■ ’ V Digitized by the Internet Archive in 2015 https://archive.org/details/populartreatiseoOOhunt PUBLISHED BY R I C H AR 0 CH I FFI H iCPCLASCO « 0.1 fljLUltlt.. A POPULAR TREATISE ON THE ART OF PHOTOGRAPHY, INCLUDING % DAGUERREOTYPE, ALL THE NEW METHODS OF PRODUCING PICTURES BY THE CHEMICAL AGENCY OF LIGHT. ROBERT HUNT, SECRETARY OF THE ROYAL CORNWALL POLYTECHNIC SOCIETY. ILLUSTRATED BY ENGRAVINGS. CONV . N H * H5t GLASGOW: PUBLISHED BY RICHARD GRIFFIN AND COMPANY. MDCCCXLI. INTRODUCTION. The announcement of the discovery of a process by which light — the most subtile of the elements, the mysterious agent of vision — was made to pencil, on solid tablets, the objects it illuminated, and permanently fix the fleeting shadow, possessed, at the same time, so much of the mar- vellous and beautiful, as to excite more than common wonder. Nor was this feeling in any way lessened by the examination of the first specimens of the new art, which were presented to the world by Mr. Talbot, — so perfect in their outline, so minute in their detail, were these early photographic * delineations. But when the delicate pictures produced by M. Daguerre became public, words were weak to express the general admiration of those beautiful things, which infinitely sur- passed, in their exquisite finish, the most perfect specimens of human art ; and which, in their distribution of light and shadow, and the magic of atmospheric effect, had a charm almost equalling that of the living landscape in its richest aspects. At the same time, these pictures con- vinced one of the strict fidelity of those productions which the pencils of some of the old masters have given to the world. All men, even the most unintellectual, are sensible to the influences of beauty, whether presented to their view in the completeness of Nature’s works, or in the approaches of human effort towards perfec- tion. There is, with men in general, a soul-felt wish to approach to the excellence they admire, but it is too frequently trammelled by the bonds of ignorance and sensuality. The desire being but seldom seconded by persevering industry, is unfortunately too often abortive — the approach to perfection being granted only as the reward of labour. The photographic processes appeared, when first reported, to be so * Photographic, or as they are sometimes called, Photogenic drawings, signify light delineated, or light generated pictures. IV INTRODUCTION. simple, that most persons conceived they could procure, without trouble, specimens of equal beauty with those exhibited by the artist and the philosopher ; and the desire of which we speak, was at once manifested in an unusual degree. It requires but the slightest consideration to convince us, that an element inappreciably subtile, must, in its action on chemical preparations, be affected by the most trifling change ; and that differences beyond detection by any other test, would become glaringly evident under the influence of light. Failure damped the ardour of the pursuit, and owing to the uncer- tainty of the results with the sensitive paper, and the delicacy of the manipulation required for the silver plate, coupled with its most unfor- tunate expense, the enthusiasts of the moment wearied, and at length resigned the task they had felt so certain of accomplishing, displeased that they had met with difficulties, where none were anticipated. This is scarcely to be regretted ; for that which is too easily obtained is rarely prized, and becoming common, it does not afford that stimuli which impels men onward in their pursuits, and leads to the improve- ment of every department of science or . of art. Photography does not possess the advantages of perfection, any more than other human inventions. Had it been left where we found it when the discovery was announced, it would have remained a beautiful, but almost useless thing —a philosophic toy, which lent a little assist- ance to the cultivation of taste, but afforded none to the economy of manufactures : whereas it now promises to be of important use to many of the arts of industry. The multiplication of pictures from an original photograph is the great end of the art. The attempts made to engrave the Dagu- erreotype plate are all of them, to a certain extent, failures, the finer details being lost, and the “ cerial perspective ” entirely destroyed. Indeed, the employment of strong nitric acid, to etch the tracery marked out by the magic finger of light, appears much as if we were to employ a smith to rivet the downy feather to the wing of the butterfly. It appears natural to suppose that the picture drawn by light must be multiplied by the same agent ; and that it is to processes on paper, similar to those of our countryman, Mr. Talbot, or on some transparent substances, as glass, in the way suggested by Sir John Herscliel, that we must direct our attention, if we wish to arrive at photographic pub- lication. INTRODUCTION. V “ All noble growths are slow,” is .the remark of an American moral philosopher, of which a thousand examples proclaim the truth ; and, looking at the progress of our new art in the very infancy of its being, what may we not expect from its maturer age. It is with the view of arranging all the various processes which have been devised, into a systematic form, that the present treatise has been undertaken. Desirous of promoting the study of the art of photo- graphy to the utmost of my ability, the greatest care has been taken in verifying the different manipulatory processes which I have introduced. Nothing is inserted which has not been put to the test of many ex- periments, from which it is hoped that this publication will render real assistance, not only to those whom it may induce to experiment in photography, but even where the practice of the art has given a con- siderable degree of certainty in manipulation. I arrogate not to myself any superiority in this respect, — far from it; the constant difficulties I have encountered, particularly from missing the exact proportions for producing any desired effect, was the first inducement to the present arrangement; and finding it of much use myself, and learning from many quarters that some popular information on the subject was required, I have studied to form a manual which might be extensively useful. It appears necessary to state in this place, that throughout the volume the terms positive and negative , as suggested by Sir John Herschel, are used to express, respectively, pictures in which the lights and shades are as in nature, and in which they are the opposite, lights being repre- sented by shades, and shades by lights. The Frontispiece contains examples of both these kinds of pictures. From the same authority the terms direct and reversed are also borrowed, to indicate pictures in which, as regards right and left, the objects appear as they do in the original, or the contrary. ROBERT HUNT. CONTENTS. Page HISTORY OF PHOTOGRAPHY, 1 PROCESSES ON PAPER, 6 1. On the Selection of Paper for Photographic Purposes, ... 6 2. Negative Photographs, 9 A. — On the Preparation of the Sensitive Paper with the Salts of Silver, . 