The Qptical LANTfERtsr 
 
 THE 
 
 "DEMONSTRATOR'S" 
 
 OPTICAL LANTERN. 
 
 FOR 
 ELECTRIC 
 OR 
 
 ---Wy LIME-LIGHT, 
 
 I Ij NEWTON'S PATENT. 
 
 Illuslrated Catalogue 
 Six Slumps. 
 
 LANTERNS AND SLIDES 
 
 Ol-' EVERY DESCRIPTION-. 
 
 HIGHEST QUALITY ONLY. 
 
 PATF..VTF.KS AND MAXUFACTURI-KS 
 
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 ALL ARTISTS, when drawing for Reproduction, 
 
 SHOULD USE 
 
 WINSOR& NEWTON'S 
 ALBANINE 
 
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 PROCESS BLACK 
 
 Tlic-se Pigments arc respectively — ^ 
 
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 PHOTOGRAPHICALLY BLACK, and 
 Drawings made with them may be relied upoii 
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 true relative values. 
 
 ALBANIN 
 
 f)i ^ Pure.Unehangeabl 
 fo^foSraphic White, reli: 
 •"Use in Drawings for 
 Reproductive Arts 
 
 ''■drawings in Btackmi 
 "<s Winsor & Ne wto 
 ,^1'ECIAL PROCESS BI 
 'Conjunc tion uuith AhS. 
 
 ^iNSOR & NEWTON 
 
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 >\ ENCJLAXD. 
 
 Price Is. per bottle. 
 
 WINSOR k NEWTON, Ltd, 
 
 Rathbone Place, LONDON; 
 
 and Fulton Street, NEW YORK. 
 
^ Advertisements. 
 
 J. H. STEWARD'S 
 
 HIGH.-CLASS' 
 
 OPTICAL LANTERNS 
 
 For OIL, GAS, LIME or 
 ELECTRIC LIGHT. 
 
 THE . 
 
 Davenport-Steward 
 
 ARC LAMP 
 
 For Direct or Alternating Currents 
 
 The BEST LAMP for 
 
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 £2 15s. £4 10s. 
 
 Arc Lamp £5 5s. 
 
 steward Institution Lanterns with 
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 ENLARGING LANTERNS. 
 
 ILLUSTRATED CATALOGUES POST LREE. 
 
 406, Strand, W.O.; 457, West Strand, W.C.; 7, Gracechurch 
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The Optical Lantern. 
 
 ROSS' 
 
 NEW I'ATENT. 
 
 SCIENCE 
 LANTERN. 
 
 RnQQ' COMBINATION 
 '^"^^ LANTERN. 
 
 This instrument has been designed to 
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 Principal Features and Advantages : 
 
 Complete change from Horizontal to Vertical, and vice 
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 Great comfort and ease of movement when used for 
 drawing diagrams, iScc. 
 
 Stage plate of aluminiujn instantly removable for clean- 
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 Complete absence of the usual stage springs, j-et every- 
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 The jet (either ' blow-through ' or ' mixed ') has all 
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 The ventilation is most perfect. 
 
 (PATENT.) 
 
 ROSS 
 
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 ENLARGING 
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 This apparatus, whilst bearing a 
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 has*; is designed to give the utmost 
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 Hat surface. 
 
 PATENTED PROJECTION 
 
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 Optical Works : Clath.-vm Commox, S.W. 
 
 Contractors to Her Majesty's Governments, British and Colonial ; also to the principal 
 l''orcign Co-rcrninc7its. Established iSio. 
 
THE 
 
 Optical Lantern 
 
 FOR 
 
 InsfruTtion anb ^Imuscmcnt. 
 
 BY 
 
 Andrew Pringle, f.r.m.s, f.r.p.s., etc., 
 
 Late Vice-President Royal PhotogTaphic Society ; 
 President Photographic Convention of the United Kingdom, i8Sgr 
 Author of Practical Photo-Micrography," I^antern Slides 
 by PhotograpJuc Methods" and foint-AntJior of 
 " Processes of Pure Photog-rapiiy." 
 
 THIRD EDITION, REVISED AND CONSIDERABLY 
 ENLARGED. 
 
 IConbon : 
 
 HAMPTON & CO., 13, Cursitor Street, E.G. 
 1899. 
 
PRINTED BY HAMPTON AND CO. 
 13, CURSITOR STREET, 
 LONDON, E.C. 
 
PREFACE TO THE THIRD EDITION. 
 
 Since the issue of the Second Edition, in 1891, many 
 and notable improvements have been made on exist- 
 ing instruments, and several new apparatus and pro- 
 cesses have been introduced. The object of this Third 
 Edition is to bring the book up to date; and I venture 
 to hope for the work in its latest form acceptance as 
 flattering as was accorded to my previous efforts. 
 
 Being pretty fully occupied for a time with other 
 matters, I was happy enough to secure the valuable 
 help of Mr. W. B. Bolton in revising and adding to 
 the matter contained in former Editions. 
 
 A. P. 
 
 Bexley Heath, 1899. 
 
PREFACE TO THE FIRST EDmON. 
 
 It may fairly be said that this book fills a place not occupied by 
 any other at present before the public. For the present treatise 
 deals with the Optical Lantern only; it does so, I trust, as fully as 
 is required for any purpose ; no attempt is made, as in other books 
 on the "lantern,'' to give instructions for lantern-slide making, still 
 less for the production of negatives. The present is, in fact, a 
 lantern book, and a lantern book only. As such I hope it will^ 
 be useful. 
 
 I have written less for the popular public lecturer than for the 
 photographer and the teacher. If in any degree my writings 
 popularise and simplify the use of the optical lantern, specially if 
 they tend to give the lantern a locus standi as a permanent part of 
 the paraphernalia of the lecture room, I shall be happy in the 
 consciousness of having done some good. 
 
 ANDREW PRINGLE. 
 
 PREFACE TO THE SECOND EDITION. 
 
 The success of the First Edition, published in America, and the 
 many requests of friends in England have induced me to offer 
 this Second and cheaper Edition. I have tried to bring the 
 book up to date, but as I have not been able to make personal 
 experiment with every instrument on the market, I have elected 
 to confine my remarks to such as I have used myself, no doubt 
 to the unavoidable exclusion of many excellent appliances. 
 
 A. P. 
 
Introductory 
 
 General System 
 
 CONTENTS. 
 
 CHAPTER I. 
 
 CHAPTER H. 
 
 The Condenser ... 
 
 CHAPTER III. 
 
 13 
 
 The Projecting Lens 
 
 CHAPTER IV. 
 
 The Lantern Body 
 
 CHAPTER V, 
 
 24 
 
 CHAPTER VI. 
 
 Double and Triple Lanterns, Dissolving Views, Effects, &c. 29 
 CHAPTER VII. 
 
 Illuminants— Oil, Incandescent Gas, Acetylene 34 
 
 CHAPTER VIII. 
 Illuminants (continued) — Limelight, Jets, Accessories, 
 
 Saturators, &c 44 
 
 CHAPTER IX. 
 
 Preparation of Oxygen and Hydrogen, and Storage in Bags 58 
 CHAPTER X. 
 
 Storage of Gases in Cylinders. Regulators. Gauges ... 68 
 
 CITAPTER XL 
 
 The Electric Light 
 
 72 
 
 CHAPTER XH. 
 
 Screens and Frames 
 
 82 
 
vi. CONTENTS. 
 
 CHAPTER XIII. 
 
 Supports. Slidk Carriers. Reading Lamps. Signals ... 87 
 CHAPTER XIV. 
 
 Lanterns and Arrangements for Experiments 90 
 
 CHAPTER XV. 
 
 Animated Photography and Projection 97 
 
 CHAPTER XVL 
 Practical Working of the Lantern, Oil Lamps, Blow- 
 Through, AND Mixing Jets ... ... ... ... 113 
 
 CHAPTER XVII. 
 
 Preparations for Lecture ... ... ... ... 119 
 
 CHAPTER XVIII. 
 Management of Lantern Durinc^, Lecture ... ... 123 
 
 CHAPTER XIX. 
 
 Deportment on the Platform ... ... ... ... 125 
 
 CHAPTER XX, 
 
 Arrangements for the Lantern in a Lecture-Room ... 128 
 CHAPTER XXI. 
 
 Enlarging with the Optical Lantern ... ... ... 131 
 
 Table for Enlargements .. ... ... ... ... 136 
 
 Memoranda for a Lantern Display ... ... ... 137 
 
 APPENDIX. 
 
 New Oxygen Generators ... ... ... ... ... 138 
 
 New Lanterns ... ... ... ... ... ... 142 
 
 The Radiant Limelight Jet ... ... ... ... 146 
 
The Optical Lantern. 
 
 CHAPTER I. 
 
 INTRODUCTORY. 
 
 WHEN we consider that in a coat pocket one can with ease 
 carry a dozen lantern slides, and when we realise that 
 each of these slides can be projected as a picture of (say) 
 twenty feet square in size by means of an optical lantern, the utility 
 and importance of this scientific optical appliance must be very 
 evident. We should also consider the ease with which these 
 lantern slides may be made, either by photography or by hand ; 
 we have also to take note of the variety of subjects amenable to 
 reproduction as lantern slides, and we should further note that we 
 are not even tied down to lantern slides as objects for the lantern, 
 but that we may by our lantern project enlarged images of opaque 
 objects, chemical and physical experiments, and microscopic 
 preparations. When, finally, we find that all this is accomplished 
 by an instrument which can be purchased by a moderate purse and 
 carried by the feeblest hand, we are bound to own the merits of 
 such an instrument, more especially because we know that what 
 it pretends to do it does well. 
 
 No defence of, nor plea for, the optical lantern is needed in 
 addressing circles or societies addicted to photography, for there 
 are but few photographic memberhoods that have not their optical 
 lantern, or at least their occasional lantern display. But most 
 singular is the slowness of the progress in lantern use among 
 
8 
 
 THE OPTICAL LANTERN. 
 
 societies and in educational institutions not purely photographic. 
 Since the first edition was written great advances have been made, 
 and most learned societies and educational institutions use the 
 lantern ; there is still, however, much room for improvement in 
 the instruments used or in the system of use. We know of estab- 
 lishments possessing good lanterns but without means for darken- 
 ing their lecture-rooms, except with considerable trouble, during 
 the daytime. The danger of misconduct by students in a darkened 
 room has, as predicted in our earlier editions, proved to be 
 imaginary. From a purely ocular point of view, diagrams, maps, 
 blackboard designs, and the like, are inferior to the much larger 
 and clearer lantern illustrations, and the shortsighted student must 
 miss much of what the more fortunate normal-eyed may catch. 
 But taking higher ground, it may be asserted that where it is 
 important to study things as they really are, and not as the teacher 
 is able to draw them, or thinks they are, or wishes them to appear, 
 the optical lantern, with or without photography, must have the 
 strongest claims on our attention. However well a physiological 
 or pathological object may be drawn by hand, such a drawing is 
 impotent to carry conviction and to impress memory in compari- 
 son with an enlarged photograph or an Enlarged image of the 
 object itself. When was ever a spectrum, drawn by hand and 
 coloured by any method, able to compare with the actual spec- 
 trum projected on a screen by means, for instance, of an electric 
 arc ? 
 
 Many professors have admitted to the writer the claims of 
 the optical lantern as a useful scientific appliance ; some have 
 owned that photographic lantern slides would be far superior to 
 any other known method for educational demonstration ; but each 
 and all of these able men have been deterred from inaugurating 
 the system we recommended by the most extraordinary and 
 imaginative misgivings as to the cost of installation, and the 
 difificulty and even danger of working the apparatus. The diffi- 
 culty, strange to say, most commonly foreseen, was that of 
 darkening the lecture-room — with a touch of doubt as to the 
 conduct of young students in a darkened room — and there was 
 the usual dubiety as to the safety of the gases used for the lime- 
 light. 
 
 The prime object of this book is to remove dangers and diffi- 
 culties from the imagination (for there alone they exist) of many 
 
INTROD UCTOR V. 
 
 9 
 
 who are prepared to admit the advantages, but who question 
 the practicability, of using the optical lantern as an educational 
 and recreative instrument. 
 
