supposed to be sulphide of silver in an allo- tropic state ; or it may possibly be a bisulphide of silver ; or a double sulphide of silver and ammonium. A similar result occurs when the print is placed in a bath of hyposulphite of soda containing unstable sulphur salts, or unstable compounds of sulphur and oxygen, or free sulphur in a nascent state or state of fine division, SUL 345 exhibiting a milky turbidity in the bath, produced by the addition of an acid to it. The ordinary fading of positives appears to be nothing more than the sulphur toning process carried to the yellow stage, in consequence of the presence of a destructive sulphur salt which cannot be removed from the paper. The combination of organic matter with the silver may have something to do with the result, and with the com- position of the yellow substance. Photographs of a black tint, pro- duced by development, and containing a much greater quantity of material than sun-prints, and that in a form much more nearly metallic, are found to be more permanent than the latter. Sulphuric Acid. SO 3 =40. There are many combinations of sulphur with oxygen, all of them being acids. Sulphuric acid con- tains the highest proportion of oxygen. It can be obtained pure — that is, without the presence of water — but cannot be kept in the anhydrous state ; so that any chemical formula to designate its general constitution does not truly indicate its exact composition. The method of preparing this acid on a large scale is so instructive in the chemistry of the reactions of elementary bodies brought into atomic contact, that we would fain have embodied the details of the whole process here, but space forbids. In Dr. Miller's "Elements of Chemistry," Vol. II., the whole matter is explained in his usual lucid manner. In Photography the principal use of sulphuric acid is confined to its employment along with nitric acid, or nitrate of potash in the manufacture of pyroxyline. There are other minor appliances, such as cleaning of glass plates, &.C., which we need not here farther refer to. They will be found under their proper headings. The impurities contained in the commercial sulphuric acid are generally not of such a nature as to interfere with its photographic properties, either in the manufacture of pyroxyline or in the cleaning of glass plates. Water is the most dreaded impurity in making pyroxyline, and that can be detected very easily by taking the specific gravity, which should range from 1836 to 1845. Adulterations of dissolved salts to increase the specific gravity of a weak acid would not pay the manufacturer, on account of the cheapness of the acid itself. He has, therefore, no object to serve by adulterating it in this way. Sulphuric acid (hydrated only) possesses most powerful chemical affinities. These, and the impurities often present in it, have little to do with Photography. Sulphurous Acid. S0 2 =32. This acid is produced by the 346 TAL TAN combustion of sulphur in oxygen. At ordinary temperatures it exists as a gas, bat at the freezing point becomes liquid. At a lower temperature it may be solidified, and then forms a white mass. Water which has been recently boiled absorbs about 30 volumes of it. The solution possesses bleaching and deoxidising properties. Talbotype. A negative process so named from the inventor, or Mr. Fox Talbot. See " Calotype." Tangent. The tangent to a curve at any point P is defined thus : Take any other point Q, and draw a straight line through P,Q. This straight line cuts the curve in the points P, Q, and is called a "secant." Now let the point Q, move along the curve towards P. When it has approached to within a distance less than any assignable distance from P, but without actually coinciding with P, the secant P,Q,. becomes a tangent to the curve at P. This definition applies equally either to the case of a curve of double curvature (like a corkscrew), or to a curve which lies upon a plane. The Tangent-Plane to a surface at a point P is found by cutting the surface by any two planes which pass through P, finding the tangent lines through P to each of the plane curves thus pro- duced, and drawing a plane through those two tangent lines. If two points, Q, K, be taken upon the surface, and a plane PQE drawn through them, and we suppose Q, it, to move towards P, then the plane PQE, when Q, and E have approached P to within a distance less than any assignable distance, is not necessarily the tangent plane at the point P, as it is sometimes stated to be in books of geometry. Tannin; Tannic Acid. Tn.=C 54 H 22 0 34 =618. An astrin- gent principle contained in various vegetable substances, but prin- cipally in infusion of galls. It is obtained in a pure form by treating powdered galls with washed ether, i.e., ether containing 10 per cent, of water ; this is allowed to filter through the galls, and the filtered liquid divides itself into two strata, the upper one being ether, and the lower a concentrated aqueous solution of tannic acid. This is evaporated in vacuo over sulphuric acid, and pure tannin remains as a bulky pale-yellow residue, which is exceedingly soluble in water., but less soluble in absolute ether and alcohol. The aqueous solution of tannic acid reddens litmus paper, and exhibits the properties of an acid, displacing carbonic acid from the carbonated alkalis with effervescence, and forming salts called tannates. TAN 347 Tannin combines energetically with gelatine, and forms an inso- luble precipitate when added to solutions of isinglass, or glue. When oxidised, tannin becomes converted into gallic and carbonic acids. Its chief use in Photography ' is in the dry collodion processes (?•"•)• . .... A concentrated solution of tannin is precipitated by nitric and hydrochloric acids, but not by oxalic, tartaric, lactic, acetic, or citric acids. Tannin, when added in excess, gives a dark blue or black colour to solutions of the j^ersalts of iron, but produces no im- mediate change in solution of the pure protosnlts. The black pre- cipitate produced in the former case is common writing ink, and is composed of one atom of peroxide of iron and three atoms of tannic acid. Tannin Process. Photographers are indebted to Major Russell for the discovery of this, perhaps the best of nil dry processes. Several modified formulas have been suggested and worked out by Major Russell himself, and by others ; but space will only permit us to describe briefly the two principal modifications upon which all others are based. 1st Process with Common Bromoiodised Collodion. — The glass plates should be thoroughly cleaned as usual, and if large, coated with a thin solution of gelatine, or better still, with the " coating fluid," described under that heading. The object of this substratum is to prevent the collodion film from leaving the glass during wash- ing, development, &c. It also serves another purpose, in prevent- ing the sensitive medium from being contaminated with impurities in or on the glass. Almost any bromoiodised collodion serviceable in the wet process will give good results with tannin. Coat the plate as usual, but allow the film to set for rather more than the ordinary time, before immersing it in the nitrate bath, which should be slightly acid. ;uhI of the strength of from thirty to forty grains to the ounce. When the plate is fully sensitised, place it for about five minutes in a bath containing distilled water. It should then be transferred into another bath containing common water. Not less than four of these baths containing a large supply of water should be used, and in each of them the plate should remain at least five minutes, in order to withdraw all the free nitrate of silver from the film. It is necessary that the whole of this should be removed or con- verted into a haloid salt, otherwise the film will become brown when the tannin is applied. A final rinsing with distilled or filtered rain water is useful before applying the tannin. 348 TAN The plate is now ready to be treated with a solution made in the following proportions : — Tannin 12 grains. Distilled water .... 1 ounce. Dissolve and filter, then add one drachm of alcohol for each fluid ounce of solution. The addition of alcohol serves two purposes : the first is to enable the liquid to penetrate the collodion film more rapidly, and the second, to prevent decomposition of the tannin solution, should it be deemed desirable to keep it for some weeks or months. While the plate is still moist from the washing water, a portion of the above tannin liquid is to be poured on and off the plate several times for at least two minutes. The residue of the tannin thus used may be returned through a filter into the stock -bottle and used again. But a surer and better method of applying the solution evenly is by immersing the plate for three or four minutes in a dipping- trough containing the tannin. When taken therefrom, the plate, after being drained for a minute or so, should be stood up to dry in a warm and dark cupboard, on several folds of clean blotting-paper, which absorbs the superfluous drainage -liquid, and prevents it from reflotving over the plate by capillary attraction. When dry, the plates are fit for use, and may then be stowed away in dry plate- boxes, to be used when wanted. Artificial heat should not be used for drying tannin plates, unless great precautions are taken to apply the heat uniformly to every portion of the plate. Plates thus prepared will keep well for many months, perhaps indefinitely, before exposure, but not nearly so well in the interval between exposure and development. The proper time of exposure is dependent on so many circum- stances, such as the actinic intensity of the light, the focal length and the aperture of the lens, the mode of development, &c, that it is not possible for any one to lay down even approximate rules. Within our own practical experience the exposure may vary from one second to twenty minutes, with the ordinary bromoiodised col- lodion treated as above directed, but under different conditions of lighting and development. To Major Eussell again we are indebted for the following devel- oping fluids, which are not only very suitable for the tannin but for all other dry processes : — (1.) Pyrogallic acid . . .96 grains. Absolute alcohol . . . 1 fluid ounce. Filter and keep in a well stoppered bottle. TAN 349 The solution will scarcely become discoloured or deteriorated after six months' keeping. Five minims of it contain one grain of pyrogallic acid. (2.) Nitrate of silver . . . 10 grains. Citric acid . . 10 to 60 ,, Distilled water ... 