9 a. Nitrated Paper , 10 b. Muriated Paper , 11 c. Iodidated Papers , 18 d. Bromidated Papers , 19 e. Phosphated Papers , 21 f Papers Prepared with other Salts of Silver , 21 g. Dr. Schafhaeutl’s Negative Process , 23 B. — On the Methods of Using the Photographic Papers prepared with the Salts of Silver. — Negative Kind, 24 a. On talcing Copies of Botanical Specimens , Engravings , <§•A saturated solution of any hydriodic salt is made to dissolve as much iodine as possible, and of this liquid two drachms are mingled with four ounces of water. Care is required that one side only of the paper is wetted, which is by no means difficult to effect, the fluid is so greedily absorbed by it ; all that is necessary being a broad shallow vessel to allow of the paper touching the fluid to its full width, and that it be drawn over it with a slow steady movement. When thus wetted, it is to be quickly dried by a warm, but not too bright fire ; of course daylight must be carefully excluded. Papers thus iodidated do not lose their sensitiveness for many days, if carefully kept from light. On examining the sheet, after the Daguerreotype processes in the camera, and of mercurialization have been completed, a very perfect picture is found upon it: but it is still capable of vast improvement, which is, by the following simple plan, accomplished in a way which is at once magical and beautiful. Dip one of the Daguerreotype pictures, formed on the sulphuretted paper, into a solution of corrosive sublimate : the drawing instantly disappears, but, after a few minutes, it is seen unfolding itself, and gradually becoming far more distinct than it was before ; delicate lines, before invisible, or barely seen, are now distinctly marked, and a rare and singular perfection of detail given to the drawing. It may appear, at first sight, that the bichloride of mercury dissolves off the metal, and again deposits it in the form of chloride (calomel). But this does not account for the fact, that if the paper has been prepared with the nitrate of silver, the mercury disappears, and the drawing vanishes, the deposit taking place only on those parts upon which light has acted but feebly ; as, for instance, on the venations of leaves, leaving those portions of surface which were exposed to full luminous influence, without a particle of quicksilver. When the paper has been either a 76 MISCELLANEOUS PROCESSES. chloride or iodide, the effect is as above, and the thickness of the deposit is as the intensity of the light has been ; consequently, the semi-tints are beautifully preserved. If the drawing remains too long, in the solution, the precipitate adheres to the dark parts and destroys the effect. The singularity of this operation will be more striking if the picture has been soaked some time in a solution of the hyposulphite of soda, and then dipped into the bichloride of mercury. As the drawing disappears, a series of circles, formed of a white powder, appear to arise from the paper, generally commencing at the centre, and slowly extending over the whole surface : the powder is afterwards deposited, and the sheet is buried in the precipitate ; but on taking the paper from the liquid, and passing a stream of water over it, the precipitate is entirely removed from all the parts, except the lights of the picture. I have also found the invisible photographic image becomes evident, without the aid of mercurial vapour, by simply soaking for some time in a solution of corrosive sublimate. When these papers are prepared with due care, they are extremely sensitive, and if used for copying engravings during bright sunshine, the effect is instantaneous . The great difficulty is to present the paper to the sun, and withdraw it with sufficient celerity. In the weak light of the camera a few minutes during sunshine, is quite sufficient for the production of the best effects. One great advantage of these pictures over those procured on the plated copper is, that the mercury does not lie loosely as on the tablets, but is firmly fixed, being absorbed by the paper ; therefore these pictures may be kept without injury in a port- folio. If, instead of immersing the paper in a vessel full of sulphuretted hydrogen gas, a stream of the gas is made to play upon it, it assumes a most richly irridescent surface ; the various colours are of different degrees of sensibility — but for surface drawings, they may be used — and in copying of leaves or flowers, beautiful pictures, which appear to glow with the natural colours, are procured. 2. — Photographic Processes without any Metallic Preparation. There are many preparations, which are effected by light in a similar manner to the salts of silver. Several have been tried as photographic materials, but as yet without much success, with the exception of the bichromate of potash, which was first announced as a useful photographic agent, by Mr. Mungo Ponton, in the Edinburgh New Philosophical Journal, from which I quote Mr. Ponton’s own account. When paper is immersed in the bichromate of potash, it is power- fully, and rapidly acted on by the sun’s rays. When an object is laid in the usual way on this paper, the portion exposed to the light speedily becomes tawny, passing more or less into a deep orange, according to PHOTOGRAPHS WITHOUT ANY METALLIC PREPARATION. 77 the strength of the light. The portion covered by the object retains the original bright yellow tint which it had before exposure, and the object is thus represented yellow upon an orange ground, there being several gradations of shade, or tint, according to the greater or less de- gree of transparency in the different parts of the object. In this state, or course, the drawing, though very beautiful, is evanes- cent. To fix it, all that is required is careful immersion in water, when it will be found that those portions of the salt which have not been acted on by the light are readily dissolved out, while those which have been exposed to the light are completely fixed on the paper. By the second process the object is obtained white upon an orange ground, and quite permanent. If exposed for many hours together to strong sunshine, the colour of the ground is apt to lose in depth, but not more so than most other colouring matters. This action of light on the bichromate of potash differs from that upon the salts of silver. Those of the latter which are blackened by light, are of themselves insoluble in water, and it is difficult to impregnate paper with them, in a uniform manner. The blackening seems to be caused by the formation of oxide of silver. In the case of the bichromate of potash, again, that salt is exceed- ingly soluble, and paper can be easily saturated with it. The agency of light not only changes its colour, but deprives it of solubility, thus rendering it fixed in the paper. This action appears to consist in the disengagement of free chromic acid, which is of a deep red colour, and which seems to combine with the paper. This is rendered more prob- able from. the circumstance, that the neutral chromate exhibits no similar change. The best mode of preparing paper with bichromate of potash, is to use a saturated solution of that salt ; soak the paper well in it, and then dry it rapidly at a brisk fire, excluding it from daylight. Paper thus prepared, acquires a deep orange tint on exposure to the sun. If the solution be less strong, or the drying less rapid, the colour will not be so deep. A pleasing variety may be made by using sulphate of indigo, along with the bichromate of potash, the colour of the object and of the paper being then different shades of green. In this way also, the object may be represented of a darker shade than the ground. Paper prepared with the bichromate of potash, though as sensitive as some of the papers prepared with the salts of silver, is much inferior to most of them, and is not sufficiently sensitive for the camera obscura. This paper, however, answers quite well for taking drawings from dried plants, or for copying prints. Its great recommendation is its cheap- ness, and the facility with which it can be prepared. The price of the bichromate of potash, is about two shillings per pound, whilst the nitrate of silver is five shillings the ounce. As the deep orange ground of these pictures prevents the permeation of the chemical rays of light,, it is very easy to procure any number of fac-similes of an engraving, by transfer from the first negative photo- graph. The correct copies have a beautiful sharpness, and, if carefully managed, but little of the minute detail of the original engraving is lost. 78 MISCELLANEOUS PROCESSES. The colour of these photographs may be very agreeably varied, by soaking the finished drawing in a weak solution of the nitrate of silver, by which the chromate of silver is formed, — a salt of a bright red col- our — , or i n a solution of the bichloride of mercury, by which a chromate of mercury is formed, which is of a rich purple colour. When the drawings are again dry, they must be washed in water having a very small portion of common salt in it, to remove the silver or