 There are many persons, especially photographers and public 
 lecturers, as well as philanthropists striving for the good of their 
 fellow-men, who use the optical lantern, but they use instruments 
 of such poor quality, and appliances so nearly obsolete, and they 
 so little understand the means forgetting the best results from the 
 appliances they have, that the writer feels it no impertinence on 
 his part to try to put " lantern matters " before the public in a 
 succinct, and, as he hopes, clear and intelligible manner. He will 
 endeavour to cover all essential ground, to reject all side issues, 
 and to deliver himself in terms precluding possibility of mistake. 
 
CHAPTER II. 
 
 GENERAL SYSTEM. 
 
 The essentials of the optical lantern as a means for the projection 
 of enlarged images from smaller objects on a suitable surface are 
 very simple. 
 
 The optical system consists essentially of (i) a radiant ; (2) a 
 condensing system for collecting and concentrating on the object 
 the light rays furnished by the radiant; (3) a projecting system for 
 collecting, projecting, enlarging, and focussing the image of the 
 object as illuminated by the rays from the condenser; (4) a 
 "screen," or surface, for receiving the enlarged image and making 
 it visible to the eye. 
 
 The entire system will be easily understood from Fig. i. In 
 this figure enough is drawn to show the general optical principles 
 involved. The projection lens is made with an unusually large 
 back combination not necessary with ordinary photographic 
 lenses. 
 
 I 
 
 Fig. I. 
 
 R, the radiant ; C, the condenser ; S, the lantern slide ; P, the projection 
 lens ; I, the screen or plane where the enlarged image is received. 
 
 We may begin by describing what, in most cases, is the object 
 to be illuminated and to be reproduced on a scale so much larger 
 and at some considerable distance from the site occupied by the 
 object. In the vast majority of cases this object is a transparent 
 
GENERAL SYSTEM. 
 
 positive picture on glass, and the size of the picture actually to be 
 projected is usually about 3 inches, as the diameter of a circle, or 
 inches, as the diagonal of an oblong or square. For conve- 
 nience, the glass plate is, in this country, invariably* 31^ inches 
 square, an opaque mask, having an aperture (circular or oblong), 
 being used as a protection to the film of the "sHde," and as a pre- 
 ventive of unsightly and superfluous light on the screen. The 
 " slide " may be produced by photography, or it may be a hand- 
 sketched design ; for the present we will assume it to be a 
 "transparent positive" on glass 3^ inches square, with a mask 
 
 having an aperture (circular or oblong) of the above dimensions 
 
 about three inches. 
 
 The radiant may be one of several kinds, but our chief con- 
 sideration is that it may be a point of light, approximately, as the 
 incandescent point on a lime cylinder : or it may have a surface of 
 incandescence, or be formed of several such surfaces, as in a 
 multiple-wick lamp. The more nearly the radiant approaches to 
 ^ point the better, provided that the incandescence is sufficiently 
 brilliant. The point of incandescence must be precisely in the 
 focus of the condenser if the best result is to be obtained; en- 
 larging the area of incandescence is a make-shift, and often a verv 
 bad one. Still, as oil lamps are used, and are often convenient, 
 we must consider wicks as our radiant as well as limes or arcs. 
 
 The function of the condenser is to arrest as large a portion as 
 possible of the rays of light from the radiant, and to cause them 
 to fall equally upon all parts of the slide so as to produce, in the 
 first place, uniformity of illumination. Passing through the slide 
 the rays converge to a focus, at or near which point the projection 
 lens should be placed in order that as much as possible of the 
 light passing through the slide shall also be transmitted through 
 the objective to the screen. From this it follows that, to obtain 
 the highest result, the focal lengths of the condenser and projec- 
 tion lens must, within certain limits, bear a definite relation to one 
 another. 
 
 The sharpness of the picture on the screen depends upon the 
 slide and the screen being in the planes of the posterior and anterior 
 conjugate foci of the projection lens ; and as the screen, once 
 erected, is to be considered immovable, while the position of the 
 
 *The American size is often 4^ by 3X inches, or "quarter-plate." 
 
 B 2 
 
12 
 
 THE OPTICAL LANTERN. 
 
 projection lens varies with its focal length in its relation to the 
 slide, the lens requires to have a certain amount of range of focus 
 — that is to say, must to a certain extent be adjustable as regards 
 its position with respect to the slide. More technically, the 
 projection lens should have a rack and pinion focussing move- 
 ment. 
 
 An optical lantern is simply a device for holding the radiant, 
 the condenser, the slide, and the projection lens in suitable posi- 
 tions with regard to each other ; and that the radiant may not 
 directly illuminate the screen, nor interfere with the view of the 
 spectators, the lantern usually takes the form of an oblong box — 
 the light and part of the condenser being inside the box, and 
 the sHde-carrier and projection lens attached to the front. A 
 lantern-box ought to be fairly light tight ; and it ought to be as 
 small as possible consistently with giving room for the radiant 
 and not itself becoming too hot. In our opinion optical lanterns 
 are made ridiculously large, though there has recently been a 
 tendency to materially decrease the dimensions, but we hope in 
 the future to see our lanterns for lime-light much more con- 
 venient and less weighty and expensive. With oil lamps we 
 have to deal with the very important matter of draught or ven- 
 tilation, which must govern the size of the lantern-box to a 
 considerable extent. 
 
 The slide is usually held in the lantern, or passed through it, 
 by means of what is called a "carrier"; the screen is often 
 sustained by a "frame," or suspended from a roller. These 
 details are not essential to the system, and will be treated later, 
 each in its proper place. 
 
CHAPTER III. 
 
 THE CONDENSER. 
 
 The condenser is used to collect rays of light that would without 
 it be lost, so far as illumination of the slide is concerned. The 
 radiant is at or close to the principal focus of the condenser, and 
 the pencils of light travelling forward from the condenser in slightly 
 converging lines should illuminate the entire aperture of the slide 
 and fill the back combination of the projecting lens. Perfect 
 parallelism is not attainable, because our radiant is greater than a 
 point. The first matter we have to consider with regard to a con- 
 denser is its area, or, more conveniently, its diameter. A large 
 condenser will illuminate a larger area of slide than a small con- 
 denser. But a large condenser requires to have a long focus 
 practically, so that we lose light, as will be shown later. And, in 
 the matter of focal length of condensers, we have always to re- 
 member that there is a limit of shortness of that focal length, for 
 such a radiant as incandescent lime, not to mention an electric arc, 
 cannot, without grave danger to the condenser, be brought nearer 
 than, say, 2)^ inches from it. 
 
 In practice, then, we have to choose between condensers 
 having diameters of 3}^, 4, and 4^ inches. If the reader will 
 examine our figure, he will see that, in order to illuminate the 
 
 X s 
 
 Fig. 2. 
 
 whole surface of a slide, the condenser must have an area at 
 the very least equal to the surface area of the slide. If now our 
 slide have a mask with a round aperture 3 inches in diameter, and 
 
14 
 
 THE OPTICAL LANTERN. 
 
 if the slide is pretty close to a 3^ inch condenser, the whole of 
 the aperture in the slide will be illuminated, as in Fig. 2. 
 
 If, in fact, the whole of a 3-inch condenser were effective, 
 and if the slide could be placed close up against it, a 3-inch 
 condenser would suffice to illuminate a 3-inch aperture shde. 
 But in practice the entire surface of a condenser cannot be 
 made available, and we cannot as a rule place the slide close up 
 
 Fig. 3. 
 
 against it, and so we may take it that a 3^-inch condenser is 
 the smallest practically available for a 3-inch aperture slide. In 
 Fig. 3 we show the effect of an exaggerated distance between 
 condenser and slide, A being a longitudinal and B a vertical 
 section on the optical axis of the system. But suppose we have. 
 
 Fig. 4. Fig. 5. 
 
 or intend to use, an aperture, say of the shape known as "cushion 
 shape." A reduced plan of such a mask is given in Fig. 4, and 
 we have placed inside it the disc of illumination from a 3J5^-inch 
 condenser, on the same scale, but supposed to be not quite touch- 
 ing the slide. Plainly, taking the condenser of 3^ inches at its 
 best, it cannot illuminate a cushion which in actual measurement 
 has diagonals of 3^ inches. It will, however, be found that a 
 condenser of 4 inches diameter will cover the ordinary cushion- 
 shape aperture, or the dome (Fig. 5), and so we may accept 4 inches 
 
THE CONDENSER. 
 
 15 
 
 as sufficient diameter for a condenser for any picture on a 3 -inch 
 slide, provided that the shde be placed near enough to the con- 
 denser. 
 
 Some people seem to think that they will improve matters and 
 make quite sure by using a 4^-inch condenser. There are still 
 occasionally slides made, of the same size as a " quarter plate " — 
 i.e., 4^ X inches. These, with their masks, require a 4)^ -inch 
 condenser sometimes, but we believe this size of slide is now 
 practically obsolete; certainly it has no advantages over the 
 commoner size, 3^ inches square. There is a positive disadvan- 
 tage in a 4^-inch condenser, which we must point out. It must 
 almost necessarily have a longer focal length than a 4-inch, if 
 aberrations and absorptions of light are to be avoided, and these 
 in a condenser are most serious defects. The focal length may be 
 taken for our present purpose as the distance between the radiant 
 and the condenser, 
 
 In Fig. 6, A is the radiant, B C the condenser, and A D the 
 focal length of B C. With a 3^-inch condenser of any ordinary 
 type the distance A D may be about 2}^ inches ; with a 4-inch, 
 about 2^ inches ; with a 4^-inch, it will be about 3}^ inches or 
 more. Now, according to the well-known law, the intensities of 
 the light acting at D vary inversely as the squares of these dis- 
 tances — that is to say, the 4-inch condenser has the advantage 
 over the 4^-inch one, in approximately the proportion of four to 
 three. We cannot with safety shorten the focus of our con- 
 densers beyond the limits named (2^^ inches), because with lime 
 or electric light there would be the great danger of cracking the 
 back combination by the heat of the radiant. So, to sum up, we 
 may say that a 4-inch condenser is on the whole the best for 
 general use. If only round apertures are to be used in the slides, 
 a 3^-inch condenser will be better; if larger slides must be used, 
 it must be larger. (See Chap. XXI.) 
 
 B 
 
 Fig. 6. 
 
i6 
 
 THE OPTICAL LANTERN. 
 
 We now turn to the construction of the condenser. The 
 simplest form is known as a bull's-eye, and consists of a single 
 piece of glass plano-convex in form. Practically it is useless for 
 our purpose ; it is not achromatic, has gross aberrations, and is 
 curvilinear. 
 
 The construction usually found nowadays is that of two plano- 
 convex glasses, placed with their convex sides nearly in contact, 
 as in C (Fig. i). The pair of glasses are mounted in a brass cell 
 which fits in the front of the lantern, and it is important that there 
 should be air holes in the circumference of the cell, so that when 
 the condenser gets heated the air may escape from between the 
 glasses, and "sweating" be thereby avoided. The glasses, more- 
 over, should be quite loosely mounted in the cell, so that on being 
 heated they may have room for moderate expansion. 
 
 Fig. 7. Fig. 8. 
 
 In the Almanac of the British Journal of Photography, 1888, we 
 find the late Mr. J. Traill Taylor, a prime authority, recommending 
 a form of condenser which we believe to be older than the one last 
 described by us. This form consists, as will be seen from Fig. 7, 
 of a plano-convex or slightly meniscated lens in close proximity to 
 a double-convex. Mr. Taylor then rightly points out the loss of 
 light certain to arise with this form of condenser unless the focus 
 be long, which will entail loss of intensity, and he proceeds with 
 his usual ingenuity to suggest the interposition of a third lens of 
 plano-convex or meniscus form between the light and the doublet 
 combination previously representing the entire condenser. The 
 figures are Mr. Taylor's, and almost explain themselves. 
 
THE CONDENSER. 
 
 17 
 
 111 Fig. 'J, ab represent rays that are lost, while in Fig. 8 they 
 are refracted and utilised by the addition of the third and smaller 
 lens. Mr. Taylor therefore concludes in favour of triple con- 
 densers. 
 
 Our experience is confined to the form first figured (Fig. i) 
 and the part of Mr. Taylor's Fig. 7. Of these we prefer the 
 two plano-convex glasses (Fig. i). 
 
 Fig. 9. 
 