1 ounce. Dissolve and filter. The amount of citric acid is left very indefinite. The heat of the weather (which always renders a larger proportion necessary), and the conditions of exposure, affect the question. It would therefore be desirable to take the minimum weight of citric acid in the above formula, for mixing with the nitrate solution, and have another bottle containing : — Citric acid . . . . .20 grains. Distilled water . . .1 ounce. When occasion required, let this be added to the developer, according as to how the action is more or less rapid. When an exposed plate is about to be developed, if it has been previously coated with an understratum of gelatine, gutta-percha, or india-rubber, no particular care farther than the ordinary share of delicate manipulations required in the wet collodion process are necessary ; but, if the plate has not been previously so coated, then it will, in most instances (especially when large plates are used), be absolutely required to run a camel's hair brush, dipped in any of the collodion negative varnishes, all round the edge of the film, to the depth of about J -inch, before development commences, otherwise the developer, &c, may get underneath the film and detach it entirely from the glass. The above precaution is a wise one under any circumstances. The next step is to moisten the surface of the film with equal parts of alcohol and distilled water ; then to wash it for about a minute under a tap, till all tendency to the formation of greasy -looking lines in the direction of the drainage, when the plate is held perpendicularly, has disappeared. W ben the water appears to flow freely over the surface, mix (for a plate 8x5 inches, and for others in proportion) half a drachm of developing solution No. 1, with one and a half ounce of distilled water. Pour this over the plate two or three times, and then add to it two drops of solution No. 2, but this latter must be intimate ly mixed with the pyrogallic liquid before the compound mixture is applied to the plates. It will now be readily seen whether the picture has been under or over exposed. If, after pouring the de- veloper on and oil' the film several times, nothing but the high lights 350 TAN" TAR appear, then add more pyrogallic acid, and a drop or two of solution of nitrate of silver not containing citric acid. If, on the other hand, the picture bursts out quickly without such addition, then add more acid silver, or still better, a few drops of plain citric acid solution. Thus, by a judicious management of pyrogallic acid, nitrate of silver, and citric . acid, the development of a dry tannin plate, no matter what has been the time of exposure, is very much under control. When the development is finished, the negatives are, of course, fixed and washed as usual. Eor the fixing, hyposulphite of soda is much preferable to cyanide of potassium, because the latter in many cases weakens the half tones . Another method of developing tannin plates has also been dis- covered by Major Russell, whereby the time of exposure is much reduced. See " Developer, Alkaline." One formula with bicar- bonate of soda is given under the heading referred to, and its mode of application described. Another formula preferred by some is the following : — Dissolve 6 grains of commercial carbonate of ammonia in 2tt ounces of distilled water, and 1§ ounce of alchohol, and keep in a stock -bottle. A drachm or more, according to circumstances, of this is mixed with a plain 2 or 3 grain solution of pyrogallic acid, and applied to the film according to the directions given for the formula with bicarbonate of soda. Tannin Process with bromised Collodion, or Russell's Process. — Major Russell considers this process at least twice as sensitive as the best common bromoiodised collodion, and eighteen times more sensitive than simply iodised collodion, when each has been treated in the manner that best suits its requirements. The formula for the collodion itself is given under " Collodion, Bromised" q. v. All the operations are the same as those already described, excepting that the strength of the nitrate bath should be 60 grains, and the time of immersion 15 minutes. Dry plates carefully prepared by this method, and developed by the alkaline developer, are nearly, if not quite as sensitive as wet ones. Taupenot Process. This is a collodio-albumen process for ob- taining negatives, so called from its inventor Dr. Taupenot. See " Collodio-albumen." Tartaric Acid. Tar. = C 8 H 4 O 10 -f 2 HO = 150. This acid exists free in many acid fruits and plants, and is generally obtained from cream of tartar, in the form of white crystals, which are soluble TEN 351 in about 4 parts of water at 60°, and also in alcohol. It is a very powerful organic acid in Photography, and should be used with caution. It is deliquescent in damp air. Paper washed with a solution of tartaric acid is said to be slightly sensitive to light. Tent. When views are taken by the wet collodion process, in which the free nitrate of silver is not removed from the plate by washing, it becomes necessary to operate either in a dark room, or van, or tent, at or near the spot whence the view was taken ; for otherwise the latent image is destroyed by the evaporation of the moisture from the sensitive plate, and by the consequent solution of the iodide of silver by the concentrated nitrate in the film. The dark tent used by travelling photographers is of various forms and sizes, and more or less portable. For large pictures 12 x 10 or so, perhaps the best form is that of the ordinary military tent, having a pole at each end and fastened to the ground with ropes and pegs. It should be made with black calico, lined with yellow. In one of the gable ends there should be a yellow window, and the entrance should be in the opposite gable. Inside there may be a table and all the necessary conveniences. This kind of tent has of course no pretensions to portability, and a travelling van is prefer- able, since much time is necessarily occupied in erecting and taking down a tent of this form. For stereoscopic pictures, or pictures not exceeding 8x6, a much simpler and more portable form of tent will answer the purpose. A deal tray about 2t't. Gin. long, 1ft. 9in. wide, and 4 ins. deep is screwed upon a short tripod stand. At the corners the four up- rights of a light iron frame are inserted, which is made thus : — the rods being about the thickness of stair rods. A covering made of black calico lined with yellow is thrown over 352 TES 9 TET this frame, and hangs down to the knees. In this covering at the back is a yellow window nearly the whole height of the iron rods and about 6 .inches wide ; the yellow curtain of this window or aperture may be drawn backwards and forwards at pleasure. The tray has a shelf at the back which carries the bottles, etc. To use this tent the operator stands beneath the projecting part of the top of it, with the tray in front of him, and draws the lower part of the curtains tightly round him under his elbows, and fastens them by means of hooks to the inside of the front part of the tray, so as to exclude day-light. He has then his hands at liberty, and manipulates in the usual way. By putting a tent of this kind, to- gether with the chemicals and apparatus, upon a wheelbarrow, or suitable truck, or basket with wheels which ship and unship, the photographic tourist becomes independent of help from others. Test Papers. Make an infusion of commercial litmus, and steep blotting paper in it ; dry it, and cut it into narrow strips. It is of a deep indigo colour, and is reddened by being immersed in any acid solution, or exposed to acid fumes. This blue litmus paper, as it is called, is therefore a test for acidity. It may also be used as a test for alkalies by dipping it into very dilute sulphuric acid, and drying it. In this state it is reddened litmus paper, and its original blue colour is restored by immersion in any alkaline solution. A volatile acid, such as acetic, should not be used in making reddened litmus paper. Another kind of test-paper for testing alkalinity is made by steeping blotting paper in an infusion of turmeric ; this is of a yellow colour, which is changed to brown by the action of an alkali. It is not considered so good as reddened litmus. But the best kind of test paper for testing alkalinity is made by steeping blotting paper in a strong infusion of the petals of the red rose. The red colour of this kind of paper is changed to green by an alkali. When a solution to be tested is very feebly acid or alkaline a few minutes must be allowed before the change of colour in the test- paper is perceived. Test-papers, as usually sold by chemists, are made up in little long narrow books. They are indispensable to the photographer for testing the condition of the nitrate bath. Tetrathionic Acid. S 4 0 5 = 104. This substance is one of the many oxides of sulphur described by Berzelius. It is formed when iodine is added to hyposulphite of soda. A clear solution re- mains, which contains iodide of sodium, and a salt of this peculiar acid. THE TON 353 It was at one time common to tone photographic prints in a solution prepared as above, but as the colouration of the proofs was due to sulphur, the process has been very properly abandoned. Thermometer. (Gr. Ofpfiri heat, fierpov a measure). This is an instrument for measuring temperature by recording the expansion produced in a liquid by heat. The mercurial thermometer consists of a glass tube of fine and equal bore, having a bulb at one end. This tube is filled with mercury at a high temperature, above boiling point, and its end hermetically sealed. On cooling, the mercury contracts and leaves what is called a Torricellian vacuum above it in the tube, i. e., a space filled with the vapour of mercury. To graduate the thermometer, it is first immersed in melting snow, the temperature of which is found to be invariable, and the height of the mercury marked ; and next in water boiling in a thin polished metallic vessel at a barometric pressure of 30 inches, a temperature which is also found to be invariable, and the height of the mercury marked again. These two graduations are called the freezing and boiling points of water. The space between them is divided differently in different thermometers, as shown in the table at the end. The thermometer used for testing the temperature of liquids is furnished with a hinged back, so that the bulb and lower part of the tube only can be inserted in the liquid. If equal parts of hot and cold water are mixed together the thermometer indicates accurately the arithmetical mean between the temperatures. Tincture of Iodine. This is composed of 48 grains of iodine added to one fluid oz. of alcohol sp. gr. 835 The iodine is dissolved and the tincture poured off into a well stoppered bottle. Tints, Photographic. Solutions of various colours, in which positive prints are immersed, after being washed and fixed, in order to dye the paper an agreeable tone, and destroy the unpleasant whiteness of the lights of the photograph They are manufactured and sold, in a concentrated state, by Messrs. Bailey, of Wolver- hampton. Very great dilution is necessary before use. Toning-Bath. In printing positives by direct light, the purple tint due to the subchloride of silver is removed by the fixing agent, and nothing remains but a red compound of silver and organic