mercury from the white parts of the paper. The most interesting variety of photographic paper, prepared with the bichromate of potash, is a kind described by M. E. Becquerel. He states, — It is sufficient to steep a paper prepared in Mr. Ponton’s man ner, and upon which there exists a faint copy of a drawing, in a solution of iodine in alcohol, to wash this paper in alcohol, and then dry it ; then the parts which were white become blue, and those which were yellow remain more or less clear. M. E. Becquerel has pursued his investigations into the action of the chromic acid on organic compounds, and has shown that the mode of sizing the papers influences their colouration by light, and that with unsized paper, colouration is effected only after a long time. Per- ceiving that the principal reaction resulted from the chromic acid con- tained in the bichromate of potash, on the starch in the size of the paper, it occurred to M. E. Becquerel, that, as starch has the property of forming with iodine a combination of a very fine blue colour, it should produce deep shades of that tint, whilst the lights still remained an orange-yellow. His method of proceeding is to spread a size of starch, very uniformly over the surface of the paper. It is then steeped in a weak alcoholic solution of iodine, and afterwards washed in a great quantity of water. By this immersion it should take a very fine blue tint. If this is uni- form, the paper is considered fit for the experiment: in the contrary case it is sized again. It is then steeped in a concentrated solution of bichromate of potash, and pressed between folds of blotting paper, arid dried near the fire. To be effective, it should be very dry. It is now fit for use. When the copy is effected, which requires in sun- shine about five minutes, the photograph is washed and dried. When dry, it is steeped in a weak alcoholic solution of iodine, and afterwards, when it has remained in it some time, it is washed in water, and care- fully dried in blotting paper, but not at the fire, for at a little below 100° Fah. the combination of iodine and starch discolours. If it be considered that the drawing is not sufficiently distinct, this immersion may be repeated several times ; by this means may be ob- tained the intensity of tone that is desired, which intensity cannot be changed at will by employing a more concentrated solution of iodine. When the paper is damp, the shades are of a very fine blue, but when it is dry, the colour becomes deep violet. If while the drawing is still wet it be covered with a layer of gum arabic, the colour of the drawing is greatly preserved, and more beautiful when it is dry. When a paper DR. SCHAFHAETJTL’S PROCESS ON CARBONISED PLATES. 79 is thus prepared, at first it loses a little of its tone, but it afterwards preserves its violet tint. An interesting variety of photographic drawings may be formed by purely vegetable preparations. These are not equal in point of beauty or delicacy to any of those varieties before mentioned, but they form a very satisfactory series of examples of the well known influence of solar light upon some of our beautiful dies and colours. The subject is re- plete with interest to the dyer and the artist, and is certain of being useful to many of our most refined processes of manufacture. Amongst the vegetable colours which are readily affected by light, may be named the alkaline tincture of many of the lichens, particularly of the Lichen Rocellus — the tincture of gum guaiacum, and of the common heart’s-ease, Viola Tricolor — together with several of the alco- holic solutions of the colouring matter of many of the dahlias. It will be found in most instances that the vegetable colour is bleached out ; consequently the object is of the original colour, and darker than the ground. No means of fixing these drawings have yet been discovered. A very speedy way of taking a copy of any object, when we have no ordinary photographic paper at hand, is to take a piece of the best blue demy paper, and dip it in either a weak solution of muriatic acid, or of any of the hydriodic salts — the object being placed upon it, it is then exposed wet to sunshine, and the uncovered parts are soon completely whitened. With some kinds of demy the effect is very rapid. Simple washing in a large quantity of cold water is all that is required to ren- der the drawing secure. 3. — Dr. Schafhaeutl’s Process on Carbonised Plates. Metallic plates are covered with a layer of hydruret of carbon, pre- pared by dissolving pitch in alcohol, and collecting the residuum on a filter. This, when well washed, is spread as equally as possible over a heated even plate of copper. The plate is then carbonised in a closed box of cast iron, and, after cooling, passed betwixt two polished steel rollers, resembling a common copperplate printing press. The plate, after this process, is dipped into a strong solution of nitrate of silver, and instantly exposed to the action of the camera. The silver is, by the action of the rays of the sun, reduced into a perfect metallic state, and the lights are expressed by the different density of the milk-white deadened silver ; the shadows by the black carbonized plate. In a few seconds the picture is finished, and the plate is so sensitive, that the reduction of the silver begins even by the light of a candle. For fixing the image, nothing more is required than to dip the plate in alcohol mixed with a small quantity of the hyposulphite of soda, or of pure ammonia. This process, from its. description, might be considered easy of ac- 80 MISCELLANEOUS PROCESSES. complishment ; and from its extreme sensibility, complete in all the details of picturesque effect. It is neither the one nor the other. The preparation of the plate requires the skill of an artist, combined with the knowledge of a chemist ; and even these are not always sufficient to ensure a perfect surface. The revival of the silver is not to be depended on : sometimes it does form a continuous sheet over the parts acted on by the light, but often it is only spangles ; and frequently a metallic arborescence will commence in the light parts, and run rapidly into the portions in shadow. The fact is, that light has the property of effecting the revival of the silver spread upon any carbonaceous body, but caloric having the same effect, and being indeed rather more active in the operation than light is, any slight increase of temperature pro- duces a revival of the metal over the parts in shadow. Reference to the early volumes of Nicholson’s Journal will afford ample evidence of these facts, which I have also recently proved. These volumes contain some papers by Count Rumford on the revival of gold and silver from their solutions, by light, when spread upon charcoal or carbonaceous earth. This philosopher has conclusively shown, that this revival is more dependent on the action of heat than light, which accounts, in some measure, for the apparent effect of candle light. It is, however, possible, that this process may, with some modifications, become of importance. 4. — A New Construction of the Photographic Camera Obscura. A photographic camera should possess, according to Sir John Her- schel, “ the three qualities of a flat field , a sharp focus at great inclina- tions of the visual ray , and a perfect achromaticity .” There can be no doubt but these qualifications are very essential, — the two first particu- larly are indispensable, and there is but one objection to the latter. We can only produce perfect achromaticity by a combination of glasses, and my experiments go to prove that by increasing the thickness of the object-glass, and the number of reflecting and refracting surfaces, we interrupt a considerable portion of light, and consequently weaken the action on the photographic material, whatever it may be. It is with considerable reluctance that I express myself somewhat at variance with so high an authority as Sir John Herschel, gifted as he is with the highest power of physical research ; I am however satisfied, that we may to a considerable extent get rid of the difficulties of chromatic disper- sion, without having recourse to a combination of glasses of different refracting powers. I have long used myself, and constructed for others, a camera obscura, which appears to answer remarkably well. It is but right I should acknowledge that I am indebted to the suggestions of Dr. Wollaston, for part of my lenticular arrangement. «, Figure 26, represents the aperture of the lens ; i i', a box sliding into an outer NEW CONSTRUCTION OF TIIE PHOTOGRAPHIC CAMERA OBSCURA. 