 Fig. 9 shows a condenser devised by Mr. E. M. Nelson, and 
 made by Mr. Baker, of High Holborn, London. This condenser, 
 as will be seen, is a triple one, consisting of a meniscus next to 
 the light, another slightly larger one in the middle, and a plano- 
 convex next the slide. The focal length of this combination is so 
 short that the light is only about an inch distant from the back 
 lens, which is plainly dangerous for the latter. But this condenser 
 gives a very good illumination, and is said to have better " correc- 
 
 FiG. 10. 
 
 tions " than the ordinary condensers of commerce. Fig. 10 shows 
 another condenser devised by Mr. Nelson, which has, possibly, 
 a more extended sphere of usefulness than the shorter focus 
 one of the previous figure. This, as is seen in the illustration, 
 consists of three plano-convex glasses, and the focal length is this 
 time great enough not to promise any danger to the optical parts. 
 
i8 
 
 THE OPTICAL LANTERN. 
 
 The condenser, Fig. 9, covers an angle of over 90°, and Fig. 10 
 an angle of over 80°. Each of these has an efficient area of 
 4 inches diameter. 
 
 Mr. Nelson has recently worked out a crown doublet, a 
 triplet, and a quadruple combination of flint glass throughout, 
 which are also, we believe, made by Mr. Baker. Of these the 
 triplet and quadruple possibly give images of greater optical per- 
 fection, though they are necessarily expensive; but the crown 
 doublet, which is shown in Fig. 11, can be recommended by the 
 writer as practically perfect for ordinary projection work at a 
 moderate price. 
 
 Messrs. Swift & Son, of Tottenham Court Road, now construct 
 three sets of special lantern condensers suitable for use with pro- 
 jection lenses of 6, 8, and 12 inches focus respectively. These 
 are made from optical flint and crown glasses from the factory of 
 Messrs. Abbe & Schott, of Jena, and can be relied upon, there- 
 fore, as being of the highest quahty and excellence. The 
 combinations for use with the longer focussed lenses are worked 
 out to give perfect results with the arc light, while that for the 
 6-inch lens is adapted to the mixed limelight jet. 
 
 A condenser, especially the lens next to the slide, must be of 
 the most perfect glass — white, limpid, and free from bubbles, 
 streaks, striae, and scratches. Any mark, on the front element in 
 particular, will show on the screen, to the great detriment of the 
 picture. The greatest care must be taken to preserve the con- 
 denser from ill-usage and noxious vapours. The elements of a 
 condenser, as already stated (especially the one next to the light), 
 should be mounted quite loosely in their cells ; they ought, in 
 fact, to rattle on being shaken. And the various elements 
 (especially the back one) should be ground to edges as thin as 
 
 Fig. II. 
 
THE CONDENSER. 
 
 19 
 
 possible. Both of these suggestions are made with a view to 
 prevent breakage. 
 
 No condenser ever made will work well if the radiant be 
 not properly centred, and carefully placed at its focal point. 
 If the radiant in a lantern be out of centre the image on the 
 screen will be unevenly lighted; if it be behind or in front of 
 the condenser's focus there will be abominable fringes of colour 
 round the margins of the disc on the screen. In oil lanterns the 
 light is usually centrally placed in the optical axis, and the lamp 
 has to be pushed and pulled till the focus of the condenser is 
 found — as nearly as it may hz found with a multiple-wick lamp. 
 
 Frequently, when a projection lens of long focus is used — say, 
 8 inches or over — a condenser of longer focus will also be required, 
 which, as a rule, implies one of larger diameter. Thus with front 
 
 Fig. 12. 
 
 lenses of 8 inches or over we generally require to use our 4^-inch 
 condenser. The reason of this will be evident to any reader who 
 will trace the course of rays from an ordinary 4-inch condenser ; 
 he will find the rays cross before they reach the projection lens of 
 long focal length. 
 
 Mr. W. I. Chadwick, of Manchester, makes a triple condenser, 
 consisting of a small meniscus collecting lens and two plano- 
 convex transmitting lenses placed with their flat sides outwards, 
 as in the ordinary crown doublet, but with a series of interchange- 
 able front lenses, by means of which the condenser may be adapted 
 for use with objectives of different focal lengths. This arrange- 
 ment, which is shown in the accompanying figure, will be found a 
 great convenience as well as a matter of economy by those whose 
 practice obliges them to use objectives of varying focus. The 
 diagram, Fig. 12, shows the condenser with the interchangeable 
 parts separated for changing. 
 
CHAPTER IV. 
 
 THE PROJECTING LENS. 
 
 The "projecting," or, as it is often loosely called, the "front" 
 lens, is in certain respects of no less importance than the con- 
 denser. It so happens that ordinary photographic lenses are in 
 the main well adapted for use as projecting lenses for the lantern, 
 and so as a rule there is little fault to find with the lenses used by 
 one who is a photographer, or by a photographic society. But in 
 the hands of the general public we often find projection lenses 
 totally unsuited for their work. 
 
 A projection lens requires in particular three qualities. It 
 must be accurately corrected for the visual rays of the spectrum ; 
 it must have a wide working aperture ; and it must be free from 
 spherical aberration. On account of the correction of a photo- 
 graphic lens, wherein the visual and chemical rays are made to 
 coincide in focal point, such a lens as we have said is usually well 
 corrected for the visual rays ; but when a lens is only corrected 
 for visual rays, and the actinic rays unheeded, that lens may be a 
 splendid lantern lens, but cannot without modification be used as 
 a, photographic lens. A photographic lens of the type used for 
 portraiture — in particular one of the rapid lenses invariably used 
 in the days of wet collodion — meets practically every want of a 
 projection lens. A portrait lens of the better class has usually a 
 flat [field, and area of aperture large in proportion to its focal 
 length, so that we get a good picture on our screen with brilliant 
 illumination. Decidedly, but not necessarily much, behind a 
 portrait lens for our purpose, comes a so-called "single" lens, 
 such as is often used in photography for landscapes ; the chief 
 drawback to this class of lens being that to secure a flat field we 
 have to "stop down" the lens and so entail loss of light. Better 
 perhaps than a portrait lens is a lens made specially for the lantern 
 by several opticians, of whom we may mention Messrs. Dallmeyer, 
 Swift & Son, and Wray, all of London. We are enabled to give a 
 
THE PROJECTING LENS. 
 
 21 
 
 diagram of a lens by Messrs. Swift, which we have worked, and it 
 leaves nothing to be desired. 
 
 Some remarks as to the focal length of the projection lens 
 seem to be called for. Given a certain distance from lens to 
 screen, the shorter the focus of the lens the larger will be the 
 disc on the screen. Given a certain lens, the nearer it is to the 
 screen the smaller the disc. Given at a certain distance from lens 
 to screen a disc too large, we can make it smaller either by taking 
 the lantern nearer to the screen or by using a longer focus lens. 
 
 Fig. 13. 
 
 A 4-inch lens will give at any distance a disc just twice the 
 diameter of that given by an 8-inch lens at the same distance. 
 Mr. Chad wick gives a table that the writer has often found 
 convenient. We quote it here : 
 
 Let S = Aperture of slide-mask in inches. 
 
 F = Focal length of projection lens in inches. 
 L = Distance from lens to screen in feet. 
 D = Diameter of disc in feet. 
 Then — 
 
 DxF _ LxS ^ LxS 
 
 Another useful guide to find the distance required from lens 
 to screen, for a certain disc, is : — Calculate by how many diame- 
 ters the disc is greater than the slide opening, add one to this 
 number, and multiply by the focal length of the lens. Thus :— 
 We require a 15-feet disc with a 6-inch lens ; 15 feet is 60 times 
 
22 
 
 THE OPTICAL LANTERN. 
 
 3 inches, which we suppose to be our slide aperture; 6i x 6 = 
 366 inches = 30 feet 6 inches, distance required from lens to 
 screen. 
 
 It is for various reasons usually a mistake to choose a lens of 
 very short focal length. If the room is small there is not much 
 choice, but in a large hall we like to get as far back as we can up 
 to 80 feet; at this distance a 12-inch lens gives a 20-feet disc. 
 But beyond this we do not go if we can help it. If the lantern 
 has, on account of shortness of focal length of lens, to be placed, 
 say, 25 feet from the screen in a moderate sized hall, some people 
 behind the lantern will be prevented from seeing properly, and 
 the apparatus will probably be surrounded by part of the audience, 
 which is objectionable and even dangerous if gas-bags are used, 
 and is forbidden by many local authorities. Moreover, if the 
 screen is placed on a raised platform the lantern has to be canted 
 up and the screen tilted forward to an inconvenient degree; 
 whereas, if the lantern were, say, 60 feet distant from the screen, 
 the necessary cant and tilt would be much less, and the loss of 
 light by greater distance is not nearly so great as some thought- 
 less persons seem to fancy, for the oft-repeated formula about the 
 " intensity of light varying as the squares of the distances " does 
 not hold here at all. The loss of light in enlarging a 3-inch disc 
 to a 1 5-feet disc is, cceteris paribus, practically the same whether 
 the enlargement be produced by a 4-inch lens or an 8-inch. But 
 if our projecting lens has a focal length either so short or so long 
 that its back combination fails to grasp some of the rays proceed- 
 ing from the condenser through the slide, then surely we shall lose 
 light (see page 19). By altering our condenser's focal length we 
 may cram more pencils of light into a long focus front lens, but 
 then we shall lose some of the pencils of light between the radiant 
 and the condenser, or else have to remove the radiant 
 further from the condenser, in which case our ancient formula 
 will hold good as touching the illumination of the condenser 
 itself. Reference to Fig. i will show at once that if the 
 front lens has a very short focal length it will not catch all the 
 pencils of light passing through the slide, unless it (the lens) has 
 a very wide back combination ; and again, if we get an aperture 
 very wide in proportion to our focal length, we shall have a lens 
 impossible to correct for aberrations. So, as usual, it is a case of 
 give and take. We cannot have perfection in anything optical. 
 
THE PROJECTING LENS. 
 
 23 
 
 Speaking from experience, we may say that from a quarter-plate 
 portrait lens of about 5^4^ inches focal length to a lantern lens of 
 8 inches, we have no trouble in using our 4-inch condenser; with 
 a 10 or 12-inch lens we have had to use a 4}^ -inch condenser; 
 while with a very short focus lens we happen to possess, our 4-inch 
 condenser does not work very well. We are perhaps safe in saying 
 that, for all-round work, a lens of about 6 inches focal length, and 
 a condenser of 4 inches, as usually made, will do as well as any 
 other one-lens battery. The most useful lens we have for the lan- 
 tern has a focal length of 9 inches; at 45 feet we get a grand 15- 
 feet disc, using a circular slide mask. 
 
CHAPTER V. 
 
 THE LANTERN BOD Y. 
 
 As already stated, the functions of the lantern-body, or box, are 
 simple, and consist merely of holding the parts of the optical 
 system together and enclosing the illuminant so that damaging 
 rays of light cannot reach the screen to spoil the image, nor the 
 eyes of the onlookers, to dazzle them. 
 
 When the radiant is an oil lamp the lantern-body must be 
 fairly darge, for the lamp is sure to be of considerable size if a 
 
 Fig. 14. 
 
 powerful one with, say, three wicks; there must be a certain 
 amount of unoccupied space inside the lantern to give draught 
 and adjustment room for the lamp; and, as a rule, the lamp 
 requires a somewhat tall chimney to ensure draught. The body 
 of the lantern is almost always "jacketed," wholly or in great 
 part, in order to meet the great heat given off by the lamp; some- 
 
THE LANTERN BODY. 
 
 25 
 
 times the outer jacket is of metal, sometimes it is of wood, the 
 inner jacket being invariably metal. 
 
 Perhaps the first really business-like lantern introduced with 
 any success was the now well-known sciopticon of Mr. Marcy^ 
 which in many points formed a standard for later modifications. 
 