matter. In order, therefore, to render a sun-print presentable as a work of art the thin red tint of the shadows must be blackened or 354 TRA intensified by some means. This is effected by the toning bath, which either darkens the print by sulphuretting the silver in the image {See " Sulphur Toning "), or by substituting' gold for silver, according to its composition. {See " Printing, " &c. Developed prints do not necessarily require a toning bath, since the material of the image is sufficiently black and intense without it. Tracing-Paper. There are two kinds of tracing-paper, viz., transparent and black. Transparent tracing-paper is made by smearing the paper with boiled oil, or magilp, or colourless dam- mar resin dissolved in turpentine or benzole ; or, better still, with Canada balsam diluted with turpentine. Black tracing-paper is made by saturating a piece of blotting paper with a mixture of lamp-black ground in honey, or by rubbing a piece of sized paper with black-lead or black chalk. The tracing is first made upon the transparent paper in lead pencil, this is then laid upon the black paper, and that with its blackened side upon the paper which is to receive the final drawing. The lines on the tracing-paper are then gone over with a porcupine's quill, or other hard point, exerting a gentle pressure. A corresponding outline is thus obtained upon the paper beneath the black one. Sometimes blue paper is used instead of black ; this is made by substituting indigo or prussian blue for lampblack. The transparent paper made with Canada balsam takes ink and water colours freely. A tracing-paper is made in France from raw flax, and is called " Papier Vegetale." Transferring. A collodion picture which is not too porous and powdery may be transferred from the glass plate, when wet after the final washing, in the following manner: — Lay a thick and wet piece of blotting paper upon the film in such a way as to cover the plate all but about a quarter of an inch at one end. Turn the narrow edging of film which is outside the blotting-paper over it by means of a penknife, and then, beginning at that end, raise the blotting-paper gently off the plate ; the film will come off with it. It may be permanently fixed to a sheet of dry gelatinised paper, by laying the blotting-paper bearing the film upon the gelatinised paper, pressing the two into close contact, and letting them dry spontaneously, when the blotting-paper will come off, leaving the film attached to the gelatinised paper so strongly as to resist all attempts to remove it again by scratching or rubbing. The paper may then be waxed. Collodion positives may be transferred from glass to glazed leather by damping both the film and the leather with alcohol, press- TRA 355 ing the two into contact, and in a few minutes peeling off the leather, which brings the film with it, apparently so incorporated with the black glaze as to be incapable of being removed by scratch- ing with the nail, &c. Collodion negatives may be transferred to gutta-percha in the following manner, described by M. Leon Cassagne at a meeting of the French Photographic Society on June 19, 1857. " It is generally known that at the Imperial Printing Office of Vienna, when a good collodion negative has been obtained on glass, it is the custom to transfer it by means of a double film of gelatine and gutta-percha dissolved in chloroform. The process which I have adopted, and which has uever been described in the Bulletin of the Society, consists in first dissolving — Pure gutta-percha . . .1*92 grammes Chloroform, or benzole . . . 31 09 „ or, Gutta-percha . . . .2*56 grammes Chloroform, or benzole . . .31-10 ,, " You perceive that the quantities are not invariable. There are cases in which it is necessary to vary them. I shall not enter into details ; the operator, in each particular case, will be able to decide for himself. " When the negative on the glass is dry and in good condition, pour on the collodion side a coating of the above solution. Let it run slowly and uniformly, that it may have time to penetrate and unite with the collodion film. As soon as this coating is completely dry, strengthen it with a second, formed of the following sub- stances : — Gelatine of commerce (very white) . 30 grammes Filtered water, as much as the gela- tine can absorb, until it has swelled to the utmost. Isinglass . . . . . . 5 „ Alcohol . . . . . . 15 ,, " Melt the gelatine in the water which it has absorbed, by plac- ing the vessel containing it in hot water. Melt the isinglass in the same way in the alcohol. Mix by degrees, and with care ; stirring with a wooden spatula this species of varnish. Warm it with pre- caution, that it may not be injured by too much heat. Hold the negative, the coating of gutta-percha upwards, before a clear fire, or over a spirit lamp, until it is heated to 10 or 20° centigrade ; then pour over it immediately (removing it from the flame of the A A 2 356 TKA lamp), a coating of gelatine, as thin as its density will allow. Tt is unnecessary to say that the gelatine must be warm and perfectly liquid at the time. Leave it for an instant to cool and dry, shel- tered from dust, and you will be able to remove easily, by means of the steam from boiling water, the triple film of collodion, gutta- percha, and gelatine. This operation, which is very easy, is per- formed as soon as you see that the film is slightly softened by the steam and you should then begin to remove it from the glass at the corner from which the excess of collodion was poured off when the plate was collodionised. It often happens that the film disengages itself at this corner of the glass. It is a good plan to facilitate the entire removal of the film with a thin blade of flexible polished horn, on which, with the help of fingers, you support the film, while you detach it by degrees, either with, or without, the aid of a thin tbread of water, running drop by drop from a tap, and which insinuates itself by degrees under the collodion, between it and the glass. As soon as the entire film is raised, flatten it between two pieces of glass, having good surfaces, and sufficiently thick to act by their own weight. The collodion used must have svfficient consistency, not so much, however, as to leave striae or lines on the plate when dry. " The chloroform or benzole solution should be allowed to stand several days before being used, in order that the colouring matter, or any impurities in it, may be deposited. Filter through paper, that the solution may be sufficiently thin, shutting the top of the funnel to prevent too much evaporation, which would have the effect of thickening the solution. Benzole, of specific gravity much less than the chloroform, gives good results, but inferior to those ob- tained by chloroform, which gives a solution almost colourless, and adheres firmly when the evaporation is completed ; which also takes place more rapidly than with the benzole. " The density of the solution of gutta-percha, which is always slightly coloured, retards considerably its complete clarification. It is necessary to avoid all impurities in this solution." The following is a method of transferring dry collodion negatives to paper, described by M. Bayard at the meeting of the French Photographic Society on Feb. 20th, 1857. " Among the specimens which I have the honour to lay before you are some which have been obtained from very old negatives, and even from negatives which have been varnished. They have all been easily transferred. I cannot however promise you that it will always be so. Tt is probable that certain varnishes, and particularly fatty varnishes, may offer an impediment to the softening and removal of the collodion film. TEA 35'7 "I am afraid also that albumen and gelatine spread upon the negatives may interfere with the success of the operation ; and I must warn you that I have not yet succeeded in transferring with certainty negatives produced by Taupenot's process, either simple or modified. " The following is my process : — F " In order to detach the film of collodion from the glass, I use paper coated with gelatine. " To prepare it, dissolve in one litre of filtered rain water, 40 grammes (about 4 per cent.) of colourless gelatine. When the gelatine is dissolved, pour the solution into a dish which has been previously heated. Float the papers on the bath for one or two minutes, and hang them up by a corner to dry. When dry, keep them in a portfolio until required for use. The kind of paper which appears most suitable for this operation is Canson's thin negative paper. "If the negative to be transferred has just been taken, and is still wet, place the glass on a horizontal support, collodion side upwards, and cover it equally and evenly with water. Then, take a sheet of the prepared paper, (which should be of the same size as the glass,) float the gelatinised side for three or four minutes on a bath of water, and having carefully removed it, lay it on the water with which, the glass has been covered. Then, by inclining the glass, allow the water to drain off and the paper to become attached to the collodion. Place the glass perpendicularly, and allow it to dry spontaneously. " When the negative which you wish to transfer is old, and has not been varnished, immerse it for about a quarter of an hour, film upwards, in a dish full of water. Ten or twelve minutes after putting it into water, lay a sheet of gelatinised paper on the same water for three or four minutes. Then remove the glass by the corners in such a way as to remove with it the paper which fioats above, (the edges being of course properly adjusted to those of the glass). By proceeding cautiously the paper will adhere to the collodion. Drain and dry as before. " When the negative has been varnished, proceed as before, with this difference, viz. : — add 3 or 4 per cent, of alcohol to the water, and let the glass remain in it half an hour. " When the paper which has been glued to the collodion has become perfectly dry, (it should not be dried by the fire,) make an incision with the point of a penknife all round it, pretty close to the edges of the glass, and then immerse the negative in a dish filled to about an inch deep with water. A quarter of an hour after, 358 TRA you may endeavour to raise a corner of the negative film with the point of a knife. Should the film not come off with the paper, leave it immersed a little time longer. As soon as you find that the collodion will leave the glass, raise the paper carefully, without re- moving the glass from the water, which always moistening the collodion renders the operation more easy. When the paper has been removed with the collodion film adhering to it, press it between blotting paper, and dry it. " Negatives transferred in this way acquire great vigour for printing, and if the prints from them are found to be too strong in the contrasts, the negative should be waxed on the reverse side of the paper in the ordinary way." After M. Bayard