81 Fig. 26. case, h h ' ; k k, a third division, containing a ground glass at the back, and a door which can be raised or lowered by the screw g, the whole fitting into the frame h h'. Figure 27 is a section of the camera, a, is a lens of a periscopic form, whose radii of curvature are in the proportion of 2 to 1. This Fig. 27. meniscus is placed with its convex surface towards the plane of repre- sentation, and with its concavity towards the object. The aperture of the lens itself is made large, but the pencil of rays admitted is limited by a diaphragm, or stop, constructed as in the figure at b, between it and the plane of representation, at about one tenth of the focal length from the lens. By this arrangement objects are represented with considerable distinctness over every part of the field, but little difference being Fig. 28. observable between the edges and the centre, c is the plate of ground glass at the back, which serves to adjust the focus, and also to lay the photographic paper on, when we desire to copy any object; d, a door to shut off the light from the paper or plate, until the moment we desire to expose it to lumin- ous agency. Figure 28 represents this screen or door more perfectly, in the act of falling; e is a door at the back, through which the picture formed on the opaque glass is examined; f a pin, keeping the door, d, in its place. M 82 MISCELLANEOUS PROCESSES. With the kind of lens here recommended, and the light thus stopped off, and adjusting the camera to the focus of the violet ray, it will be found that most of the advantages of the achromatic lens are obtained, and we get rid of some of its defects. A camera of this description, capable of forming a picture 12 inches by 10, may be constructed for from thirty to forty shillings, which is about the expense of a good achromatic lens. 5. On the possibility of producing Photographs in their Natural Colours. Few speculations are more replete with interest, than that of the pro- bability of our succeeding in the production of photographic images in their local colours. M. Biot, a great authority, says, — “ substances of the same tint may present, in the quantity, or the nature of the radia- tions which they reflect, as many diversities, or diversities of the same order, as substances of a different tint ; inversely, they may be similar in their property of reflecting chemical radiations, when they are dis- similar to the eye ; so that the difference of tint which they present to the eye may entirely disappear in the chemical picture. These are the difficulties inherent in the formation of photographic pictures, and they show, I think, evidently, the illusion of the experimenters who hope to reconcile not only the intensity, but the tints of the chemical impres- sions produced by radiation, with the colours of the objects from which these rays emanate.” It may be remembered that two years since, Sir John Herschel succeeded in procuring upon photographic paper a col- oured image of the solar spectrum ; and that eminent enquirer has com- municated to me a recent discovery of great interest, which I have his permission to publish. “ I have got specimens of paper,” says Sir John Herschel, “ long kept , which give a considerably better representation of the spectrum in its natural colours , than I had obtained at the date of my paper, (February, 1840,) and that light on a dark ground; but at present I am not prepared to say that this will prove an available pro- cess for coloured photographs, though it brings the hope nearer Here we have the speculations of one philosopher representing the production of such pictures as hopeless, while the experiments of another prove these to be within the range of probabilities. My own experiments have in many instances given me coloured pic- tures of the prismatic spectrum, dark upon a light ground , but the most beautiful I have yet obtained, has been upon the Daguerreotype iodidated tablets, on which the colours have, at the same time, had a peculiar softness and brilliancy. Daguerre himself has remarked, that when he has been copying any red, brick or painted, building, the photograph has assumed a tint of that character. I have often observed the same thing in each variety of photographic material, i. e., where a salt of POSSIBILITY OF PRODUCING NATURAL COLOURED PHOTOGRAPHS. 83 silver lias been used. In the Philosophical Magazine for April, 1840, will be found a paper, — “ Experiments and Observations on Light which has permeated Coloured Media,” — in which I describe some curious results, on some of those photographs which are prepared with the hydriodic salts, exposed to luminous influence with coloured fluids super- imposed ; permitting, as distinctly isolated as possible, the permeation of the violet and blue, — the green, the yellow, and the red rays, under each of which a complementary colour was induced. During January of the present year, I prepared some papers with the bichromate of potash, and a very weak solution of nitrate of silver ; a piece of this paper was exposed behind four coloured glasses, which admitted the passage respectively of, 1st, the violet, indigo, and blue rays ; 2d, the blue, the green, and a portion of the yellow rays ; 3d, the green, yellow, and orange rays ; and, 4th, the orange and red rays. The weather being extremely foggy, the arrangement was unattended to for two days, being allowed to lie upon a table opposite a window having a southern aspect. On examining it, it had under the respective colours become tinted , of a blue, a green, and a red ; — beneath the yellow glass the change was uncertain, from the peculiar colour of the paper, — and this without a solitary gleam of sunshine. My numerous engagements have prevented my repeating the observations I desire on this salt, which has hitherto been considered absolutely insensible to light. The barytic salts have nearly all of them a peculiar colourific effect; the muriate, in particular, gives rise to some most rich and beautiful crimsons, particularly under the influence of light which has permeated the more delicate green leaves; and also in copying the more highly coloured flowers, a variety of tintings have been observed. We may always depend on producing a photographic copy of a leaf of a green colour by the following arrangement; — Having silvered a copperplate, place it in a shallow vessel, and lay thereon the leaf of which a copy is desired, maintaining it in its position by means of a piece of glass; pour upon it, so that the plate beneath the glass may be covered, a solu- tion of the hydriodate of potash, containing a little free iodine — then expose the whole to sunshine. In about half an hour one of the most beautiful photographic designs which can be conceived is produced, of a fine green colour. The fluid is yellow, and cuts off nearly all the “ chemical ” rays, allowing only of the free passage of the less re- frangible rays — 'the most abundant being the yellow. This retards the process of solarization ; but it produces its complementary colour on the plate. These facts will, I think, prove that the possibility of our being enabled to produce coloured photographs is decided, and that the pro- bability of it is brought infinitely nearer, particularly by Sir John Her- schel’s very important discovery, than it was supposed to be. 84 MISCELLANEOUS PROCESSES. 