 The double wick of the sciopticon lamp was soon found to be 
 faulty in several ways, and a third wick was by various makers 
 and in various forms added. Messrs. Newton greatly improved 
 the performance of the lanterns of that day by placing a suitably 
 curved reflector behind the wicks, their oil lamp being known as 
 the " Refulgent." In most oil lamps will be found in front of the 
 wicks a piece of flat glass to prevent breakage of the condenser. 
 In our experience a well-arranged trio of wicks is better than two 
 only; and we are informed ihat even five wicks have been 
 successfully introduced. It must never, however, be forgotten 
 that the number of wicks does not necessarily mean increase of 
 eff'ective light ; and so far as our knowledge goes, we believe the 
 best multiple-wick combination to be one of three wicks, placed 
 longitudinally in the optical axis, the two outer wicks slightly 
 inclined inward towards the centre one. The dangers with many- 
 wick lamps are, great heat and a shadow down the centre of the 
 image on the screen. 
 
 Serviceable oil lanterns are supplied by W. C. Hughes, of 
 Kingsland ; Clement & Gilmer (successors to Laverne), of Paris; 
 Chadwick, of Manchester ; Hume, of Edinburgh ; and Lancaster, 
 of Birmingham, from any of whom particulars of instruments in 
 varying degrees of elaboration and costliness can be obtained on 
 application. 
 
 But while a lantern-body for an oil lamp is necessarily of 
 considerable size, we are convinced that lanterns for use with 
 oxyhydrogen jets are usually made ridiculously large. Until quite 
 recently the lantern, as usually made, was about three times as large 
 and twice as heavy as it need be. With a lime jet there is no 
 occasion for much unoccupied space, for no such draught is required 
 as with an oil lamp ; no chimney is required at all, and the only 
 inconvenience to be feared is that of great heat. If the body be 
 well jacketed — especially if the outer jacket be of wood — the total 
 size of the lantern body may be very much less than it usually is. 
 without affecting in any way the performance of the apparatus. 
 Fig. 15 shows a lantern made by Mr. Baker, of High Holborn,. 
 
 c 
 
26 
 
 THE OPTICAL LANTERN. 
 
 and it embodies all the essentials of a perfect single instrument, 
 though it is much smaller than the average optician's lantern. 
 
 The receptacle whereinto the " slide- carrier " goes should have 
 its front provided with a spring sufficiently strong to hold the 
 carrier in position while slides are being passed through. We 
 propose to add to this part a couple of pinch screws, whereby the 
 carrier, once in position, shall be clamped there. The space 
 between the front of the condenser and the back of the " cone," 
 or nozzle tube, to which the projection lens is screwed, is some- 
 times required for a water-tank, or other scientific article, whose 
 image requires to be projected upon the screen. An " open stage " 
 is easily devised, or a lantern specially adapted for such demon- 
 strations may be used. 
 
 Fig. 15.— Single Lantern. 
 
 The projection lens is screwed, or otherwise fixed, to the 
 " nozzle " of the lantern. This nozzle should " telescope " to a 
 reasonable extent outw^ards, for on occasion a long-focus lens 
 may be required, 8, 10, or even 12 inches focal length. It is very 
 awkward to find that the nozzle will not stretch far enough for 
 such a lens, when of necessity the lantern has to be a consider- 
 able distance from the screen. (If the lantern is to be used for 
 '"enlarging" purposes in photography there is all the more need 
 for a long stretch of nozzle.) 
 
 Fig. 16 shows a very small lantern made for the writer by Mr. 
 Beard, and now on sale by Mr. J. H. Steward. This miniature 
 lantern packs into a box 7 x 6 x 51^ inches, and it has a 4-inch 
 condenser and a good projection lens. It is fitted with a mixing 
 
THE LANTERN BODY. 
 
 27 
 
 jet ; and, in fact, is capable of any work that any other single 
 lantern can do. For packing, the front simply slides into the body, 
 Fig. 1 7. The whole lantern is of iron, except the front, which is of 
 brass. In use it does not become intolerably hot if the jet is 
 
 Fig. 16.— Pringle-Beard Lantern. 
 
 properly used; but of course we do not lay our hands on it during 
 use. We have packed up our lantern within two minutes after 
 extinguishing the light. Another advantage of this miniature is 
 that it can be sold at a very moderate price. 
 
 Fig. 17.— Showing Pringle-Beard Lantern Packed Up. 
 
 Messrs Newton also make a "miniature" lantern, and Mr. 
 W. C. Hughes makes one with a cylindrical body. If the con- 
 denser be made square, the size of the body may be still 
 further reduced ; but as we can with ease carry our own lantern 
 (packed with all its fittings, lenses, carrier, and jet) in one hand. 
 There seems to be no cause for further reduction of size. 
 
 Lanterns have always side doors— sometimes on one side 
 only, sometimes on each side. The door is usually provided 
 with a window of coloured glass, by which the worker is sup- 
 
 c 2 
 
28 
 
 THE OPTICAL LANTERN. 
 
 posed to view the light without hurting his eyes. The writer 
 has never yet been able to judge of the light through such a 
 window, and has never felt any bad effect from looking directly at 
 it with half-closed eye-lids, a way by which he caii judge of 
 the quahty of the light. It is certainly necessary to have at 
 least one door, which, however, during a display, should be very 
 seldom opened. In every lantern the writer remembers to have 
 seen there has been a considerable opening at the back where 
 the jet goes in, and lecturers of any experience must have 
 noticed the mischief caused by this opening if any of the 
 audience happen to be seated behind and near the lantern. 
 A person so situated can see next to nothing on account of the 
 glare from the lantern. If the lantern have a wooden outer body 
 a curtain of thick velvet should be fixed to the top of the back of 
 the body, and hang loosely down over this objectionable opening. 
 The lantern. Fig. 15, has at the back a door sliding in runners, 
 and this door is pushed down after the jet is in position ; . but 
 ■ even this is not a sufficient protection in all cases. When the 
 lantern body is of iron a similar piece of cloth may be hooked on 
 to it by metal hooks, so bent as to make the curtain hang free 
 of the hot metal body— if the body ever becomes hot enough 
 to singe cloth, which ought never to occur if the lantern be 
 properly arranged. 
 
CHAPTER VI. 
 
 DOUBLE AND TRIPLE LANTERNS, DISSOLVING 
 VIEWS, EFFECTS, ETC. 
 
 We now come to those edifices known by various names, and 
 consisting of two or more lanterns made to work concentrically, 
 which appear to be as necessary to the professional itinerant 
 lecturer as they are useless to the scientific demonstrator or the 
 photographic amateur, except in a few very rare cases. The chief 
 uses of these multiple-lanterns are: — For what is called "dis- 
 solving " one picture into another, and for producing so-styled 
 " effects," which are peculiarly the province of the popular public 
 lecturer. Some of these "effects" are interesting, "effective," 
 sensational, or really pleasing, and ought not to be despised. 
 We allude to changes from day to night — from one season to 
 another ; while others, on which great ingenuity in slide-making 
 is spent, do not come within the province of this book, nor touch 
 those for whom this book is specially intended. There are some 
 who think that in the display of a series of ordinary photographic 
 lantern slides there is an ^advantage in " dissolving " from one view 
 to the next ; that is to say, these parties prefer to see one image 
 die gradually away and the next on the list grow gradually in a 
 weird or " uncanny " sort of way out of the ghost of the last, 
 rather than to have one picture follow another in the natural way. 
 For our part, we distincdy object to the dissolving business for 
 ordinary slides ; but provided the masks of the slides be all of 
 one size and shape, the lanterns accurately registered, and the 
 views or pictures reasonably adapted for such treatment (which 
 they very seldom are), then by means of a double lantern the 
 desideratum may be attained. And further, if it is deemed 
 requisite or desirable to have the appearance of a curtain being 
 drawn at the beginning of, and persisting throughout, the 
 lecture, it can be achieved by means of a third storey, the 
 lantern being then a triple, or more gloriously a " triunial," the 
 top storey being sometimes lighted by oil. All these luxuries are 
 
30 
 
 THE OPTICAL LANTERN. 
 
 obtained only at the expense of a double or triple set of jets to 
 look after, and to keep burning with equal brilliance— for if one 
 disc is less bright than the other the effect is simply atrocious. If 
 we are using, say, a double lantern, we have an arrangement 
 usually very ingenious, often highly intricate, called a " dissolver "; 
 if we are using gas in all three lanterns of a triunial the dissolver 
 becomes a matter, sometimes, for a life-study. Still we do not 
 seek to discourage the triunialist, nor to underrate the value of 
 good "effect" displays. 
 
 In the early days of double lanterns, the two bodies were 
 usually placed side by side, and the "dissolving" effected by 
 serrated shutters in front of the projection lenses. We are 
 indebted to the late Mr. George Smith, of the Sciopticon Co., 
 I-ondon, for this detail sketch of the dissolver attached to his 
 instrument. Of course there was a lamp in each lanlern. 
 
 a. 
 
 Fig. i8. — Smith Dissolver. 
 
 The dissolver 6? is mounted on the arm b by means of a screw, 
 as seen in Fig. i8, so as to cover alternately the lenses as it swings 
 from side to side. The horizontal part of b slips into c till the 
 length of the united axle just allows the dissolver to swing clear 
 of the lenses, and the whole is held in place by a socket spring 
 at each end of the base-brard. 
 
 The dissolver is operated by the handles at c, which are 
 adjusted at the proper angle to limit the lateral movement of a 
 to the distance between the lenses. 
 
 Now, however, nous avons change tout cela, and we place the 
 lanterns one on top of another. There must be an arrangement 
 for causing the two discs to coincide precisely on the screen; this, 
 of course, is managed by giving facilities for tipping the upper 
 lantern down or lower lantern up, or both. Or the hinged 
 parts may be entirely in front of the lantern-box proper, and the 
 
DOUBLE AND TRIPLE LANTERNS. 31 
 
 tipping may include only the front plate and all it carries. 
 The latter system is perhaps the most convenient. Double or 
 
 Fig. 19.— Biunial or Double Lantern. 
 
 biunial lanttrns are generally made all in one piece, and the two 
 lanterns cannot be separated ; but it seems customary to make 
 
 Fig. 20. — Triunial or Triple Lantern. 
 
32 
 
 THE OPTICAL LANTERN. 
 
 triples in separate parts — the top lantern almost always can be 
 removed — so that if we have a triunial we have three separate 
 lanterns. So far as we have seen, the chief variations in these 
 articles consist rather in the amount and finish of the brasswork, 
 and in the price, than in any really essential qualities. 
 
 From what has been said, it will be readily believed that the 
 arrangement for the distribution of the gases in suitable propor- 
 tions to each jet, without loss of time or uncertainty from anyone 
 to any other of three lanterns, without waste of gas, must be a 
 problem of considerable difficulty of solution. It is, however, 
 excellently solved by more than one "dissolver." 
 
 This consists of a system of tubes or pipes, arranged in pairs, 
 and communicating between each of the separate lanterns and 
 the oxygen and hydrogen supply respectively. The individual 
 
 Fig. 21.— Star Dissolver. 
 
 tubes are fitted with stopcocks to regulate the supply of each gas, 
 and, when this has been carefully adjusted, there are usually one 
 or more additional taps by means of which the flow of gases can 
 be diverted to one or other of the lanterns as may be required, 
 or, if necessary, shut off altogether, with the exception of the 
 small quantity needful to prevent total extinction, for which a 
 " bye-pass," subsequently referred to, is provided. This form of 
 dissolver was originally introduced under the name of the 
 " Maiden," from its inventor, but is now manufactured in a 
 variety of modifications by nearly every maker of lantern requi- 
 sites under the generic title of the " Star Dissolver," from the cir- 
 
DOUBLE AND TRIPLE LANTERNS. 
 
 33 
 
 cumstance that the system of tubes usually takes more or less 
 the form of a star. The number of tubes depends upon the kind 
 of lantern to be used, whether " biunial" or " triple," and the 
 dissolvers are known as " four-way " and " six-way," according to 
 the number of tubes they include in addition to bye-passes. 
 
 In all cases, of course, there must be a bye-pass on the 
 hydrogen ; in some cases there is a bye-pass on each of the gas 
 tubes. With a pair of "blow-through" jets, for instance, it is 
 necessary to have a bye-pass on the oxygen as well as the hydro- 
 gen. In testing a dissolver we have to note whether there is 
 any noise in effecting the change, whether any time is lost before 
 the second hme is fully heated, and whether any gas is wasted. 
 A dissolver that requires only one hand to work is naturally pre- 
 ferable to a less convenient one. 
 
CHAPTER VII. 
 