had made the above communication, he, in order to show with what ease these transfers could be made, placed in water a collodion negative on glass, having a sheet of gelatinised paper adhering to the film. M. Le Gray, who had lent this negative, said that he had warned M. Bayard that it was in very bad condition for transferring, as the film wanted consistency ; but that, if the experiment suc- ceeded, it would be the more conclusive. Notwithstanding these unfavourable conditions M. Bayard effected the transfer with complete success. Collodion positives may be transferred to paper by the following process, communicated to the " Photographic Notes " by Mr. Man- son, of Edinburgh : — " To make the transfer varnish : — Take of borax, 1 dram, shellac, 4 drams ; digest them in about 5 ounces of water, nearly boiling, in a covered vessel, till the whole is dissolved ; when cold it is ready for use. " To transfer the film : — Apply a coat of the varnish to the surface of the picture with a large and soft camel's hair brush, and dry it quickly by holding it over a flame, or at a fire ; when cold, apply- ing a second coating of varnish as before. " Then take a piece of black paper a little larger than the glass. Coat it, and also the picture, with varnish, and lay the two wet sur- faces together, beginning at one end, and carefully excluding every bubble of air. When nearly dry lift one end, and strip the whole from the glass. It is now ready for mounting." Daguerreotypes may be transferred to paper by the following process, described by Mr Belfield Lefevre, of Exeter, in the " Photo- graphic Notes," Vol. 2, page 343 :— " To obtain a negative by transferring to the surface of some TRA 359 more or less transparent substance the loose particles which form the lights and half-tints of Daguerre's image, is an idea which must have suggested itself to the minds of many, whilst the means by which this transfer may be effected are as simple as the idea itseif is obvious ; and yet, although from the day on which M. Arago communicated M. Daguerre's process to the Academy of Sciences, I have been a votary of the photographic art, and am not unfamiliar with photographic literature, I have seen no allusion, however remote, to any such process. Is it that the results so obtained have not been found available for the purposes of photographic printing? However this may be, as it is not in my power to pursue these researches any further, I submit the process to your judgment in its present imperfect state, and my sincere desire to contribute to the progress of Photography must be my excuse. The following, then, is a short, but I believe sufficient description, of the modus operandi. I purposely omit mentioning those precautions which are familiar to all careful operators. " 1st. — Dissolve one part of pure gelatine, and one part of clarified uncrystallisable sugar (golden syrup of the grocers) in ten parts of boiling water, and pour out the hot solution in a shallow pan. " 2nd. — Float for a few minutes on the hot solution a sheet of Hollingworth's thin negative paper, previously well dried. " 3rd. — Draw off the paper, holding it vertically, at a short dis- tance from the fire, until the superabundant liquid has ceased to drain off. " 4th. — Lay it out horizontally on a cold slab, until the gelatine has firmly set. " 5th. — Meanwhile, take the image to be transferred fresh from the mercury box., and having washed it first in the solution of hypo- sulphite, and then in water, put it on end to drain, until the forma- tion of the horizontal water line marks that the liquid on the suriace is reduced to a mere film. " 6th. — Lay the gelatine paper on the image, pressing it down firmly and evenly with a soft cloth, until it is brought at every point in perfect contact with the surface of the metal. "7th. — After a few minutes peel off the paper. Some caution will be required, as it will be found to adhere rather firmly. " If the proof has been well selected, and the manipulation suc- cessful, every particle of reduced silver will be found transferred to the surface of the gelatine, and a faint vestige of the original image will alone be traced on the black and polished surface of the silver. " I say, if the proof has been well selected, for this is a point of much importance. Of course the choice would fall on a full-bodied 360 TRA proof, with thick and creamy lights, and rich opaque middle tints ; but this alone will not be found a sufficient guide, and it will be advisable to pass a camel's hair brush gently over some portion of the washed image ; if the passing brush leaves a tract of black and burnished metal behind, the transfer may be attempted, if not, the operation will not be successful. " On examining the transferred image by reflected light, it will appear as a faint and somewhat shadowy transcript of the original drawing, in which a careful inspection in a favourable light will detect many details reproduced with great sharpness and delicacy. By transmitted light, however, the semi-transparent nature of metallic films of extreme tenuity will be found painfully evident. It is indeed a faint negative, but it differs from those obtained by ordinary processes in two most important particulars. In the first place, its lights are perfectly and absolutely pure, and in the second, its half- tints, however faint, are all represented by a metallic equivalent, really and substantially existing on the surface of the gelatine, and which, therefore, may become the basis of a chemical action, although too minute to be detected by the most careful inspection. The colour of the metallic film varies greatly, generally approaching to a reddish brown where it is most dense. This clearly points out a fact for which we should have hardly been prepared, viz., that the high lights in Daguerre's image are in reality formed of two distinct layers, the upper stratum being blanched by the action of the mercury, and probably amalgamated with it, whilst the lower retains the reddish hue which reduced silver sometimes assumes. The rosy tint which is observable in the highlights of the finest proofs, when seen obliquely, is thus explained. Considered as a basis of chemical action, the transferred image is a sheet of gelatine, on which particles of pure metallic silver, or of silver amalgam, are more or less densely strewn. To increase the opacity of these particles, so as to render them less permeable to the rays of transmitted light, is the problem still to be solved, and for the solution of which three methods are open : — First, to transform the metallic particles into some binary compound, such as an oxide, a sulphuret, an iodide, or a chloride ; secondly, to substitute for them thin chemical equivalents of platinum or gold ; and, thirdly, to render them the centre of a catalytic action, which shall group around them fresh molecules of reduced silver. The very few experiments which I have been able to make in these different directions have impressed me with the belief that no very serious obstacle is to be apprehended. Thus I have found that the action of iodine transforms the metallic film into a saffron- coloured compound which is not altered by exposure to TRA TUR 361 light. Bi-chloride of mercury changes it into a greyish powder, which is again darkened by a weak solution of ammonia, and the terchloride of gold increases considerably the intensity of the image, but forms unfortunately, with the gelatine, a compound of a truly Tyrean purple tint. " All these, however, are topics on which I need not dwell, as they will naturally suggest themselves to the minds of those who may deem the subject worthy of investigation." Transparent Cement. Dissolve 75 parts of caoutchouc in 60 parts of chloroform, and add 15 parts of mastic. Transparent Positives may be either taken on glass in the camera, by means of the ordinary wet or dry collodion or albumen processes, or they may be printed on dry plates exposed to light, in contact with a negative. In all cases a negative is necessary. To those who are adepts in the usual negative processes, no special instructions for taking transparencies are necessary. Ttie prints, if not of the desired depth of colour, may be toned after fixing and washing, by pouring on and off a slightly alkaline solution of chloride of gold and again washed. Treacle. The uncrystallisable sugar or syrup obtained from the sugar cane. Golden syrup is a ( thin light-coloured treacle, better adapted for photographic purposes than common treacle. Triplet. See " Lens." Tripoli. The waxen veins, or Spectoriss ludi Jlelmonti, found on the east coast of England, calcined; also the curl-stone of the Staf- fordshire mines, calcined. It contains 80 per cent, of silica, and is used for cleaning and polishing metals, &c. Tnrmeric. The root of an Indian plant, the Curcuma longa. The powder is orange yellow, and the tincture used for making test- papers (q. v.) The colouring matter of turmeric is called ". curcu- mine." Tnrpentine. Crude turpentine is a kind of balsam composed of a resin and a volatile oil, and obtained as an exudation from the wounded bark of various trees, but particularly the tir. It is im- ported chiefly from America. Oil of turpentine is obtained by distilling crude turpentine with water ; the residue left in the still is common resin, and the volatile oil passes over with the steam, with which it mixes. It is a limpid, colourless liquid, sp. gr. '86, and boiling point 314°. It is neutral to test-paper, and almost insoluble in water, but is taken up to a 362 ULT TJEA greater extent by absolute alcohol and ether. It mixes readily with oils, and is very inflammable, depositing a dense soot, which is lamp black, or carbon in a finely divided state. Its composition is C 10 H 8 . It is a solvent of the resins, and to some extent of caoutchouc. Oil of turpentine is acted on energetically by sulphuric and nitric acids, and chlorine ; with hydrochloric acid it forms a curious com- pound, called artificial camphor. It combines with iodine and bromine. Ultramarine. A magnificent blue pigment obtained from a rare mineral, called " Lapis lazuli." Ultramarine, Artificial. This substance, which is chiefly'com- posed of sulphide of sodium, is extensively used in the arts, parti- cularly by paper makers for giving a blue tint to paper. There is a very large manufactory of it at Dusseldorf, where some hundreds of men and women are employed. The following is the account of the process, as described by Dr. Redwood. " Mix together 1 part of porcelain clay, 1| part of sulphur, 1 part of anhydrous carbonate of soda, and keep the mixture at a dull red heat in a covered crucible as long as vapours are given off. On opening the crucible it will be found to contain a spongy mass, part of which will be of a dark -blue colour, and this is to be separated from the other part. The results of this process are not uniform, yet it is considered the best that has yet been published/' Since photographic