6.' — Invisible Photographs, and their Reproduction. It was first noticed by Sir John Herschel, that any of the ordinary photographic drawings were completely obliterated by being immersed in, or washed over with, a solution of corrosive sublimate, and the paper restored to its original whiteness, in which state it might be kept any length of time ; but the drawing is to be reproduced at pleasure, by washing it in a solution of the hyposulphite of soda. About the same time, and being then perfectly unacquainted with what Sir John Herschel had done, I fell upon the same phenomenon, but on the hydriodated papers, instead of the simple muriated. These are changed yellow by the corrosive sublimate, but present no trace of the original picture, which exists, but is invisible, and may be restored by the same means as before mentioned. Either of these photographs may thus be obliterated, and rendered again visible as frequently as we please, affording an extremely curious illustration of the action of chemical reagents. After we have completed a picture on a Daguerreotype plate, by a little brisk rubbing with the hand or a leather, we entirely obliterate it ; place it again in the mercurial vapour, and the design will be reproduced ; or plunge it into a solution of iodine, and the picture again appears very defined, but with reversed lights and shadows. The effect of these solutions of bichloride of mercury and the hypo- sulphite of soda, may be very strikingly shown on one of the papers which, after having been darkened by light, are exposed to the Da- guerreotype processes. The effect of the bichloride of mercury is to whiten the dark parts of the picture, and, of course, produce a negative drawing, which is rapidly rendered positive by immersion in the solu- tion of hyposulphite of soda. We have it thus in our power to pro- duce upon the same sheet the two distinguishing varieties of photo- graphic drawing. The corrosive sublimate may be employed for painting on the darkened photographic paper, by drawing in the lights with it. The processes named above may also be used for secret correspondence. 7. — On the Spontaneous Darkening of the White Photographic Papers. Great annoyance often arises from the rapid discolouration of the more sensitive kinds of photographic drawing paper, independent of the action of light, which appears to arise from the action of the nitrate of silver on the organic matters of the size. Unsized paper is less liable to this change. If we spread a perfect chloride of silver over the paper, it may be kept for any length of time without a;iy change of its white- SALTS OF GOLD AS PHOTOGRAPHIC AGENTS. 85 ness taking place in the dark. Wash it over with a very weak solution of nitrate of silver, and, particularly if the paper is much sized, a very rapid change of colour will take place, however carefully we may screen it from the light. From this it is evident that the organic matter of the size is the principal cause of the spontaneous darkening of photo- graphic papers prepared with the salts of silver. The most curious part of the whole matter is, that in many cases this change is carried on to such an extent, that a revival of metallic silver takes place, to all appearance in opposition to the usual force of the affinities. I have now some packets of paper prepared two years since, which have been carefully kept in the dark. Over many of these there is a perfect revival of the metal. This is very difficult to deal with. Chemistry has not yet made us acquainted with any organic body, which would separate either chlorine or nitric acid, from their metallic combinations. I can only view it in this light: — the nitric acid liberates a quantity of carbonaceous matter, which, acting by a function peculiarly its own, will at certain temperatures effect the revival of gold and silver, as we have seen in the case of Dr. Schaf- haeutl’s process and Count Rumford’s experiments. Having been informed that the paper-makers are in the habit of bleaching their sizes with sulphur and the sulphites, I have recently submitted a considerable quantity of the browned papers which I hap- pened to have by me, to analysis. In all cases where there has been a revival of the silver, or where the paper has blackened, I have detected the presence of sulphur. Consequently, when the darkening goes on rapidly, we may, I think, correctly attribute it to the formation of a sulphuret of silver, rather than to the causes above named. Where the darkening process is slow, these will, however, be found to be tolerably near to the truth. 8. — On the Use of the Salts of Gold as Photographic Agents. It is well known that gold is revived from its etherous solution by the action of light, and that the same effect takes place when the nitro- muriate of gold is spread on charcoal. Considering it probable that the required unstable equilibrium might be induced in some of the salts of gold, I was induced to pursue a great many experiments on this point. In some cases, where the paper was impregnated . with a mordant salt, the salt of gold was darkened rapidly, without the assistance of light; in others, the effect of light was very slow and uncertain. By washing paper with muriate of bary- tes, and then with a solution of the chloride of gold, a paper, having a slight pinky tint, is procured; by exposing this paper to sunshine it is at first whitened , and then, but very slowly, a darkening action is induced. If, however, we remove the paper from the light, after an 86 MISCELLANEOUS PROCESSES. exposure of a few minutes, when a very faint impression, and oftentimes not any, is apparent, and hold it in the steam of boiling water, or im- merse it**in cold water, all the parts which were exposed to the light are rapidly darkened to a full purple brown, leaving the covered portions on which the light has not acted, a pure white, producing thus a fine negative drawing. If, while such a paper, or any other paper prepared with the chloride of gold, is exposed to the sun, we wash it with a weak solution of the hydriodate of potash, the oxidation is very rapidly brought on, and the darkness produced is much greater than by the other method ; but this plan is not often applicable. I have not yet been enabled to produce with the salts of gold, any paper which should be sufficiently sensitive for use in the camera obscura. 9. — On the Action of Heat on the Hydriodic Photographic Papers. I have before alluded to some remarkable effects on these papers, produced by the “ calorific” rays. It is therefore necessary to notice the analogous action produced by artificial heat, under similar circum- stances. If a piece of darkened photographic paper, washed with an hydriodic solution, be pressed into close contact with a dark engraving, or a printed page, by means of a piece of metal, which is kept moder- ately warm, a very faithful copy is in general obtained ; but not always. There are some circumstances, not yet detected, which sometimes pre- vent a change. All the dark parts of the engraving are copied in lights, i. e ., we have a negative picture. Much appears to depend upon the composition of the ink used in printing; with some kinds I have never failed — with others I have seldom succeeded in producing this kind of drawing. 10 . — On Cofying Letter- Press, &c., on the Photographic Papers, by means of Juxtaposition. There are numerous instances in which copies are effected by mere juxtaposition, in a very remarkable manner: — Allow a piece of white muriated photographic paper to lie between the leaves of a book for a few weeks, nothing will be observed upon the paper if we then remove it ; but if we plunge it into sulphuretted hydrogen gas, the letters are all brought out in metallic silver, the other parts of the sheet becoming the black sulphuret of silver. If a sulphuretted paper is placed in the same way in a book, in a few days the printing is faintly copied. By passing the paper then through a solution of iodine, it becomes much more visible, the letters being, if viewed in one position, the darkest parts of the paper, while in another they appear the lightest. Many kinds PHOTOGRAPHIC PAPERS FOR METEOROLOGICAL SELF-REGISTRATION. 