 ILLUMINANTS—OIL, INCANDESCENT GAS, 
 ACETYLENE. 
 
 For small displays, undoubtedly the most popular, if not also the 
 most convenient, form of illuminant is the mineral oil lamp ; but 
 its use is limited to discs of moderate dimensions, say, of not 
 more than 9 feet or 10 feet in diameter, Formerly, the lantern 
 in which a lamp burning colza or sperm oil was used could be 
 considered as little better than a toy, although prior to its super- 
 cession by the more modern form a considerable amount of 
 attention had been given to the improvement of the quality of the 
 light. In its latest form the "fountain lamp," as it was called, 
 consisted of a circular or argand wick, surrounded by a slightly 
 tapering glass chimney, and so arranged that a free current of air 
 passed through the centre of the flame. The oil was contained 
 in a reservoir placed behind and slightly above the level of the 
 fiame, in which position the heat of the flame was advantageously 
 utilised in raising the temperature of the oil and thus improving 
 its illuminating power. The wick-chamber was connected with 
 the reservoir by means of a tube, "trapped" in order to regulate 
 the flow of the oil, but owing to the position of the reservoir above 
 the level of the flame it was always a difficult matter to avoid over- 
 flow, and, consequently, mess. The more usual illuminants were 
 colza, or, where expense was of less importance, sperm oil. To 
 these were frequently added camphor and other substances to 
 further improve the quality of the light. Camphine — a mixture of 
 alcohol and turpentine, and a variety of similar compounds, were 
 frequently used or recommended, but with very doubtful ad- 
 vantage, and colza oil may be safely regarded as the staple 
 illuminant. 
 
 About five-and-twenty years ago, however, Marcy, of Phila- 
 delphia, introduced an entirely new form of lantern already 
 alluded to under the name of Sciopticon, which formed the pre- 
 cursor of the modern style of instrument. Amongst other 
 
OIL. 
 
 35 
 
 important improvements it included a double-wick lamp burning 
 paraffin or petroleum oil. This radical departure from previous 
 practice, together with the arrangements made for the perfect 
 combustion of the oil, wrought such an improvement in the 
 degree and quality of illumination, that the oil-lantern began to 
 assume a practical form for exhibition purposes on a small scale. 
 The original Marcy instrument was still further improved by the 
 late W. B. Woodbury, and introduced, into this country by the 
 Sciopticon Company, and a fresh impetus was thus given to the 
 general use of the lantern. 
 
 Since that period the number of wicks has been increased to 
 three, four, and even five, though it is questionable whether the 
 gain in light with the larger number is commensurate with the 
 great increase in heat and consequent inconvenience. Probably 
 a well-arranged triple-wick lamp at the present day offers the 
 greatest combination of advantages with the least inconvenience ; 
 at any rate, that is the form we should be inclined to recommend 
 as the best all-round lamp. 
 
 A very efficient, and, at the same time, powerful lamp of 109 
 standard candle-power is known as Stocks's patent, and is obtain- 
 able through most dealers. It has four wicks, placed edgewise 
 and at such an angle with one another that a very even illumina- 
 tion is given. It is fitted with a suitable reflector and a chimney 
 adjustable as to height by means of rackwork, so that the 
 draught may be increased or reduced to produce the best pos- 
 sible result. When a powerful oil-lamp is required this will meet 
 the want. 
 
 One difficulty, and possibly the chief one from an optical point 
 of view, with the multiple-wick lamp is the tendency to uneven 
 illumination of the screen, owing to the area of the source of light 
 and its want of homogeneity. In order to secure the greatest 
 lighting-power the wicks are placed edgewise to the condenser, 
 under which conditions there is a danger of the formation of dark 
 streaks corresponding with the spaces between the flames ; but 
 this trouble is minimised as far as possible by a variety of in- 
 genious modifications of the relative positions of the wicks and 
 by placing them at such angles with one another as to cause them 
 to present a practically uniform illuminating surface. Attention 
 also to the important matter of a proper adjustment of the supply 
 of air to the flame to set up perfect combustion aids not only 
 
36 
 
 THE OPTICAL LANTERN. 
 
 the quality and brilliancy of illumination, but further helps to 
 reduce the inconvenience arising from excessive heating. 
 
 In order to reduce as far as possible the latter trouble, the 
 body of the oil lantern is usually, or we may say invariably, in 
 instruments of any pretentions to quality, "jacketed" or doubled 
 in order to provide for a constant current of comparatively cool 
 air passing between the two casings and thus reducing the heating 
 of the whole arrangement. In the original Marcy lantern, as well 
 as in many of the more modern ones, a sheet of plain glass is 
 interposed between the light and the condenser to protect the 
 latter, and in fact form a combustion chamber round which a 
 current of cooler air is constantly playing. 
 
 It is needless to say, in view of the great amount of heat given 
 off even under the most favourable conditions, that the selection 
 of a suitable sample of oil is a matter of the utmost importance. 
 Not merely on the score of quality of light, but more imperatively 
 on grounds of safety, none but the best quality of oil should be 
 used. The cheap low-flash samples are entirely out of the 
 question, and even those of medium quality, that would be per- 
 fectly safe for ordinary household purposes, may prove a source of 
 anxiety, if not of absolute danger^ when used for optical lantern 
 purposes. After all, the difference in cost is so slight that there is 
 no inducement to wilfully employ the cheaper oils. 
 
 A serious objection frequently raised against oil lanterns is on 
 the score of dirt and smell, but this we have little hesitation in 
 saying is invariably the fault of the operator rather than of the 
 lantern or system. If the wicks be kept properlytrimmed and always 
 burning at a proper height, and the inlets for air to reach the 
 flame be kept clear and free from particles of burnt wick ; but, 
 above all, if every portion of the lantern be thoroughly wiped and 
 no trace of oil allowed to run over, then there need be no fear of 
 either dirt or smell. But if these precautions be neglected, the 
 oil lantern will be a source of constant trouble and a nuisance 
 wherever used. 
 
 Many have been the attempts made to adapt ordinary house 
 gas to optical lantern purposes, but until quite recently, with 
 but scant success. The comparatively poor illuminating power 
 of coal gas, in proportion to the size of flame and amount 
 of heat given off, has been altogether against the adoption of what 
 would otherwise be perhaps the cleanest and most convenient 
 
INCANDESCENT GAS. 
 
 37 
 
 method of lighting we have. However, the introduction of the 
 Welsbach incandescent gas burner, or rather, perhaps, the improve- 
 ments made in that form of Ughting since the last edition of this 
 work was published, has brought gas within the bounds of 
 practicability for projection on a small scale, and especially for 
 enlarging purposes. 
 
 In this system an intensely hot flame on the principle of the 
 Bunsen burner is caused to impinge upon a " mantle " or thimble- 
 shaped framework of woven fabric impregnated with metallic 
 oxides, such as zirconia or thoria; the mantle is thereby rendered 
 incandescent and emits a light of extreme intensity with a 
 relatively small consumption of gas. The principal inconveni- 
 ence hes in the fragile nature of the mantle and its comparatively 
 short " life." The system is, however, easily applied to any exist- 
 ing lantern, and though we have personally not had much experi- 
 ence with it, where gas is available we have Uttle doubt that it will 
 often be a convenience for small discs. The intensity of the light, 
 the comparative smallness of its area, and its relatively low 
 temperature, are all in its favour, though we believe there is often 
 a difficulty in securing evenness of illumination owing to the 
 tendency of the image of the "mantle" to project itself on the 
 screen. When this occurs, however, we think it must be at least 
 partly owing to a want of proper adjustment of the focal lengths of 
 the condenser and projecting lens respectively. When it occurs 
 and cannot othervvise be got rid of, it may be obviated at the cost 
 of some loss of light by the intervention of a sheet of finely-ground 
 glass — in the optical system, preferably between the light and the con- 
 denser. For enlarging purposes the loss of light may be practically 
 ignored as it may be compensated by a slight increase of exposure, 
 and the same expedient will often be found useful in the case of 
 an oil lamp. 
 
 The latest aspirant for popular favour as a general illuminant is 
 acetylene gas, and this is more particularly adapted to lantern 
 purposes by reason of the ease with which it is generated as 
 required, its intensity and high luminosity, the smallness of its 
 flame and the relatively small amount of heat given off. When 
 consumed under proper conditions and with a suitable burner, it 
 will give a light equal to from 200 to 300 candles, which places it 
 far ahead of the ordinary oil lamp for small exhibitions, and brings 
 it somewhere to the level of the blow-through limelight jet. For 
 
38 
 
 THE OPTICAL LANTERN. 
 
 discs up to lo feet in diameter, or for working up to 30 feet from 
 the screen, it answers admirably, and even for larger shows is no 
 doubt superior to a badly-worked limelight. For enlarging also it 
 is particularly convenient owing to the whiteness and actinic 
 character of the light. 
 
 Acetylene is a hydrocarbon — C2 Ho — and was first prepared in 
 the pure state, and as a definite compound by Berthelot in 1859, 
 though as far back as 1836 it had been obtained by E. Davy in an 
 impure state by the action of water on the black substance formed 
 in the preparation of potassium. The earlier methods of prepara- 
 tion consisted in passing ethylene gas or vapour of ether, alcohol, 
 aldehyde, or wood spirit through a red-hot tube, or by the action 
 of red-hot copper upon chloroform. The gas thus obtained was 
 purified by passing it through a solution of cuprous chloride, by 
 which a red precipitate is formed, which, when decomposed by 
 hydrochloric acid, gives off pure acetylene. But under these 
 methods of preparation acetylene had not the least industrial 
 value, and it was not until the accidental discovery by an American 
 of a cheap method of production on a practically unlimited scale 
 that it assumed any importance in connection with the arts and 
 manufactures. 
 
 Berthelot had shown that acetylene could be formed by 
 the direct combination of carbon and hydrogen by passing 
 hydrogen gas over charcoal heated by the electric arc ; but this 
 again, with the appliances of the period was not a practical pro- 
 cess, though at the present day somewhat similar means have 
 been adopted with success. In the course of some experiments 
 an American chemist, Mr. Wilson, had occasion to fuse together 
 in the electric furnace lime and carbon, the result being a dark 
 grey, slag-like mass which was thrown as waste into a bucket of 
 water. Violent effervescence took place with copious evolution 
 of a pungent and evil smelling gas, which on investigation 
 proved to be acetylene and the fused mass from which it is obtained 
 carbide of calcium. This in fact constitutes the modern process 
 of manufacture. Carbide of calcium is first prepared by fusion 
 together in the electric furnace of lime and carbon, the latter 
 in the form of powdered coal or similar waste. This, when sub- 
 jected to the action of water is decomposed, acetylene being 
 liberated and slaked lime remaining as a residue. 
 
 2CaC + 2HoO = QHg -f 2CaH0. 
 
ACETYLENE. 
 
 39 
 
 The acetylene thus formed, if properly cooled and condensed 
 or dried, is in a state of sufficient purity for ordinary illuminating 
 purposes, though with some of the samples of inferior foreign carbide 
 it is decidedly advantageous to submit it to a further course of 
 purification. It forms a heavy colourless gas of strong unpleasant 
 odour and specific gravity 0-92, or as nearly as possible double 
 that of coal gas. It burns with an intensely brilliant, but if not 
 properly supplied with oxygen, smoky flame. The latter is 
 relatively small and the light white and extremely actinic in 
 character, possessing from fifteen to twenty times the luminosity 
 of coal gas, with which, light for light, it compares favourably 
 from an economic point of view, being equal at current prices of 
 the carbide to gas at 2s. 6d. per thousand. 
 
 Under normal conditions acetylene is not explosive and cannot 
 be made to explode, but when mixed with a certain proportion 
 of air, or when under a pressure exceeding two atmospheres, it 
 becomes highly explosive. It also forms explosive compounds 
 with copper for which reason no brass or copper should be used 
 in the fittings. For these and other reasons probably based upon 
 ignorance of the nature of the gas, various absurd and vexatious 
 restrictions were placed upon the use of acetylene as well as the 
 storage of calcium carbide by the insurance companies and the 
 County Councils ; but these have been in the main removed or 
 considerably modified as increased knowledge of the necessary 
 conditions was acquired and more perfect appliances for the 
 generation and use of gas have been devised. The only remain- 
 ing restrictions of any importance are, first, that for general 
 illuminating purposes the generator must be fixed outside the 
 building; and, second, that carbide in quantity must be stored, 
 also outside, in properly constructed air and damp proof recep- 
 tacles sanctioned by the County Council* Small generators for 
 lantern use, if satisfying the Council's requirements and quantities 
 of carbide up to a few pounds are free from restrictions. 
 