prints are readily destroyed by an alkaline sulphide, it is evident that the above colouring matter should on no account be added to photographic papers, and yet many of the foreign papers manufactured for Photography are tinted with it. Uranium. U = 60. This metal is obtained from the mineral termed Pechblende, which is an impure oxide of it, and also from uranitic mica. The process consists in acting on the oxide with potassium. It is obtained as a black powder which has a powerful affinity for oxygen. The protoxide of uranium was for some time mistaken for the metal itself, and is not by any means a costly sub- stance. There are five oxides of uranium, viz. : — Suboxide . . . . U 4 0 3 Protoxide . . . . U O Black oxide . . . U 4 0 5 Green oxide . . . . U 3 0 4 Peroxide Sesquioxide Uranic acid. URA 363 The protoxide is a grey or brown powder obtained by passing hydrogen over peroxalate of uranium at a red heat. The salts of the peroxide are reduced to salts of the protoxide by the action of light, as in the case of the iron persalts, so that in this respect uranium and iron are analogous. The hydrated peroxide is a yellow powder. Uranium, Oxide of. U 2 0 3 , N0 5 4- 6 HO = 252. This salt is obtained by dissolving sesquioxide of uranium in nitric acid. It is purified from copper, lead, &c, by passing through the solution a current of sulphuretted hydrogen, which precipitates these metals, but not uranium. The crystals are of a greenish yellow colour, and are soluble hi water, alcohol, and ether. For the so-called Wothlytype process, the nitrate of uranium is purified as follows. To an aqueous solution of it, ammonia is added as long as any precipitate is produced. The precipitate is carefully washed and dissolved in nitric acid, care being taken that the pre- cipitate be in excess, in order that the acid be completely neutralised. The solution is then crystallised. Uranium Glass. Glass is frequently coloured yellow by the addition of oxide of uranium. It possesses the property of " Fluo- rescence," q.v. Uranium Printing Process. To Mr. Burnett, of Edinburgh, we are indebted for the discovery of this process. A sheet of paper is first rendered sensitive to light by immersing it in a strong solu- tion of a salt of the peroxide of uranium (the nitrate is probably the best). It is then dried, and exposed under a negative to direct light for about the same time as an ordinary sun-print upon a chloride of silver paper. A very faint visible image is thus obtained, which is perceived by holding the paper against the light. The print is then placed either in a weak solution of chloride of gold, or in a strong solution of aceto-nitrate of silver. In the former case a picture is obtained of a purple inky tint, and in the latter case of a chocolate brown tint. The print is then washed in abundance of water, several times renewed, and the operation is complete. The theory of the process appears to be as follows : — The uranium persalt is reduced by light to a protosalt, which, when the print is placed in the gold or silver developer, becomes again oxidised, and the gold or silver reduced, either to a purple substance in the case of gold, or a brown substance in the case of silver. The redundant chemicals are then removed by washing. The principal objection to this process appears to be the difficulty of obtaining good surface vigour, and fine definition ; there is also 564 the fear of the lights becoming discoloured in consequence of the imperfect removal of the chemicals from the paper. Uranium prints developed with silver may be intensified by im- mersing them in a solution of protosulphate of iron acidified with acetic acid ; but the lights of the picture are very liable to become discoloured if no fixing agent, such as hypo or cyanide, be employed. The uranium printing process is identical in principle with the Chrysotype process of Sir John Herschel, published in 1 842 ; q. v. The above is the principle of what is now generally denominated the Wothlytype process, patented by Herr Wothly, of Aix-la- Chapelle. The improvements which he has introduced consist in having found out means to preserve the picture on the surface of the paper, and in associating the nitrate of silver or chloride of gold to be reduced with the uranium salt. Ordinary photographic paper receives a supplementary sizing of arrowroot starch ; it is then coated by attaching it to a flat board, with collodion prepared as follows : — Plain collodion .... 4 ounces Castor oil . . . 4 drops Canada balsam .... 2 drops Ammonio-nitrate of uranium . . .160 grains Powdered nitrate of silver ... 6 grains There have been devised many useless complications for prepar- ing the sensitive collodion. The above preparation is as good as any that we have met with. The mixture must be shaken up till the whole of the salts are dissolved, when it is fit for use. Being sensitive to light it must, of course, be kept in a dark place, so also must the paper coated with it until it is required for use. The prints are obtained by pressure in contact with the negative and exposed to light, as in the ordinary printing process, and the progress can be watched just the same, because the developer is here conjoined with the salt to be reduced. The proofs can be toned in alkaline chloride of gold as usual. It has been supposed that no fixing farther than washing in plenty of water, to remove the uranium and soluble silver salts, is necessary for such prints. This is a mistake. Some of the nitrate of silver has entered into combination with the organic and other salts in the collodion and paper. Hence treatment with some one of the usual fixing agents is necessary, and a thorough subse- quent washing. Proofs printed by this process certainly possess a remarkable VAR 365 richness of tone, although not superior to the best prints on albumen- ised paper. Neither can they be more permanent, because the same substances — viz., gold and silver, form the image in each instance. As to economy, the advantage is much in favour of the old method. Instead of nitrate of silver, chloride of gold may be substituted in the sensitising or developing medium. In this case, the image will consist entirely of gold, and will probably possess absolute per- manence ; but the feeble blue tone produced even in the deepest shadows is not conducive to high pictorial effect. Varnishes. An excellent practical account of the manufacture of varnishes, by Mr. J. W. Neil, will be found in the 49th volume of the Transactions of the Society of Arts. But the photographer is not so much concerned with knowing the particulars of this manu- facture in its various branches, as with knowing the best formula for making a few of the different kinds of varnish which are used in Photography. These are as follow : — Benzole Varnish. This may be applied to glass plates without the application of heat, and it dries very quickly, Leaving a tolerabi? hard film, which does not become sticky at ordinary temperatures. It is made by adding finely pulverised gum darnmar to pure benzole. The dammar readily dissolves, and the varnish may then be filtered through cotton wool to separate any solid particles there may be in it. The proportions are about 1 ounce of dammar to a pint of benzole. The varnish is applied to the plate exactly in the same way as collodion. Instead of dammar, finely powdered amber may be employed, and this varnish will be found better than amber dissolved in chloro- form, though probably not so good, and more costly than that made with dammar. Spirit Varnish. This is the best varnish for photographs upon glass, but there is some little trouble and risk in applying it. It is made thus : — Put into a glass flask Alcohol, sp. gr. '825 . . 20 fluid ounces. Pulverised white lac . . \\ ounce. Put the flask into hot water, having previously wrapped a piece of paper loosely over the mouth of it. The ingredients are soon dissolved, and may then be filtered by passing the varnish through cotton wool. Instead of white lac, seed-lac may be employed, but the varnish 366 VAR VIS is then of a darker colour. Methylated spirits may be used as the solvent. French polish diluted with an equal part of alcohol makes a good spirit varnish for negatives. Before applying the varnish the plate must be dried and heated before the fire to a temperature of about 100°; not more, or there will be a risk of destroying the picture by causing it to run in smears down the glass when the superfluous varnish is poured off the plate into the bottle. On the other hand, if the plate be not heated sufficiently, the varnish will be chilled, and produce the effect of ground glass. The plate must be warmed again while the varnish is drying. When spirit varnish is properly applied, it forms an exceedingly hard and tough film, which it is difficult to scratch or injure during the process of printing, and which does not become sticky by heat. Black Varnish. This is made by dissolving in one bottle pow- dered asphaltum in benzole, and in another, india-rubber in benzole ; then adding the latter to the former in such proportion as may seem best, the object of the india-rubber being to prevent the black varnish from cracking. The india-rubber should be cut into small pieces, and left two or three days to dissolve in the benzole, which it does without heat. Varnishes may be divided into two classes— viz., fat varnishes and spirit varnishes ; and the latter class may be subdivided into two, in one of which alcohol is the solvent, and in the other turpentine, and analogous substances. The principal fat varnish used by painters is copal, and the prin- cipal spirit varnish mastic, but neither of these is suitable for photo- graphic purposes. Varnish for maps and drawings may be made by adding turpentine to Canada balsam in about equal parts, and gently heating the mixture. The paper should be sized with gelatine before applying the varnish. Another paper varnish may be made by digesting together — amber, 300 parts ; camphor, 1 part; alcohol, 1500 parts. White lac dissolved in borax may also be used as a paper varnish. Another kind of paper varnish may be made from Xyloidine. Paper may be varnished by floating it upon albumen, drying it, and then coagulating the albumen by floating the back of the paper upon boiling water. Vinegar. See " Acetic Acid." Vision. (Latin, videre, to see.) The phenomenon of human vision will be described under two heads ; viz., monocular vision, and binocular vision. VIS 367 Monocular VisUn. "Vision is said to be " monocular " when only one eye is employed. The human eye may be considered as a sort of spherical camera obscura, in which the pupil (or little black dot in the centre of the eye), is the diaphragm, and the retina the focussing screen, this admirable natural camera being then placed within a socket lined with fat, in which it works by means of voluntary muscles that are attached to it, and by which its axis is directed towards any point with astonishing rapidity and