87 of pencil marks are readily copied on photographic papers, and gold printing soon leaves its impression on the chloride of silver. Sir John Herschel states, this effect is not produced by all pencils on the same paper ; as a preparation of the paper, nitrate of silver over borax seemed to succeed best; this possibly arises from the sulphur in the pencil. The gold leaf may act on the silver, in all probability, from its con- taining a small alloy of copper. These facts are curious, and may eventually be turned to some important uses. 11. — On the Use of Photographic Paper for Registering the Indications of Meteorological Instruments. There are so many advantages attendant on self-registration, as to make the perfection of it a matter of much interest to every scientific enquirer. The first who suggested the use of photographic paper for this purpose was Mr. T, B. Jordan, who brought the subject before a committee of the Royal Cornwall Polytechnic Society, on the 18th of February, 1839, and exhibited some photographic registers on the 21st of March of the same year. The plan this gentleman adopted was to furnish each instrument with one or two cylinders containing scrolls of photographic paper. These cylinders are made to revolve slowly by a very simple connection with a clock, so as to give the paper a progres- sive movement behind the index of the instrument, the place of which is registered by the representation of its own image. The application of this principle to the barometer or thermometer is most simple ; the scale of either of these instruments being perforated, the paper is made to revolve as close as possible to the glass, in order to obtain a well defined image. The cylinder being made to revolve on its axis once in forty-eight hours, the paper is divided into forty-eight parts by vertical lines, which are figured in correspondence with the hour at which they respectively arrive at the tubes of the instruments. The graduations on the paper correspond to those on the dial of the bar- ometer or scale of the thermometer, and may be printed on the paper from a copperplate, or, 'what is much better, may be printed by the light at the same time from opaque lines on the tube, which would of course leave a light impression on the paper; by this means we should have all that part of the paper above the mercury darkened, which would at the same time be graduated with white lines, distinctly marking the fluctuations in its height for every minute during day- light, and noting the time of every passing cloud. Mr. Jordan has also published an account of his very ingenious plan of applying the same kind of paper to the magnetometer or diurnal variation needle,* and several other philosophical instruments ; but as See the Sixth Annual Report of the Royal Cornwall Polytechnic Society. 88 MISCELLANEOUS PROCESSES. these applications have not been entirely successful, owing principally to the difficulty of finding a suitable situation for so delicate an instru- ment, it is thought unnecessary to occupy these pages with any particu- lar description of the arrangements adopted, the more so, as this is a subject which can scarcely be said to come within the meaning 'of a popular treatise. One subject, however, which, at the same time that it is highly philo- sophical, is of a more popular character, must not pass unnoticed. The registration of the ever varying intensity of the light is so important a subject, that it has occupied the attention of several eminent scientific observers. Sir John Herschel and Dr. Daubeny have applied their well known talents to the inquiry, and have, both of them, devised instru- ments of great ingenuity- for the purpose. The instrument constructed by Sir John Herschel, which he has named an ctctinograph, not only registers the direct effect of solar radiation, but also the amount of general illumination in the visible hemisphere, which constitutes day- light. One portion of the apparatus being so arranged that a sheet of sensitive paper is slowly moved in such a direction, that the direct rays of the sun, when unobscured, may fall upon it through a small slit made in an outer cylinder or case : — while the other is screened from the inci- dent beam, the paper being fixed on a disc of brass, made to revolve by watch work, is affected only by the light which “ emanates from that definite circumpolar region of the sky to which it may be considered desirable to limit the observation,” and which is admitted, as in the other case, through a fine slit in the cover of the instrument. Mr. Jordan has devised an instrument for numerically registering the intensity of the incident beam, which appears to have some peculiar advantages, a description of which I shall take the liberty of transcrib- ing. Figure 29 is an elevation of the instrument; it consists of two Fig. 29. copper cylinders supported on a metal frame, the interior one is fixed to the axis and does not revolve, being merely the support of the pre- pared paper; the exterior cylinder is made to revolve about this once in twenty-four hours by a clock movement. It has a triangular aper- ture cut down its whole length, as shown in the figure, and it carries the scale of the instrument, which is made to spring closely against the INFLUENCE OF CHLORINE AND IODINE ON SOME KINDS OF WOOD. 89 prepared paper. This scale or screen is composed of a sheet of metal foil between two sheets of varnished paper, and is divided into one hun- dred parts longitudinally, every other part being cut out, so as to admit the light to the prepared paper without any transparent medium inter- vening. The lengths of the extreme divisions, measuring round the cylinder, are proportioned to each other as one to one hundred, conse- quently the lower division will be one hundred times longer passing over its own length, than the upper one over its own length, and the lines of prepared paper upon these divisions will, of course, be exposed to the light for times bearing the same proportion to each other. Now, as the sensitiveness of the paper can readily be adjusted, so that the most intense light will only just tint it through the upper division during its passage under the opening, and the most feeble light will produce a similar tint through the lower division during its passage , the number of lines marked on the paper at any given time, will furnish a comparative measure of the intensity of solar light at that time, and may be registered as so many degrees of the Heliograph , the name Mr. Jordan has given his instrument, just as we now register the degrees of the thermometer. Of course, it is essential for these registrations, that the photographic paper should be always of the same kind. The manufacture of such paper is not so difficult as it may, in the first instance, appear to be. Provided a perfectly uniform paper, which shall be invariable in its composition, can be procured from the manufacturers, by attending strictly to the rules prescribed in the former part of this treatise, there will be no difficulty in producing sheets of any length, and in any num- ber, which shall act in all respects similarly under the influence of radiation v 12. — The Influence of Chlorine and Iodine in rendering some kinds of Wood sensitive to Light. Having on many occasions subjected the simply nitrated photographic paper to the influence of chlorine and iodine in close wooden boxes, I was often struck with the sudden change which light produced on the wood of the box, particularly when it was of deal, changing it in a few minutes from a pale yellow to a deep green. This curious effect frequently occur- ring, led me to observe the change somewhat more closely, and to pursue some experiments on the subject. These produced no very satisfactory result. They proved the change to depend much on the formation of hydrochloric, and hydriodic acids, and the decomposition of water in the pores of the wood. I found well baked wood quite insusceptible of this very curious phenomenon. The woods of a soft kind, as the deal and willow, were much sooner influenced than the harder varieties, but all the light-coloured woods appeared more or less capable of N 90 MISCELLANEOUS PROCESSES. undergoing this change. All that is necessary is, to place at