 A very large number of generators of varying degrees of 
 merit have sprung into existence, the difference consisting chiefly 
 in the degrees of precaution taken to ensure efficiency and safety. 
 The majority work on the principle of the floating gasometer, 
 which rises as it fills and on rising lifts the charge of carbide out 
 
 At the time of going to press, even these restrictions have been still 
 further modified. 
 
40 
 
 THE OPTICAL LANTERN. 
 
 of the water and temporarily arrests the formation of gas until, 
 by use, immersion again takes place and the process is renewed. 
 In this manner only a small quantity of gas is stored at once 
 but the process of manufacture becomes practically automatic and 
 continuous, so long as any carbide remains undecomposed. In at 
 least one form of generator the decomposition is set up by allow- 
 ing water to drip at an adjustable rate upon the carbide, but the 
 former plan seems in every way the preferable one. Additional 
 arrangements have to be made for cooling and drying the gas in order 
 to keep the supply pipes clear and also to prevent the admixture 
 of air with the gas and the formation of an explosive compound, 
 and it is in these details that the various machines chiefly differ. 
 
 Fig. 22.— " Incanto" Acetylene Generator. 
 
 The earliest and also one of the most efficient of generators is 
 the "Incanto" of Messrs. Thorne & Hoddle, of Westminster. 
 This is made in various sizes for general purposes but the diagram 
 (Fig. 22) shows their pattern A which is specially designed for 
 lantern purposes. It consists, as will be seen, of an outer water 
 tank in which is fitted a rising gas holder in the centre of which 
 is placed the carbide holder or generating chamber. When a 
 charge of carbide is placed in the chamber and the apparatus 
 filled with water, the gas is given off and collects in the holder, 
 which immediately begins to rise and continues to do so until 
 the carbide holder which rises with it is lifted out of the water, 
 when the generation ceases, to be renewed when sufficient gas 
 
ACETYLENE. 
 
 41 
 
 has been used to allow the carbide to dip into the water again 
 If the instructions supplied with the apparatus be strictly followed 
 no attention is required after charging until the gas is exhausted 
 and the charge requires renewings and the whole may be left alone 
 when not in use. The smallest size takes a charge of \}{ lb. of 
 carbide equal to about 6 cubic feet of gas, and will work continu- 
 ously with one burner for five hours or with a double one for two 
 
 Fig. 23. — Abingdon Safety Generator. 
 
 hours and a half. Other sizes will work respectively for twelve and 
 thirty hours with single burner. 
 
 In Fig. 23 is shown the "Abingdon Safety Generator," as specially 
 constructed for lantern purposes, with the generating chamber 
 removed for charging, which operation may be performed while the 
 gas is being used, as the gas-holder is fitted with a "water seal," 
 which effectually prevents the escape of gas, either during use or 
 when the generating chamber is removed. This renders the 
 apparatus practically continuous in its action. Fig. 24 shows the 
 same apparatus with the carbide cages removed from the generator 
 and still further explains the internal arrangements. It is needless 
 to say that in these, as in several other generators which we have 
 not space to describe, the arrangements for drying and purifying 
 the gas and for safeguarding against explosion are of the most 
 perfect description, and the employment of acetylene under these 
 conditions, whether for general illuminating or merely for lantern 
 purposes, is absolutely safe in any ordinary hands. When it is 
 
 D 
 
42 
 
 THE OPTICAL LANTERN. 
 
 Stated that the total weight of the apparatus varies, according to 
 size, from 8^ lb. to i6 lb., its advantage over gas-bags or cylinders 
 for small exhibitions is manifest. 
 
 Fig. 24. — Abingdon Generator (showing Internal Arrangements). 
 
 In order to secure perfect combustion, and therefore the 
 best result with acetylene, a special form of burner is necessary, or 
 one passing a very small quantity of gas as compared with ordinary 
 coal gas. With the latter the usual consumption per hour is about 
 5 feet, but with acetylene it is necessary to reduce the quantity to 
 I foot by using a Bray's 00000 burner, the small consumption of 
 
 Fig. 25. — Three-burner Acetylene Jet. 
 
 gas and the diminutive flame being compensated by the extreme 
 brilliancy of the light. This appears to be the maximum consump- 
 tion of gas permissible from a single burner, though they may be 
 had to consume no more than a quarter of a foot per hour. But 
 to secure the very best illumination possible for lantern purposes 
 very convenient arrangements of two or three burners have been 
 
ACETYLENE. 43 
 
 devised, and with these two and three hundred candle-power illu- 
 mination is easily attained. 
 
 Fig. 25 shows a 3 burner fitting made by Messrs. Thorne and 
 Hoddle, for which they claim 300 c.-p. This goes into the 
 lantern in much the same way as a limelight jet, and is similarly 
 
 Fig. 26. — Moss Lamtern Jet. 
 
 adjustable and particularly easy to focus. In Fig. 26 we have the 
 recently patented " Moss Lantern Jet," made by the Abingdon 
 Acetylene Illuminating Company, which, besides the other adjust- 
 ments, provides for the separate regulation of the individual 
 burners, which means the getting of the highest efficiency out 
 of each. This we believe gives 240 c.-p. 
 
 D 2 
 
CHAPTER VIII. 
 
 ILLUMINANTS ( Continued)— LIMELIGHT, JETS, 
 ACCESSORIES, SATURATORS, ETC. 
 
 The radiant most commonly used, and, as we think, tlie best to 
 use in the optical lantern, is known as the lime-light, or sometimes 
 as the oxy-hydrogen lime-light. A point on the surface of a disc 
 or cylinder of unslaked lime is rendered incandescent by the 
 action of an oxy-hydrogen blow-pipe. The lime is very " refrac- 
 tory " i.e., very difficult to cause to burn — but when it is rendered 
 
 Fig. 27.— Oxy-Calcium Jet and Tray. 
 
 incandescent the light given out is very brilliant. There are "soft" 
 limes and " hard " limes, the soft ones being rendered incandes- 
 cent more easily than the hard ones, and so the former are used 
 when the heating power of the blow-pipe is not very great, as, for 
 instance, with the so-called oxy-calcium light, or the "blow- 
 through" or "safety" jet. But to get the best result possible 
 with the lime-light—/.^., to get the greatest brilliance over the 
 least area— we must use the hardest lime and a blow-pipe of 
 proportionate heating power. 
 
 The oxy-calcium light calls for little remark ; it is much less 
 brilliant than the next higher light, the blow-through. In the oxy- 
 calcium system we have a reservoir of spirits of wine and a tube 
 from reservoir to nozzle, where there is an ordinary wick. The 
 
JETS. 
 
 45 
 
 spirit is lighted at the wick ia the usual way, and a stream or jet 
 of oxygen gas is driven through the flame to the lime, which is 
 rendered incandescent, and gives a fairly bright light, estimated 
 approximately at "150 candles." Fig. 27 shows the ordinary 
 form of oxy-calcium jet and tray. 
 
 A form of spirit jet, introduced by Mr. J. M. Turnbull, of 
 Edinburgh, is shown in Fig. 28. The reservoir contains methy- 
 lated spirit, and oxygen is blown through the flame. In order to 
 secure the best effect, the oxygen nipple should project an eighth 
 of an inch inside the flame, and, of course, soft lime is required. 
 The arrangement is shown by which the jet is kept level when 
 the lantern is tilted. It gives a good light for small discs not 
 exceeding ten or twelve feet. 
 
 Fig. 28.— Turnbull Spirit Jet. 
 
 The blow-through jet comes next in order of brilliance of light 
 attainable. The nozzle is made in slightly varying forms. In 
 this form of jet we use a fairly large column or stream of 
 hydrogen gas, either direct from the usual house supply or under 
 a low pressure when compared with that of the mixing-jet. In 
 or near the centre of this quietly burning stream of hydrogen is a 
 small nipple, through which oxygen is forced at some pressure 
 greater than that on the hydrogen; the oxygen is bloivn through 
 the hydrogen flame, whence the name given to this jet, which is 
 also sometimes called the "safety." A form of blow-through jet 
 which gives as good a light as any known to us is one made 
 by Messrs. Newton. The estimated power of a good blow- 
 through jet is 250 candles; but the disadvantages of this sys- 
 tem are that the area of incandescence is generally large when 
 
45 
 
 l^HE OPTICAL LANTERN. 
 
 the greatest total of light is given off, and the light is apt to be 
 red in tint. 
 
 An excellent jet on the blow-through principle has been 
 more recently introduced by Erin's Oxygen Company, and this 
 leaves little to be desired when circumstances admit of the 
 employment of the weaker form of light. 
 
 The same gases are used in the mixing-jet, but both are 
 usually under high-pressure, and they are mixed in a chamber 
 before they emerge from the nipple where they are lighted. The 
 reason for the nomenclature is thus evident, and on the com- 
 pleteness of the mixture of the gases depends in a great measure 
 the amount of heat, and consequently of light produced. 
 
 Durmg the winter of 1895-6 a couple of evenings were 
 devoted by the members of the Photographic Club to a trial of 
 gas jets and several instruments of very high efficiency were shown 
 and photometrically measured by Mr. C. E. Hearson. The 
 
 Fig. 29.— Gwyer Jet. 
 
 best results. — and there was practically but little to choose 
 between them — were given by a new jet devised by Mr. Gwyer of 
 Bristol, and manufactured by Messrs. Willway & Sons, of the 
 same place, and two or three others manufactured by Messrs. 
 Ottway & Sons, of Clerkenwell, but operated at the trial by 
 different gentlemen. Mr. Gwyer's jet, of which we can person- 
 ally speak in the highest terms, is shown in Fig. 29. It is made 
 of various powers, the No. 2 shown in diagram being of, approxi- 
 mately, 2000 candle-power. But, as already remarked, there is 
 not much to choose between it and Mr. Ottway's, which is shown 
 in Fig. 34. 
 
 In order that we may obtain the very best results, certain 
 principles must be attended to in the construction of all kinds of 
 jets where the gases are under pressure, however low. 
 
 In the gas-way there should be no sharp turn or acute-angle 
 corners; even in the mixing-chamber the "jostling" of the gases 
 may be too violent. We do not ourselves recommend the putting 
 
4CCESS0J^!/ES. 
 
 47 
 
 of any obstacles whatever in the gas-way. Many jets by the best 
 makers contain near the mixing-chamber pieces of fine gauze. 
 Whether this is intended to insure effective mixing of the gases, or 
 whether it is put there with an idea of greater safety, we cannot 
 say ; but we invariably remove and throw away the gauze. Some 
 makers have themselves admitted to the writer that they know of 
 no advantage belonging to the gauze, but that it was put there in 
 deference to public opinion only. In connection with this point 
 we have to note that Messrs. Newton and others have effected a 
 decided improvement in mixing-jets by placing in the mixing- 
 chamber, alternately, perforated discs and rings of metal. Many 
 other substances have at one time or another been put in the 
 gas-way — cotton, wool, shot, powdered pumice-stone, and even 
 lengths of cane — but for what object, unless to cause failure and 
 danger, we have never yet been able to ascertain. 
 
 Fig. 30. — Chadwick Safety Valve. 
 
 With ether tanks and such arrangements there must be value 
 in any device that will prevent a suck-back of gas, or will ex- 
 tinguish gas if sucked back lighted. We would not, however, be 
 misunderstood with regard to our position on the ether-oxygen 
 question. Accidents have certainly occurred with ether^ but in 
 these cases the ether has almost always been "loose," either in the 
 liquid state or escaping as vapour. For "saturators" the volatile 
 fluid should always be taken up by absorbent material such as 
 wool or cotton waste, as in devices described later (pp. 54 et seq.). 
 With the volatile so used and with saturators of the latest designs 
 we do not expect to hear of any accident, provided reasonable 
 care be used. 
 