precision. The following account of the human eye is so excellent, that we extract it verbatim from a " Treatise on Optics," published by the Society for the Diffusion of Useful Knowledge : — " The human eye, of which a vertical section is given in the fol- lowing figure, is nearly of a globular form, with a slight elongation or projection in front. It con- sists of four coats or mem- branes, viz., the Sclerotic, the Cornea, the Choroid, and the Retina; of two fluids or humours, the Aqueous and the Vitreous ; and of one lens, called the Crystalline. The Sclerotic coat, a a a, is the outer and strongest coat, to which the muscles for giving it motion are attached. It constitutes the white of the eye. It is joined to the Cornea, b b, or the clean and transparent circular membrane through which we see. The cornea, which is equally thick throughout, is very tough, and consists of several layers or folds to give it strength, so as to defend the delicate parts within from external injury. On the inner surface of the sclerotic coat is a delicate membrane, called the Choroid coat, which is covered with a black pigment. On the inner side of this lies the Iletina, rrrr, which is the innermost coat, and is a tender reticular membrane, formed from the expansion of the optic nerve, which enters the eye at 0, a little more than one-tenth of an inch from the axis on the side towards the nose. At the end of the axis of the eye, and in the very centre of the retina, there is a small hole, with a yellow margin. It is called the foramen centrale, or central hole, though it is not a hole but merely a transparent spot, free of the soft pulpy matter of which the retina consists. "A flat membrane of a circular form, ef \ called the iris, and seen through the cornea b b, divides the interior globe of the eye into two 368 VIS very unequal parts. It has a circular opening; in its centre called the pupil, which expands when the light which enters the eye is diminished, and contracts when the light is increased. The space before the iris, called the anterior chamber of the eye, contains the aqueous humour, from its resemblance to pure water ; and the space behind the iris is called the posterior chamber, and contains the crystalline lens, c c, and the vitreous humour, which fills all the rest of the eye. The crystalline lens is suspended in a transparent capsule, or bag, by what are called the ciliary processes, g g. This lens is more convex behind than in front, as the figure shows ; and it con- sists of concentric coats composed of fibres. It increases in density from its circumference to its centre, for the purpose of correcting its spherical aberration. The vitreous humour, V V, occupying the largest portion of the eye, lies immediately behind the crystalline lens, and fills the whole space between it and the retina, rrrr. The following are the dimensions of the eye, as given by Dr. Young and M. Petit :— English inches. Length of the optical axes ..... 0 91 Vertical chord of the cornea . . . . . 0 45 Versed sine of ditto . . . . .011 Horizontal chord of the cornea . . . . . 0 - 47 Opening- of pupil seen through the cornea . . 0*27 to 0*13 Diminished by magnifying power of cornea to . 0'25 to 012 Radius of the anterior surface of the crystalline lens . . 0 30 R adius of the posterior surface . . . 0 22 Principal focal distance of the lens .... 1*73 Distance of the centre of the optic nerve from the central hole at the end of the axis ..... 0*11 Distance of the iris from the cornea . . . O'lO Distance of the iris from the anterior surface of the crystalline 0'92 Range of the eye, or diameter of field of vision . . . 110° " Dr. Brewster and Dr. Gordon took the following measures of the crystalline and cornea from the eye of a woman above fifty years of age, a few hours after death; — Diameter of the crystalline .... 0*378 Diameter of the cornea . . . . , 0-400 Thickness of the crystalline .... 0172 Thickness of the cornea . . . . 0 042 " The following are the refractive powers of the humours of the eye, according to different observers : — Aqu ous Humour. Hauksbee . 1 33595 Jurin . . 13333 Rochon . 1329 Young. . 1-3333 Brewster . 13366 Crystalline Lens. Outer Coat Centre Mean 1-3767 13990 1 3839 Vitreous Humour 1- 1-332 13394 VIS 369 " Prom the last of these measures we may deduce the following indices of refraction : — Index of Refraction. For rays passing from the aqueous humour into the outer coat of the crystalline lens .... T0466 For rays passing from the aqueous humour into the crystal- line, taking its mean index of refraction . . T0353 For rays passing from the outer coat of the crystalline into the vitreous humour ..... 0 93 " From the dimensions of the eye given above, and by means of the preceding indices of refraction, it will be easy to trace, by the method already described, the progress of rays through the humours of the eye, whether they fall upon it in a parallel or a diverging direction. " Let MN, for example, be an object at a considerable distance from the eye, EFO. Kays of light diverging from the points M N, will be converged by the refraction of the humours to points m, n, upon the retina, where they will form an inverted image of it, in the same manner as an image is formed in a camera obscura. That such an image is actually formed on the back of the eye may be easily proved by paring away the sclerotic coat of the eye of an ox with a sharp knife, till it is sufficiently thin to allow the image to be seen through it. " In what manner the retina, thus impressed with a distinct image of an external object, conveys to the mind, through the medium of the optic nerve, of which it is the expanded termination, a knowledge of the existence, the position, and the magnitude of that object, is not known, and probably never will be. Certain facts, however, or laws of vision, have been deduced from observation, and merit our attentive consideration. " 1. On the direction of visible objects. — When the mind sees the extremity M of any object MN, by means of rays flowing from M and collected at m, the retina, receives these rays at different degrees of obliquity, and yet the point M is seen only in one direction, namely, in the direction of the central rays of the cone, whose apex is at in. This, however, does not arise from the ray being the resultant, as it were, or the mean of the directions of all the other rays ; for if we close up all the pupil excepting a small opening at its U B 870 VTS margin, the point M will be represented at m only by the most oblique rays of the conical pencil, and yet it will still be seen in the same direction as before. Hence we conclude, that when a ray of light falls upon any point m of the retina, in any direction, however oblique to its surface, the object will be seen in the direction of a line perpendicular to the retina at the point m. As the surface of the retina is a portion of a sphere, these perpendiculars must all pass through one point which may be called the centre of visible direc- tion ; because every point of an external object will be seen in the direction of a line joining that centre and the given point. The truth of this we have established by marking the perfect stability of the image of any object, when it is seen by different points of the retina when the eyeball alone is moved. Hence the centre of visible direction is a fixed point in the vitreous humour ; and as it never changes its place during the rotation of the eyeball, it must be coincident with the centre round which that rotation is performed. In consequence of this coincidence, and in virtue of the law of visible direction, an arrangement of consummate skill, the great Author of nature has provided for the perfect stability of every point in the images of external objects. "2. Cause of erect vision. — As the humours of the eye act exactly like a convex lens of an equivalent focal length, an inverted picture of external objects will, for the reasons already assigned, be formed upon the retina. Many philosophers of eminence have perplexed themselves very unnecessarily, into attempting to deduce erect vision from inverted images. The law of visible direction removes at once every difficulty ; for as the lines of visible direction must necessarily cross each other at the centre of visible direction, those from the lower part of the image must -go to the upper part of the object, and those from the upper part of the image go to the lower part of the object, and hence an erect object is the necessary result of an inverted image. " 3. Distinct and indistinct vision in the same object. — When we look intensely at any point of an object in order to examine it with care and attention, we direct to that point the axis of the eye, and consequently, the image of that point falls upon the central hole in the retina. Every other point of the same object is seen indis- tinctly, and the indistinctness increases with the distance of the point from that which is seen distinctly. The only perfectly distinct point of vision, therefore, is that where there is no retina ; but we are not entitled to ascribe this to the absence of the nervous matter, as the gradual increase of distinctness towards the central hole does not appear to be accompanied with a gradual diminution in the thickness of the retina. VIS 371 "4. Indistinctness of vision at the base of the optic nerve. — It was discovered by M. Mariotte, that when the image of any object fell upon the base of the optic nerve, the object disappeared. In order to prove this experimentally, fix. on the side of a room, and at the height of the eye, three wafers, two feet distant. Stand oppo- site to the middle wafer with one eye shut, and, beginning near the wall, retire gradually from it, (looking always at the outside wafer which is on the same hand as the covered eye,) till the middle wafer disappears. This will be found to take place at about five times the distance at which the wafers are placed, and when it does happen, the other wafers will be plainly seen. If we use candles in place of wafers the middle one will not disappear, but it will become a cloudy mass of light. The base of the optic nerve, therefore, is not insensible to light, it is only unfit for giving distinct vision of those objects whose images fall upon it. M. Le Cat considered the size of this portion of the retina to be about one-third or one-fourth of a line; but Daniel Bernoulli found it to be about one-seventh part of the diameter of the eye." The foregoing explanation is clear and accurate, with the excep- tion of some remarks on the direction of visible objects, in para- graph 1. These it is important for us to correct. When a pencil of light from a luminous point M enters the eye and is brought to a focus at m upon the retina, the line of visible direction is not the axis of the cone of rays within the vitreous humour of the eye, but a line drawn perpendicular to the retina at the point, m, and which passes through the centre of the eyeball, or, which is the same tiling, through the centre of visible direction, as correctly stated in the latter part of the paragraph referred to in terms which evidently contradict the former part of the same paragraph. If the reader will draw a figure showing the course of an oblique pencil through the eye, he will perceive that the axis of the final cone of rays within the vitreous humour is a line joining the point m, with a point some- where near the pupil of the eye, while the line of visible direction is a line joining m with a point near the centre of the eyeball. When the retina receives a blow at m, no matter how obliquely, the direction of the supposed force is referred by the mind to a line perpendicular to the retina at m. Binocular Vision. When both eyes are employed simultaneously vision is said to be " Binocular." The principles of it are easily explained, and will be understood by means of the following figure, and a few words of explanation. 