the bot- tom of an air-tight box, a vessel containing a mixture of manganese and muriatic acid, and fix the piece of wood at some distance above it. Different kinds of wood require to be more or less saturated with the chlorine or iodine, and consequently need a longer or shorter exposure. The time, therefore, necessary for the wood to remain in the atmosphere of chlorine, can only be settled by direct experiment. Wood is impreg- nated very readily with iodine, by putting a small portion in a capsule a few inches below it. It does not appear to me at present, that any practical result is likely to arise out of this peculiar property : it is only introduced as a singular fact, which is perhaps worthy a little more attention than my numerous engagements have left me time to devote to it. 13. — Process for Preparing the Hyposulphite of Soda. As the solution of this salt is found to be the best fixing agent yet recommended, and as it is not commonly kept by the retail chemist, I insert the following directions to enable any person to prepare it for himself : — • Form, in the first instance, a solution of caustic soda, by dissolving a pound of soda in a quart of boiling water, and mixing it, while hot, with half-a-pound of fresh burnt lime, slacked with another quart of boiling water. The mixed solution is to be carefully covered from the air until cold. The clear liquor is then to be poured off, and made to dis- solve, by boiling in an earthen vessel, as much sulphur as possible. The deep yellow solution formed is to be decanted off into a deep vessel, and a current of sulphurous acid gas passed through it until it becomes quite colourless. This is very easily done by mixing, in a retort with a long beak, some linseed oil and sulphuric acid. On applying a little heat, the sulphurous acid gas is given off in great abundance. By plunging the beak to the bottom of the vessel, it* passes through, and is rapidly absorbed by the solution. If it is desired to crystallize the hyposulphite, the fluid should not be allowed to become quite free of colour. Whilst still a little yellow, it should be filtered and evaporated in a porcelain or earthen vessel, over a quick fire, to the consistence of a syrup. The liquid thus evaporated is mixed with half its volume of alcohol, and well shaken. The alcohol takes up all the sulphuret, and floats above ; the lower solution is left to cool under the alcoholic one. The hyposulphite of soda must be preserved in well stopped glass bottles, and never be exposed to any bright light. It is best to keep it in small bottles, as the action of the oxygen of the atmosphere has a tendency to form a sulphate and precipitate the sulphur. 01 CONCLUSION. Haying now completed my description of all the published photogra- phic processes of any real value, and my own observations on each of them, nothing more remains to be done, but to draw attention to the importance of the art, both practically and philosophically considered. A great number of important considerations press themselves upon me, but I shall only remark on those which have the most popular ten- dency. We have seen that the beams which proceed from the sun, in the form of white light, are each one of them a combination of distinct rays, not only differing in colour, but having very opposite powers. For- merly these rays were divided into three classes, and called the “ calo- rific,” the “ luminous,” and the “ chemical,” under an impression that these particular functions were confined to the particular parts thus named. Recent observations have shown that the calorific action is not confined to the red or “ calorific ” rays, but that it extends over the greater part of the spectrum. The “ luminous,” or yellow rays, have long been considered as being destitute of any chemical action ; but Sir John Herschel, in a communication he has been kind enough to make to me, and which I have his permission to publish, says, “ I have ob- tained photographic actions on certain papers, not argentine, which are limited not to the * chemical,’ not to the 4 calorific,’ but to the ‘ lu- minous ’ rays, i. e. which seem to be produced exclusively, or nearly so, by those rays which affect the organ of sight. These papers are pre- pared with substances of vegetable origin ; and though at present I do not see how this can become serviceable in the arts, it strikes me as scientifically of considerable importance.” This fact is to me singularly interesting, the more so, that in my experiments on the effects of light on vegetation, I have detected powers in the “ luminous” rays which are highly destructive to the germination and the growth of plants. This inquiry I am now pursuing, and I hope the coming summer will enable me to add something to the many very remarkable discoveries recently made by Sir John Herschel and others. The chemical action of light, so far fromjbeing confined to the most refrangible rays, we now know extends over the whole spectrum, visible and invisible, the action only being shifted from one ray to another, according to the substance upon which its peculiar functions are ex- erted. It is extremely difficult to explain many of the phenomena of light by either of the rival theories ; and as we proceed in our inquiries, the question of the materiality or immateriality of light becomes more and more complicated. A matter of much interest arises out of these considerations, which is, are the different rays in similar electrical states, or do they vary in this respect with their refrangibility ? Those philosophers who have adopted the undulatory theory of light, put the 92 CONCLUSION. question aside with a smile, or show how completely the electrical notion is at variance with their theory. There exists many very great difficulties in solving the problem, but although a good theory will often aid us in discovering the truth, we must not allow our researches to be stopped, because they may appear inconsistent with the received notion. If we could establish the fact of peculiar electric action existing in the different rays of light, we should then have the means of reducing to something like system, the many anomalous features which come under our notice in prosecuting our studies into the character of the solar light. “ In this instance,” says M. Arago, “ it is upon the unforeseen that we are especially to reckon,” and every new discovery goes to prove the correctness of this. Few words need be expended to show the utility of the photographic art to manufactures. We may expect in a few years to find the designs with which we ornament our porcelain, and the beautiful fabrics of the loom, infinitely superior to those with which they are at present adorned, and if not directly formed by the operations of light, they will be copied from those incomparably faithful pictures which photographic processes enable us to obtain. From the first publication of the photographic operations we may date an improvement of taste. No one who has accustomed himself to the exquisite finish of these productions, will be enabled to endure any artistic design which is not of superior excellence. Hence will be created a new era in the arts, and I have little doubt but the effects of photography will soon be apparent by improvements in linear perspective, and the general disposition of light and shadow in the productions of modern painters. It has been said that photo- graphic drawings fail in artistic effect . That they fail in producing those exaggerated effects which are found no where in nature, but on the canvass of some modern artists of eminence, is most true. But nature in her rudest forms is more beautiful than any human produc- tion; and in her choice arrangements, how infinitely beyond us. If the photographic art does nothing more than teach our artists to subdue the violence of contrasts in which they have of late indulged, under a mistaken idea of producing a superior effect, it will have been of great service to all that relates to refined taste. Of all effects, the most un- truthful is the modern artistic effect. I shall conclude with a few words from the speech of M. Arago. “ To copy the millions and millions of hieroglyphics which completely cover the great monuments of Thebes, Memphis, Carnac, &c., would require scores of years, and legions of artists. With the Daguerreo- type a single man would suffice to bring to a happy conclusion this vast labour. Arm the Egyptian Institution with two or three of Daguerre’s instruments, and on many of the large engravings in their celebrated work, the fruit of our immortal expedition, vast assemblages of real hieroglyphics would replace fictitious or purely conventional characters. CONCLUSION. 