 There should be no obstruction, then, in the gas-way ; but if 
 there is to be obstruction at all, the gauze alluded to will do little 
 harm. The discs and rings of Messrs. Newton are valuable, and 
 a certain safety-valve may give confidence to a timorous worker. 
 We show such a valve in Fig. 30 ; by means of a simple internal 
 
48 
 
 THE OPTICAL LANTERN. 
 
 arrangement gas is allowed to pass one way only. We believe 
 Mr. W. I. Chadwick devised this. 
 
 A very important point toward the production of a good result 
 is the angle at which the gas-stream impinges on the lime. A 
 simple diagram (Fig. 31) will illustrate this. 
 
 A B 
 
 Fig. 31. 
 
 A and B may be taken as extreme cases. If the angle of 
 impact be such as suggested at A, the nipple of the jet would 
 come between the point of radiance and the condenser, and, in 
 fact, there would be a shadow of the nipple on the condenser. 
 Moreover, the lime would be worn into a very deep "pit," which 
 always hurts the light and endangers the condenser, because the 
 litde brilliant pit might have its heat focus on the condenser, and 
 even the flame itself might glance off the lime at such an angle as 
 actually to strike the condenser. On the other hand, in B the 
 gas-stream impinges on the lime in such a slanting manner that 
 the impact would be very imperfect, and there would be a want of 
 "grip," so to speak. Mr. Lewis Wright, a great authority in such 
 
 Fig. 32. — Newton Mixing Jet. 
 
 matters, worked out the theory and practice of this subject, and 
 as a result Messrs. Newton produced a mixing-jet which has given 
 excellent results in our hands (Fig. 32). As this jet shows some 
 novel features we shall have to recur to it. 
 
 The bore of both tubes and nipples varies in different makers' 
 
J CCESSOI^IES. 
 
 49 
 
 jets, but it is impossible to lay down any laws on the matter. Per- 
 fection of light depends not on any absolute or isolated item, but 
 on the proper adaptation of one part of a system to another. In 
 particular, it may be said that the pressure of gases regulates all 
 other conditions. A small-bore tube may give a better light than 
 a large bore, at a less expenditure of gas, if the pressure happens 
 to suit the smaller tube. A large-bore nipple works at a decided 
 disadvantage if the pressure of gas be not up to a corresponding 
 point. Doubtless the very best light is to be got from a large 
 bore with a great pressure. Perhaps the greatest mistake made in 
 ordinary practice is having a bore too large for the pressure. For 
 general use the calibre of both tubes and nipple should be small 
 rather than large. 
 
 Everyone who has worked with lime-light knows the un- 
 pleasant effect of " hissing," " buzzing," " whistling," or "roaring" 
 by the jet. These noises may be due to various causes — improper 
 proportion of the gases to each other, incorrect position of the 
 lime in relation to the nipple, pitting of the lime, &c. ; but the 
 commonest cause is roughness in the tubes or nozzle, protrusion 
 of shreds of metal into, or square corners in the gas-way — in fact, 
 any rough edge or unevenness inside the tubes. Sometimes the 
 noise may be stopped by cleaning, or broaching, or riming out of 
 the tubes, but sometimes jets are so badly made that noise is in- 
 evitable at any pressure over a very low one. Any jet may be 
 noisy under certain conditions, and should not, ipso facto, be 
 rejected on account of noise. If it be a good one otherwise, but 
 is noisy, it should be examined, cleaned out (rimed out, if neces- 
 sary) ; failing these remedies, if the noise is intolerable it may 
 be rejected, but reduction of pressure is likely to obviate the 
 trouble. 
 
 The " nipple " of a jet sometimes screws on to the nozzle, 
 sometimes is in one piece with it ; sometimes is entirely of brass, 
 sometimes has a platinum tip let into it. Some first-class lan- 
 ternists find no advantage gained by the platinum tip, while 
 others advocate it. If the tip is used, it must be properly ad- 
 justed to the brass, so as not to form a projection into the gas-way. 
 Sometimes in very powerful jets the platinum-tipped nipples lead 
 to trouble, on account of the platinum separating or burning out 
 from the brass. In our opinion the bore of the nipple should be 
 an uninterrupted cone in shape. 
 
50 
 
 THE OPTICAL LANTERN. 
 
 All taps and fittings, especially of a mixing or high-pressure 
 jet, should be of the best manufacture and finish. The nozzle is 
 often removable from the mixing-chamber — a very good plan — 
 but the collar by which the junction is made must fit accurately 
 and be tightly screwed up. There should never be any leakage, 
 even under the highest pressures. 
 
 Jets are sometimes made " interchangeable " ; that is, are sup- 
 posed to act either as a blow-through or mixing by a change of noz- 
 zles. We do not recommend this form of jet — the same kind of 
 " chamber" is not well adapted for the two systems. 
 
 It is always important to know at the very instant, and without 
 possibihty of mistake, which tube conveys the oxygen and which 
 the hydrogen. Often, too, it is important to know by touch as 
 well as by sight. Accordingly we recommend that one tube be 
 blackened and the other left bright. Further, we advise that the 
 two taps be of different shapes. Our own practice is to have the 
 H tube black and its tap the common T shape, while the O tube 
 is bright and has an "arm-tap," which, moreover, we roughen or 
 nick with a file. 
 
 As the impact of the burning gases on a limited area of the lime 
 sooner or later bores a small hole in the surface — in technical 
 language, the lime becomes "pitted," or a " pit " is formed — and 
 as this pit not only spoils the illumination but endangers the con- 
 denser, it is important to have a good system and apparatus for 
 turning constantly or frequently a new part of the lime's surface 
 to the action of the gases. The more powerful the blow-pipe 
 heat the more frequent the necessity for a fresh lime surface. 
 With the most powerful jets it used to be considered almost 
 essential that the lime be constantly turning ; this was usually 
 effected by clockwork. With an average mixing-jet and pressure 
 the lime ought to be turned at least once every minute. But with 
 a blow-through jet of ordinary power the lime-point does not 
 attain its full incandescence till the gases have played upon it for, 
 say, one second, and in this case the lime ought not to be turned 
 so often. We have some very crude methods of turning the lime 
 as well as some very neat ones. The old inconvenient method 
 consisted in opening the lantern door and turning the " chair " 
 round with the hand. Nowadays the chair is usually operated by 
 a rod and some kind of cog-wheel arrangement, the rod being long 
 enough to protrude backwards to the outside of the lantern, in a 
 
ACCESSORIES. 
 
 51 
 
 manner shown in all our figures of lanterns with jets. Mr. Place, 
 of Birmingham, designed an exceedingly good quick-motion 
 action for the lime turning. Messrs. Newton further improved 
 this by adding a check, or click action, the effect of which is that 
 the points presented consecutively to the gas-jet follow a regular 
 spiral course round the lime cylinder, the eighth pit being directly 
 below the first. 
 
 Fig. 32 shows a detail not yet explained, but, in the opinion of 
 competent judges, a valuable addition to a jet. Below the jet 
 proper may be seen a rod with a T-piece at one end, and operat- 
 ing toothed wheels under the jet tubes. In each of the two 
 tubes is placed an extra tap, that on the oxygen tube being a 
 complete " cut-off," while that on the H tube is a " bye-pass," 
 allowing a small quantity of hydrogen to pass even when the tap 
 is turned off as far as it will go. The two taps work equally, and 
 are simultaneously operated by means of the rod and the toothed 
 wheel attached to it. If now these two taps be turned full on by 
 means of the cross, or T-piece, and if the light be then adjusted 
 at its best by means of the ordinary jet taps, it may be turned 
 down by the "cut-off" arrangement — the hydrogen will continue 
 to burn a small flame, and the lime will be kept hot, and when the 
 full light is required all that is necessary is to turn up the " cut 
 off" to its full extent. Thus — first, gas is not wasted; second, an 
 experienced hand may regulate the light, turn it down and leave it 
 — any person, however inexperienced, may then turn up the " cut 
 off" and find the light just as the expert arranged it ; third, the 
 proportions of the g^ses being maintained, the total brilliance of 
 the illumination may be regulated by one motion — where miscel- 
 laneous slides are being passed through a lantern (some dense, 
 others thin), this faciUty of regulation will be found of value, 
 This " cut off," devised by the author, is made by Messrs. Newton, 
 of London, and others. 
 
 Several manufacturers make arrangements similar in intention 
 but dissimilar in execution. 
 
 Lime-cylinders are very apt to crack during an exhibition, and 
 a lime once cracked never gives a perfect light. Considering that 
 draughts of cold air acting on the lime cause this cracking — 
 undoubtedly the case often — Mr. E. G. Wood, of London, devised 
 a " shield " of metal, a little wider than the lime-cylinder, and long 
 enough to protect it even when turned to its highest point. Of 
 
52 
 
 'J'HE OPTICAL LANTERN. 
 
 course an opening in the side of the shield allows the gas to 
 impinge upon the lime.* 
 
 Most persons of a mechanical turn of mind will notice the 
 very insecure way in which the jet is usually fixed to the pin, or 
 standard, forming part of the "lime "tray. The jet is fixed in 
 such a position that the slightest force on the long arm of the 
 lever may uncentre the whole jet. Few attempts have apparently 
 been made to improve this matter, but, Mr. Pumphrey, of Bir- 
 mingham, has an arrangement worthy of notice. (Fig. 33.) 
 
 Fig. 33.— Pumphrey Mechanical Stage. 
 
 Here, in Mr. Pumphrey's "mechanical stage," we have the 
 usual backward and forward sliding motion of the tray, and also a 
 raising and lowering rack as well as a "traverse" and a double 
 grip of the jet. The figure also shows the receptacle for lime-^/i"(r.r 
 — seldom used now, so far as we know. 
 
 Lately several makers have produced jets and trays with 
 traversing and other motions for the centering of the jet. We 
 would specially mention in this connection the " Premier " jet 
 and tray of Mr. J. H. Steward, and an arrangement of Messrs. 
 Newton & Co. 
 
 Fig. 34 shows the rack-work and other arrangements now used 
 by the writer. The jet is by Messrs. Ottway, and is mounted on a 
 strong upright. The whole jet is steadily and accurately raised or 
 lowered by means of a rack and pinion ; the side adjustment is 
 regulated by a special mechanical movement easily accessible to the 
 
 *The fireclay lime-case described in the appendix acts also as a protection 
 for the lime against cold draughts. 
 
ACCESSO/^ES. 
 
 53 
 
 hand. When the proper position is found, the whole jet is firmly 
 clamped in place by a screw for the purpose. The position of the 
 lime with regard to the nipple is regulated from the outside of the 
 lantern. The " tray " carrying all this apparatus slips backwards 
 and forwards in grooves of triangular section in the lower part of 
 the lantern body. This figure also shows the writer's "stand-by" 
 arrangement, whereby, during a lecture, a fresh lime can be 
 brought into action without lowering of the light. This wall be 
 iound very convenient for lectures of considerable duration. 
 
 Fig. 34. — Ottway Jet and Mechanical Stage. 
 
 When a good supply of coal gas can be obtained from the 
 ordinal y house main, it may be convenient to use this supply and 
 dispense with a cylinder of coal gas. A jet has been devised by 
 the Manchester Oxygen Company (Jackson's patent) to work 
 under these conditions, and is known as the " Injector." By 
 special arrangements in the mixing chamber a full stream of 
 oxygen is caused to suck a sufficient supply of coal gas through 
 the jet, and the result is a limelight of much greater power than 
 is obtained with an ordinary blow-through jet. It is necessary 
 to have a full pressure of oxygen and a good free supply of coal 
 gas, and the regulator on the oxygen cylinder must be adapted to 
 give extra pressure, or may be omitted altogether. There is no 
 danger of oxygen being forced into the house pipes, because the 
 stronger the draught of oxygen the greater is the suction on the 
 coal gas tube. Extra strong rubber tubing should be used for 
 
54 
 
 THE OPTICAL LANTERN. 
 
 the cylinder, and the gas-way for the coal gas should be of full 
 size. We have seen a remarkably fine light produced with this 
 jet; it is often convenient, and always saves the expense of com- 
 pressed coal gas or saturators. 
 