2 b 2 372 VIS VOL Let L, R, be the two eyes employed simultaneously. Then only one point can be seen distinctly at any instant of time, and that is the point to which both the optic axes are directed and at which they meet, as shown by the point A in the figure. If B, C, be other points, then the mind is simply warned of their existence by their images upon the sides of the retinae, and distinct vision of them is not obtained. But the eyeballs are capable of being turned in their sockets with extreme rapidity and precision, and the optic axes may be united at several points in suc- cession by muscular motions which are almost instantaneous, so that it is by the comparison which the mind is able to form of the efforts made by the voluntary muscles in enlarging or contracting the angles LAE, LBR, LCR, that the most certain estimate of the comparative dis- tance of near objects is obtained. Hence arises the important difference between monocular and binocular vision, for by means of the latter, additional certainty is given to our appreciation of distance, and hence arise all the remarkable phenomena of stereoscopic effect. As a proof that only one object is seen distinctly at a time, it is only necessary to hold up the forefinger at a distance of a foot from the nose, and look intently at some object several yards beyond it ; the finger will then appear double and transparent, distant objects being seen through it indistinctly. No idea of the distance of an object can be obtained from the changes which take place in the focussing arrangements of a single eye to suit the different distances of objects between the limits of a few inches and infinity. Visual Rays. See " Perspective," Vitriol. " Blue vitriol" is sulphate of copper ; " green vitriol" proto-sulphate of iron ; " oil of vitriol" sulphuric acid ; and " white vitriol" sulphate of zinc. Volume. Thespaceoccupiedbyanysolidbodyiscalled its "volume." Volume has three dimensions, viz,, length, breadth, and thick- ness ; Area tioo dimensions, viz., length and breadth ; Length only one dimension ; and Number no dimensions. VUL WAS 373 Vulcanised India-rubber, or Gutta Percha. These contain sulphur, added chiefly for the purpose of diminishing the stickiness of the surface. Washing Prints. The following mode of washing prints appears to be as good as any that has yet been devised. The account of it was given in " Photographic Notes." " After a print has been fixed in hypo it is of the utmost impor- tance to remove every trace of hypo from the paper, l'or if any be left it will cause the print to fade. In order to remove the hype, the print is generally washed in water changed several times, and allowed to remain in water several hours. But when a great number of prints are to be washed at the same time, a separate dish cannot be provided for each, so it is customary to put a number of prints together in the same vessel of water. When this is done they soon settle to the bottom, and lie there so closely packed that the water cannot easily get between them and soak out the hypo. It is only when the water is agitated by being changed at intervals, and for a few minutes after each change, that it gets fairly between them and acts to advantage. It is desirable, therefore, to keep the water in a state of constant agitation, so that the prints may not lie together ; and this is more particularly desirable since it is found that damp favours the action of hypo in causing a print to fade. The object is therefore to remove the hypo as quickly and effectually as possible. " Various plans have been proposed for accomplishing this end ; and when a constant supply of water is obtainable, as it is in most large towns, it has been a common practice to leave a tap constantly running into the vessel which contains the prints, the overflow pass- into a sink beneath. But the following plan is a great improve- ment on this, because the prints are caused to circulate freely round the vessel, without sticking together, so long as the tap is kept running : — "The vessel is made as in the figure, with sides slightly inclined outwards, like a washing tray. It should be square, and not too deep, or too large for the supply which is to run into it. The jet of water from the tap is directed ob- liquely against the further side of it near the left-hand corner ; it is then reflected to the adjoining side, and thus a rotatory motion of the fluid in the direction of the arrows is esta- blished and maintained. The vessel is of course always full, and the 374 WAS surplus runs over the edge, as shown in the figure. The prints immersed are in this way kept in a state of constant rotation, not en masse, but each print following- its own particular course, without adhering to its neighbour. In order to insure the change of the water at the bottom of the vessel it may be well to make a little hole about the size of a cribbage peg, as shown in the figure." The above method of washing paper proofs is extremely simple and effective. Washing Machines. In order to dispense with the very tedious labour of washing photographic prints by soaking them in many successive changes of water, several ingenious contrivances have been suggested for the purpose of expediting the complete removal of the hyposulphite in which the pictures have been fixed. Mr. Grisdale's patent centrifugal machine is undoubtedly the best that has yet been suggested, and as it represents the best type of all others of the same class, we subjoin his description, copied from the specification in the Patent Office : — " My invention relates to a peculiar construction and arrangement of centrifugal machinery or apparatus for washing photographic prints, and consists, according to one arrangement, in the employ- ment of a peculiarly constructed revolving drum in combination with a trough, in which such drum is partially immersed. The prints to be washed are taken from the water in which they have been placed on their removal from the fixing or other bath, and are packed in one or more piles, which piles are placed round the circumference of the drum, each pile being composed of alternate prints and sheets of wire gauze or other open or reticulated fabric, so that no two prints shall be in contact with each other. These piles are held in their places on the drum by means of open frames or gratings, which bear against the opposite surfaces of each pile, and are secured to the arms of the drum by screws or otherwise, the whole or a portion of such frames or gratings forming part of the drum itself. Or, ac- cording to another arrangement, the piles above described may be laid flat upon a disc, which is made to revolve either vertically or horizontally in a trough or cistern, provision being made in the horizontal arrangement for allowing the piles to be brought in or out of contact with the water as required ; or in lieu of the photographic prints being disposed in the form of piles or packs round a drum or revolving disc, they may be laid separately or individually round the surface of a drum, a webbing of open or reticulated fabric being wound on such drum simultaneously with the placing of the prints thereon, so as to interpose a thickness of the fabric between each WAS WAT 375 succeeding layer of prints. The process of washing consists in alternately driving out the moisture from the prints by the centrifugal action of the revolving drum or disc, and saturating the prints again. During the first part of the process the prints are not immersed, but when the second part of the process, namely, the saturation, is to be effected, the trough or cistern is to be supplied with water, or the prints may be brought down into the water, and caused to revolve therein until thoroughly saturated, when the water may be run off from the trough again, or the drum or disc elevated and the moiscure expelled by centrifugal force as before." About a quarter of an hour's washing in the above described machine is considered sufficient for the object aimed at. Its action is so simple that a boy can work it with perfect ease, and we bave proved from practical experience that its efficiency is all that can be desired. There only remains one subject of doubt, and that is as to how far the centrifugal force has a tendency to disintegrate the tex- ture of tender fibred paper placed between alternate layers of wire or other gauze. We think this objection will be a serious one when, the very brittle thin Rive papers are used for printing. Water. HO = 9. Water may be considered as the protoxide of hydrogen, being composed of one atom of oxygen and one of hydrogen ; or, which is the same thing, of two volumes of hydrogen and one of oxygen. Water in its ordinary state exists either as rain, river, or spring water. Rain-water always contains carbonic acid, ammonia, organic matters, and sometimes nitric acid. It becomes putrid when kept. If collected in leaden vessels, oxide of lead is readily formed and a small quantity dissolved by it. Rain-water is not suitable for the nitrate bath or developer, although perhaps better than spring- water, since it does not contain salts which form precipitates with nitrate of silver. River and spring waters contain various salts and organic impuri- ties ; the principal salts being chloride of sodium, sulphate of lime, and carbonate of lime dissolved in carbonic acid ; these produce pre- cipitates or turbidity in the water when nitrate of silver is adiled to it. Carbonate of lime may be thrown down by boiling the water, which expels the carbonic acid which keeps it in solution. Water is said to be M hard " when it contains salts of lime. Water may be purified by distillation and condensation of the steam, provided it does not contain volatile oil or empyreumatic matter, which impurities are generally present in the distilled water obtained by condensing the steam from steam boilers. Perfectly 376 WAT WAX pure distilled water for delicate chemical operations is obtained by the redistillation of ordinary distilled water in silver