03 Again, these photographic delineations having been subjected, during their formation, to the rules of geometry, shall enable us, with the aid of a few simple data, to ascertain the exact dimensions of the most elevated parts, and of edifices the most inaccessible.” Beyond this, what need be said of the vast importance of Photo- graphy? March 17th , 1841. SUPPLEMENTARY CHAPTER. A new Photographic Process by the Author, for procuring Pic- tures with the Camera Obscura in a few Seconds. On referring to the section in which I have treated of the application of the Daguerreotype to paper, it will 'be seen, that I was the first to show, that both iodine and mercury could be used in the same way with papers properly prepared, as with the prepared silver tablets. I have been lately induced to extend my inquiries, and particularly to examine the manner in which chlorine and bromine would act on papers prepared as I have before directed. Many extremely curious results, which are omitted from their not having any practical bearing, led me to examine the effect of the mercurial vapour on the pure precipitated iodides and bromides. I was long perplexed with some exceedingly anomalous results, but being satisfied from particular experiments, that these researches promised to lead to the discovery of a most sensitive pre- paration, I persevered in them. Without stopping to trace the progress of the inquiry, I may at once state, that I have the satisfaction of being enabled to add to the present treatise, an account of a process which serves to prepare •papers that are much more sensitive than Daguerre’s iodidated plates. The exquisite delicacy of these new photographic papers may be imagined when I state, that in five seconds in the camera obscura , I have , during sunshine , obtained perfect pictures ; and that , when the sky is overcast , one minute is quite sufficient to produce a most decided effect. The action of light on this preparation, does indeed appear to be instantaneous. On several occasions I have procured, in less than a second, distinct outlines of the objects to which the camera has been pointed, and even secured representations of slowly mov- ing bodies. With this great increase of sensitiveness, we of course secure greater sharpness of outline, and more minute detail. It should be understood that the process is a negative one, from which positive pictures may be procured on the ordinary photographic paper by transfer. To prepare this very sensitive paper, we proceed as follows : — Select the most perfect sheets of well glazed satin post, quite free from specks of any kind. Placing the sheet carefully on some hard body, wash it over on one side by means of a very soft camel’s hair pencil, with a solution of sixty grains of the bromide of potassium in two fluid ounces SUPPLEMENTARY CHAPTER. 95 of distilled water, and tlien dry it quickly by the fire. Being dry, it is again to be washed over with the same solution, and dried as before. Now, a solution of nitrate of silver, one hundred and twenty grains to the fluid ounce of distilled water, is to be applied over the same surface, and the paper quickly dried in the dark. In this state the papers may be kept for use. When they are required, the above solution of silver is to be plentifully applied, and the paper placed wet in the camera, the greatest care being taken that no day-light, not even the faintest gleam, falls upon it, until the moment when we are prepared, by removing the screen, to permit the light, radiated from the objects we wish to copy, to act in producing the picture. After a few seconds, the light must be again shut off, and the camera removed into a dark room. It will be found, on taking the paper from the box, that there is but a very slight outline, if any, as yet visible. Place it aside, in perfect darkness , until quite dry, then place it in the mercurial vapour box described in the former pages, and apply a very gentle heat to the bottom. The mo- ment the mercury vaporizes, the picture will begin to develope itself. The spirit lamp must now be removed for a short time, and when the action of the mercury appears to cease, it is to be very carefully again applied, until a well defined picture is visible. The vaporization must now be suddenly stopped, and the photograph removed from the box. The drawing will then be very beautiful and distinct ; but much detail is still clouded, for the development of which it is only necessary to place it cautiously in the dark, and allow it to remain undisturbed for some hours. There is now an inexpressible charm about the picture, equalling the delicate beauty of the Daguerreotypes ; but being still very susceptible of change, it must be viewed by the light of a taper only. The nitrate of silver must now be removed from the paper by well washing in soft water, to which a small quantity of salt has been added, and it should be afterwards soaked in water only. When the picture has been dried, wash it quickly over with a soft brush, dipped in a warm solution of the hyposulphite of soda, and then well wash it for some time in the manner directed for the ordinary photographs, in order that all the hyposulphite may be removed. The drawing is now fixed, and we may use it to procure positive pictures, many of which may be taken from one original. The transfers procured from this variety of negative photographs, have more decision of outline, and greater sharpness in all their minute detail, than can be procured by any other method. This is owing to the opacity produced by the curi- ous combination of mercury and the bromide of silver, which is not', I believe, described in any chemical work. This very beautiful process is not without its difficulties; and the author cannot promise that, even with the closest attention to the above directions, annoying failures will not occur. It often happens that some accidental circumstance, generally a projecting film, or a little dust, will occasion the mercurial vapour to act with great energy on one part of the paper and blacken it, before the other portions are at all 96 SUPPLEMENTARY CHAPTER. affected. Again, the mercury will sometimes accumulate along the lines made by the brush,* and give a streaky appearance to the picture, although these lines were not at all evident before the mercurial vapour was applied. The action, however, of this new photographic preparation is certain ; and although a little practice may be required to produce finished designs, yet very perfect copies of nature may be effected with the greatest possible ease and certainty. I have stated that the paper should be placed wet in the camera : the same paper may be used dry, which is often a great convenience. When in the dry state, a little longer exposure is required, and instead of taking a picture in four or five seconds, two or three minutes are necessary. I cannot conclude without remarking, that it appears to me that this process, when rendered complete by the improvement of its manipu- latory details, will do much towards realising the hopes of those who were most sanguine of the ultimate perfection of photography ; and will convince others who looked upon the art as a philosophical plaything, that the real utility of any discovery is not to be estimated from the crude specimens produced in its infancy, ere yet its first principles were evident to those who pursued it with an eager hope. Falmouth, April 19 , 1841 . BELL AND BAIN, PRINTERS, GLASGOW. ■SE&AU CjOf4 e j- iV 1 *) -HfW MdjBfttMtR LIBRARY