vessels at a low temperature. It is then perfectly neutral to test paper, and a cubic inch, of it at 60° weighs 252*45 grains. Professor Tomlinson gives a very ingenious method of detecting the presence of greasy matter or empyreumatic oil in distilled water. A small piece of camphor thrown on the surface spins round if the water is free from these impurities, but if it contains them this motion is prevented. Water is very slightly compressible, and when submitted to sudden and violent compression becomes luminous. It freezes at 32°, and boils at 212°. In freezing, or crystallising, it expands in bulk, therefore ice floats. Water, in freezing, rejects impurities, or salts held in solution, therefore melted ice is very pure water. The combustion of 2 volumes of hydrogen and 1 of oxygen produces 2 volumes of steam. At a mean atmospheric pressure, and tempera- ture 212°, the bulk of steam is 1689 times that of water. Wax. This substance is supposed to be produced by the bee from the honey which it collects. Its composition is stated by Lowig as C 34 H 34 0 2 ; but it varies, although in every case but one there are as many equivalents of carbon as of hydrogen. Bees' wax in its ordinary state is yellow, but is bleached by being exposed in thin ribands to the joint action of air, light, and moisture. Sometimes, however, it is bleached more expeditiously by mixing with it nitrate of soda and dilute sulphuric acid, and then traces of sulphate of soda and nitric acid are generally retained. Wax melts at 150°, and is almost entirely soluble in boiling alcohol and ether; but, on cooling, the alcohol only retains a small quantity, and the ether none. When caustic potash is added to melted wax, a soapy gela- tinous substance is formed which is soluble in a large quantity of water. On adding an acid, an oily liquid forms, which solidifies on cooling, and is soluble in hot alcohol ; it is probably the " Ceraine " of Ettling. Succinic acid is obtained by digesting wax for some days with nitric acid. Many vegetables secrete wax from their leaves and stems. In- stances of this occur as myrtle-wax, palm-wax, Japan-wax, Ocuba- wax, sugar-cane-wax, cork-tree- wax, &c. White wax is commonly sold in round flat cakes, and is frequently adulterated with tallow, stearine, &c. Wax may be mixed in any proportions with oil of turpentine. Positive prints, or sheets of paper rubbed with this mixture, acquire a high polish on the evaporation of the turpentine. The ancient "WAX WHI 377 fresco paintings upon the walls of houses were varnished and pre- served with wax. See " Encaustic Painting." Waxed Paper Process. See " Calotype Process." Waxing Apparatus. In the paper negative processes the paper is generally waxed either in the first or last stage of the operation. The best mode of doing this is to procure an oblong apparatus of zinc or pewter, made exactly on the principle of a hot water plate. This is filled with boiling water, and placed over the flame of a spirit lamp to maintain the temperature. The sheet of paper or paper negative to be waxed is then laid upon the bottom of the upper part of the apparatus, and a cake of wax rubbed over it, until the paper has completely imbibed the wax. It is then removed, and a fresh sheet introduced. \\ hen a sufficient number of sheets have been treated in this way, they are held, one at a time, before the fire, and the superfluous wax which runs off is collected in a saucer. They are afterwards ironed between sheets of blotting paper, with a moderately hot iron, until no shining patches of wax appear on either side of the paper. Weights and Measures. It is a very unfortunate circumstance that these are not uniform all over the world. In our country the standard of lineal dimensions is the length of a pendulum vibrating seconds in the latitude of Greenwich Observatory. From this all measures of capacity and weight can be calculated, from a given bulk of distilled water taken at a certain temperature. The French linear standard is one metre. It is the ten-millionth part of a quarter of the earth's meridian, and measures 39 371 English inches. This is divided or multiplied by ten, and thus constitutes a most convenient method for calculating distances. The French measures of capacity and of weight are derived from the same standard. See Tables in Appendix. Whey. See " Serum of Milk." White Enamel. Glass rendered white and opaque by the addition of oxide of tin. When ground it forms a beautiful substance to print upon, the negative being copied by a lens, and the wet collodion process employed. White Lead. Carbonate of lead. This substance should never be ground by hand, but by machinery, as the minute particles which escape into the air are fearfully injurious to the health. The pallid, sickly appearance of painters is mainly attributable to the 378 YEL extensive use of white lead in paint. Zinc paint is not open to the same objection. White Fire. Mix together 24 parts of saltpetre. 7 parts of sulphur. 2 parts of realgar (red sulphide of arsenic). Wood Spirit. See " Pyroxilic spirit." Wothlytype Process. See " Uranium printing." Xyloidine. Same as pyroxyline q.v. Yellow Calico, Paper, &c. When used for photographic pur- poses the tint should verge on orange rather than green. Two layers at least should always be employed, because white light passes through the innumerable interstices between the fibres of the calico. Yellow calico is generally bleached by light, and requires to be renewed from time to time. 379 APPENDIX. TABLE of Elementary Bodies, with their Equivalents ; {The non-metallic elements are printed in Italics.) Symbol. Name of Substance. "3 > % a 1 w & | til XT „ _ , .Name of Substance. Equivalent. Al. Aluminium .... 13-75 Mercury( Hydrargyrum ) 100 Sb. Antimony (Stibium) . 122 Mo. Molybdenum 48 As. Arsenic 75 ! Mi. Nickel . 28 Ba. Rarin in XJiXL llliil ..... 68-5 Nb. 1^ lnniii m ? Bi. Bismuth 213 N Nitrogen 14 B J-tKtl W IV ..... 11 Os. Osiniii m 100 Br. Bromine 80 1 0 Oxygen . 8 Cd. Cadmium .... 56 Pd. Palladium . 54 Ca. Calcium 20 P Phosphorus .... 31 C Curbon 6 Pt. Platinum 98-6 Ce. Cerium 46 K Potassium (Kalium) . 39 01. Chlovtne ..... 355 R Rhodium 52 Cr. Chromium .... 263 Rb. Rubidium 85 Co. Cobalt 30 Ru. Ruthenium 52 Ta. Oolumbium (Tantalum) 185 Se. Selenium . 40 Cu. Copper (Cuprum) . . 317 Si. 15 Di. Didymium .... 48 Act. Silver (Argentum) 108 Er. Erbium ? Na. Sodium (Natrium) . . 23 F Fluorine 19 Sr. strontium .... 438 G Glucinum .... 5 S 16 Au. Gold (Aurum) . . . 196-6 Te. Tellurium .... 64 H Hydrogen 1 Tb. p n. Ilmenium .... ? Th. 60 i Iodine 127 Sn. Tin (Stannum) . . . 59 Ir. Iridium 99 Ti. 24 Fe. Iron (Ferrum) . . . 28 W. Tungsten (Wolfram) . 100 La. Lantanium .... 44 U 60 Pb. Lead (Plumbum) . . 1035 V Vanadium .... 68 Li. Lithium Y 32 Mg Magnesium .... 12-16 Zn. 32-7 Mn Manganese .... 27-5 Zr. Zirconium .... 23 380 BAUME'S HYDROMETER, OR AREOMETER. table showing the relation between specific gravities and Degrees of Batjme's Hydrometer, for Liquids heavier than water. S.G. B. S.G. B. S.G. B. S.G. B. S.G. B. 1 .AAA A V _L- xuo — 1ft 1 -9ftfi J. iiOO QO €)Li 1 -^01 1 .QA1 JL oul — f\A O* 1 .AA7 1 x J. ±0"± 1 -9Q8 J. ^yo qq — Do JL 0£\J — 1Q 1 .Q9q DO 1 .A1/f X O 1 x<±o IS 1 -30Q — 31 1 -^39 X OOu — ^0 — o\j 1 .Q^7 X - 4 o o Chloride of Calcium . . o „ ) Snow . • ... • • JP arts |from32°to-30° Diluted Nitric Acid . . 4 „ ) Snow 3 parts") from + 32 o to -51° Potash 4 „ ) 387 ENGLISH MEASURES. LINEAL MEASURE. The imperial yard bears to the length of a pendulum vibrating seconds of mean time in vacuo, at the sea level, in the latitude of London, the ratio 36 : 391393. 1 inch 12 = 1 foot 36 = 3 = 1 yard 72 = 6 = 2=1 fathom 198 = 16j= 5j= 1 pole, perch, or rod 7920 = 660 = 220 = 40 = 1 furlong 63360 : 5280 = 1760 = 320 = 8 = 1 mile IMPERIAL MEASURE OF CAPACITY. The imperial gallon is the volume occupied by 10 pounds avoir- dupois weight of distilled water, weighed in ah- at 62° Faht., the barometer being at 30 inches. Equivalents in prrains. Distilled water 62^> Faht. 1 pint 8750 2 = 1 quart 17500 8 = 4=1 gallon .... 70000 16 = 8=2=1 peck . . . 140000 64 = 32 = 8 = 4 = 1 bushel . . 560000 512 = 256 = 64 = 32 = 8 = 1 quarter . 4480000 APOTHECARIES MEASURE OF CAPACITY. 1 minim 60 = 1 fluid drachm . 480 = 8 = 1 fluid ounce . 9600 = 160 = 20 = 1 pint . 76800 = 1280 = 160 = 8 = 1 gaUon Grains. Distilled water at 62° Faht. 091 . 547 . 437-5 . 8750- 70000- IMPERIAL CUBIC MEASURE. Equivalents in Equivalents in prams, cubic inches. Distilled water at 62° Faht. 1 gallon = 277-274 = 70000 1 quart = 69-3185 = 17500 lpint = 34-65925 = 8750 16 ounces = 27*72740 = 7000 1 ounce = 1-73296 = 4375 388 ENGLISH WEIGHTS. The grain is the same in Troy, Apothecaries, and Avoirdupois weights. AVOIRDUPOIS WEIGHT. Equivalent in grains. 1 dram 27*34375 16 = 1 ounce 437*5 256= 16= 1 pound .... 7000- 3584= 224= 14= 1 stone . . 98000- 28672 = 1792= 112= 8 = 1 cwt. . 784000- 473440 = 35840 = 2240 = 160 = 20 = 1 ton . 15680000- TBOY OR JEWELLER'S WEIGHT. 1 grain. 24 = 1 pennyweight. 480= 20= 1 ounce. 5760 = 240 = 12 = 1 pound. APOTHECARIES WEIGHT. Symbols. 1 grain . gr, 20 = 1 scruple 3 60= 3= 1 drachm ..... 3 480 = 24 = 8 = 1 ounce I 5760 = 288 = 96 = 12=.! pound. ... lb Observe : 437*5 grains are considered equal to 1 ounce of nitrate of silver, the only legal table of weights being the Avoirdupois. The other tables are only occasionally used for compounding medi- cines, &c, but probably they will soon be discontinued. 389 FRENCH WEIGHTS. Troy Weight. Troy Grains. Lbs. Ozs. DrmB. Grains. Millegramme •0154 Centigramme •1543 Decigramme 1-543 = 1-5 Gramme 15-432 = 154 Decagramme 154-323 = 2 34 Hectogramme 1543 234 = 3 1 43 Kilogramme 15432-348 = 2 8 1 12 Myriagramme 154323-488 m 26 9 4 3 FRENCH MEASURE OF LENGTH. English. English Inches. Miles. Furlongs. Yards. Feet. Inches. Millimetre = •03937 Centimetre = •39371 Decimetre = 3-93708 Metre = 39-37079 = 1 0 3-7 Decametre = 393-70790 = 10 2 9-7 Hectometre — 3937-079 = 109 1 1 Kilometre = 39370-79 = 4 213 1 10 Myriametre = 393707-9 6 1 156 0 0 FRENCH MEASURE OF CAPACITY. English. English Cubic Inches. Gallons. Pints. Fluid ozs. Drms. Minims. Millitre •0610 = 169 Centilitre •6103 = 2 50 Decilitre 6-1027 = 3 4 13 Litre 61-027 = 1 15 2 11 Decalitre = 610-27 = n 1 12 5 51 Hectolitre = 6102-7 = 22 0 7 3 8 Kilolitre = 61027- 220 3 13 7 30 Myrialitre = 610270- 2204 4 10 3 390 G\l Gvl OJ OJ CM CM (M Cvl C\l OJ C\l CM C\l 00050H(MCO^iOCOI>00 05 0 CMCMCMCMCMCMCMCMCMCMCOCOCOCOCOCOCOCOCOCO^ I>Gli^CO^COOOO^^iOt^05rHCCxo00050H(MCO^)0(£iNOOOi 05OOOOOOOOOOOHHHHHHHHHH COQOO(MCOiOI>0)HCOi01>OOOCQTf(COOOOHO:iC CONOOOlOHCqCO^iOCCNOOOiOHtMeO^iOCON t^t^r>t^GoooooaooooooooooooooiOi<3io:oiOia5a5 WCOOOO(M^COXO(MCOiOI>OiHMiONOOO(M^ OOOr-iHHHHtMCKMOlWiClWMWCCM^^^ 5 ^iiOCON00050HMCO^iOCOI>OOQOH(MCO^iO xO^^^^^ < ^CO ( X> ( X>^0 ( X> ( X>CC>COCDl>'l>.t^J>.I>.I>. COiOI^050CM^COOOOCM^COOOC5rHCOiOJC^C2rHCO C0OC0C01>t>I>I>I>MC00000000005aJ050505OO OOOOOOOOOOOOOOOOOOOOrHi— I G^CO-^iOCDt^OOOiOt— IClCO^iit)COI>.OOOiOi— I a5i— I o q