 Tanks or " saturators " for use with ether and other voi;.itile 
 liquids are frequently used, and, as they are very convenient when 
 hydrogen is not obtainable, we shall describe one or two con- 
 trivances, drawing attention, however, to the danger incu --ed if 
 carelessness creeps in, and to the fact already mentioned that we 
 do not expect any gain of light over that of a good mixing jet. 
 The principle on which saturators all work is this : Oxygen is 
 passed over ether — which may be either sulphuric or "petro- 
 leum "• — or other suitable volatile liquid, and in passing becomes 
 saturated with the vapour, and is then conducted to the H tap of 
 the jet, while part of the oxygen passes directly and without 
 saturation to the O tap. In some tanks the ether lies in divided 
 chambers, over which the oxygen passes; in others flannel or a simi- 
 lar material is thoroughly saturated with the liquid, and the oxygen 
 passes over or through layers of the damp fabric. Of the earlier 
 forms the " Broughton " tank was of the former type, the apparatus 
 being divided into a number of compartments into which the 
 ether was poured; while in Ives's saturator the container was 
 packed with flannel to absorb the ether. The late Mr. Albert W. 
 Scott introduced a saturator for use with ordinary benzoline or 
 similar liquids, in which flannel was used to absorb the liquid, and 
 a small lamp was fitted outside the container to slightly warm the 
 surrounding air, and so promote the volatilisation of the liquid. 
 
 But, despite a vast amount of attention lavished upon saturators 
 by, amongst others, the late Rev. T. F. Hardwich, it was long 
 before they gained any marked favour, owing to the great uncertainty 
 attached to their use. One of the first of the really reliable ones 
 was the "Optimus" of Messrs. Perken, Son & Rayment, which 
 fitted inside the lantern, and in which not only were the condi- 
 tions of working scientifically dealt with, but a higher degree of 
 efficiency was attained than heretofore. With this form of 
 saturator, however, unless specially made for the lantern, or the 
 lantern for it, it was a troublesome job to adapt them to one another. 
 
 Of late years very much more attention has been turned to satu- 
 rators, with the result that perfect control of action, absolute safet} , 
 and an efficiency, at least equal to mixed gases under the best con- 
 ditions, has been attained. In fact, given a really good form of 
 
S.4TC/A'JT0A'S. 
 
 55 
 
 mixing jet, many operators are of opinion that an efficient saturator 
 will afford a better result than the mixed gases. 
 
 In Fig. 35 we give a diagram of the " Pendant Saturator," 
 
 manufactured by Messrs. Willway &: Sons, of Bristol, one of the 
 latest and most efficient instruments of the kind. This is con- 
 structed to work outside the lantern, being suspended from a nail 
 on any convenient part of the stand, and is therefore available for 
 use with any lantern without special adjustment or adaptation. 
 It can also be used in conjunction with any mixing jet, which, 
 however, should be preferably packed. Gwyer's jet, manufactured 
 by the same firm, is specially recommended. In this instrument 
 the difficulty arising from the blowing of ether into the jet tubes, 
 or sucking it back into the cylinder tube, has been entirely over- 
 come, and absolute steadiness of light ensured. One great 
 trouble with saturators has been in connection with the filling, 
 both in using the right quantity of Hquid, and more especially 
 in getting rid of the less volatile and useless residuum that remains 
 after the really valuable portions have been taken up by the oxygen. 
 The first portion of this difficulty is overcome by the provision 
 of an overflow plug, shown at the left hand side of the diagram ; 
 no matter whether quite exhausted, or only partially emptied 
 when last used, there is no uncertainty as to how much ether 
 is required for refilling, as it is simply poured in at the 
 "filler plug" until it runs out at the overflow. Then, again, 
 with most saturators it is extremely difficult to get rid of the heavy 
 residuum already alluded to, the only available means being to 
 dry it off by heat, which is a tedious operation. In the " Pen- 
 dant " saturator this task is performed by the simple process of 
 
 Fig. 35. — Pendant Saturator. 
 
56 
 
 THE OPTICAL LANTERN. 
 
 blowing out, the lower plug on left hand side of the diagram 
 being opened, and an ordinary bicycle tyre pump screwed on to 
 the "filler" aperture at top, when in a very short time the whole 
 of the waste liquid is blown out. The frequency of the blowing 
 out will depend on the hquid used, and the amount of work done, 
 but working every evening with ether of s.g. 717 the makers 
 recommend the blowing out to be done once a week. If gaso- 
 line be used instead of ether the blowing out will require to be 
 done more frequently. These are the only two liquids recom- 
 mended with this saturator, benzoline and methylated spirit being 
 alike unsuitable. The blowing out should be performed in the 
 open air. 
 
 With this saturator the makers claim to be able to get at least 
 as good a light as with mixed cxygen and hydrogen, and as they 
 employ it in their extensive works in preference to the mixed 
 gases for testing jets, their practical conclusion should carry some 
 value. Charged with one pint of ether, the " Pendant " will keep 
 a large-bore jet supplied for four hours at least. 
 
 Fig. 36. — GRU)IROi\ bATURATOE, 
 
 In the " Gridiron " Saturator (Fig. 36), we have another form 
 for use inside the lantern. This is compactly and solidly made, 
 and, when finally fixed in position and centred, is perfectly rigid 
 and permits the lantern to be tilted in any direction with safety 
 and without fear of the light working out of centre. It is silent 
 
S.4 TURA TORS. 
 
 S7 
 
 in action and will work with one charge for about two hours and 
 a half; and it is claimed for this instrument, also, that it gives as 
 good a light as the mixed gases with wide- bore jet. The three 
 screws in the base of the stand serve the purpose of levelling or 
 of raising or lowering the light, but for accurate centring a 
 mechanical tray (not shown in the diagram) is supplied at a slight 
 extra cost. This instrument, as well as other appliances for 
 lantern purposes, is made by Mr. F. Brown, of Gate Street, 
 Lincoln's Inn. 
 
 Amongst other saturators of modern type, which we have not 
 space to fully describe, we may mention the "Lawson," which is 
 capable of good work. 
 
 In using a saturator it is necessary, in the first place, to exercise 
 care in the selection of the liquid employed, which should be of 
 the highest grade obtainable. In recommending 717 ether, it 
 will be obvious that any higher specific gravity must contain 
 more water, and that this water remains behind to saturate the 
 material forming the packing of the saturator, and to raise the 
 specific gravity of the next charge, and lessen its efficiency. And, 
 moreover, after a few charges have been exhausted, the saturator 
 will become so charged with water as to be useless. Quite simi- 
 larly with benzoline or other liquids, the lighter portions are 
 taken up by the oxygen, and the heavier non-volatile portion re- 
 mains behind to be pumped or dried off. 
 
 Always be careful to properly fill the tank, especially for a 
 long run; as with a nearly exhausted saturator there is a great ten- 
 dency to small explosions, which, if not dangerous with modern 
 instruments are at least startling. Be careful also to pump out or 
 dry off the tank at regular intervals in order that it may always be 
 in an efiicient working condition. 
 
 In turning off the light always be careful to turn off the satu- 
 rator taps before shutting off the general supply of gas. 
 
 If these simple rules be followed, and a good saturator used, 
 it will be found decidedly more convenient than using the gases 
 in separate cylinders, whether at home or travelling. 
 
CHAPTER IX. 
 
 PREPARATION OF OXYGEN AND HYDROGEN, 
 AND STORAGE IN BAGS. 
 
 Although during many years past improvements in the methods 
 of producing and storing oxygen in a compressed state in cyHnders 
 has virtually put an end to the practice of home manufacture, v^re 
 feel that a work like the present would not be complete without 
 some instruction for the benefit of those who are beyond the range 
 of oxygen pumping stations, or who from any other reason are 
 unable to maintain a regular supply of compressed gas. It has 
 therefore been decided to continue in the present edition the 
 instructions which have appeared in previous issues. 
 
 Chlorate of potash heated to a fairly high temperature will 
 yield oxygen, but the evolution of the gas in such a case is apt to 
 be unsteady and difficult to control. If, however, " black oxide 
 of manganese " (manganese dioxide) is added to the potash salt, 
 the gas comes off at a lower temperature and more steadily. But 
 when the manganese is used, a certain amount of chlorine comes 
 off with the oxygen; and this chlorine, being destructive to india- 
 rubber bags, and even to metal fittings (as taps and tubes), should 
 be as far as possible eliminated before the oxygen reaches the bag, 
 or gasholder, where it is to be stored. The chlorate of potash 
 and oxide of manganese — the mixture being sometimes called 
 "oxygen mixture" — are heated in a retort made of metal; the 
 oxygen, with its accompanying chlorine, is passed through water 
 containing a substance which abstracts the chlorine; thereafter the 
 oxygen is sometimes dried over sulphuric acid, sometimes passed 
 direct into the bag or other receptacle for storage. 
 
 The chlorate and manganese do not combine in the oxygen 
 mixture, nor does the manganese undergo any permanent change 
 during the heating, but is probably converted for the time being 
 into permanganate of potash, which is at once decomposed. 
 After the operation of gas-making the manganese oxide remains, 
 
PREPARATION OF OXYGEN AND HYDROGEN 59 
 
 and can be used again, if it is considered worth the trouble of 
 washing and drying what remains in the retort. Manganese 
 dioxide gives off oxygen at a red heat, but is not, so far as we 
 know, used for this purpose. 
 
 The usual instruction for making oxygen gas is to take 4 parts 
 of KCIO, (chlorate of potash) and i part of MnO., (manganese 
 dioxide) and mix them. These proportions are ^a very good 
 average ; but our own practice is to take the required amount of 
 KCIO3, and stir up with it MnO^ until every crystal of the former 
 is black with the latter. The mixing should be done with a 
 wooden or bone rod or spatula, not with a metal instrument. As 
 a rule the KCIO, is pure enough as sold, but sometimes the 
 Ignorant or unscrupulous trader adulterates his MnO., with soot or 
 charcoal, and sometimes by accident sulphide of\ntimony is 
 substituted for or mixed with MnO,. Either soot or antimony 
 present m the retort with the chlorate would be a source of 
 great danger, but the mixture may be very easily tested. Let a 
 few grains be placed in a test tube and held over a flame, and if 
 on gentle heating little sparks are seen, accompanied perhaps by 
 a slight crackling, the mixture is correct ; but if there is a flash or 
 a httle explosion, it is dangerous. When KCIO3 is bought in 
 bulk there is almost always mixed up with it a quantity of organic 
 matter, bits of wood, paper— we have even seen moss ■ these 
 must be carefully removed, as they might lead to trouble in the 
 retort. 
 
 One pound avoirdupois (16 ounces) of the oxygen mixture will 
 yield '4 cubic feet of oxygen— theoretically it ought to yield rather 
 more, but we never could get more out of it, using ordinary 
 chlorate of potash of commerce, which is good enough for the 
 purpose. Some years ago it was suggested that in place of, or in 
 default of MnO, common kitchen salt may be used ' And 
 certainly this is the case ; if the chlorate be pounded along with 
 the salt, or the salt carefully mixed with the pounded chlorate the 
 oxygen comes off very gently and steadily, and the yield of oxygen 
 m proportion to the chlorate is good. Iron rust or sand and 
 doubtless other substances, may be used for the same purpose as 
 MnOo, and we note that Mr. Hepworth* uses both MnO, and salt 
 We find nothing works better than KCIO, blackened with MnO,. ' 
 
 " "The Book of the Lantern." By T. C. Hepworth, F.C.S. 
 
 D 2 
 
6o 
 
 THE OPTICAL LANTERN. 
 
 The mixture being ready is placed in the retort. Fig. 37 
 shows the kind of retort that we can recommend from experience. 
 The body is conical in section, made of wrought iron, brazed 
 and riveted. The neck screws on to the body and a washer ot 
 mill-board or felt impregnated with asbestos is placed between 
 body and neck. Sometimes the bottom is made of copper, but 
 this costs more and is unnecessary. At the top may be seen a 
 small cylindrical projection; in this is placed pretty tightly a cork, 
 and so a safety-valve is obtanied. If any stricture occur in the 
 gas-way the cork will be blown out. The neck of a retort should 
 never have a square turn, but always be rounded, as in Fig. 37. 
 
 Fig. 37.— Oxygen Retori. 
 
 No violent heat is needed to drive off the oxygen, in fact, too 
 much heat, especially at first, retards operations, seeming to 
 cause the mixture inside to form a cake, with an outside more 
 or less impervious to heat. The retort may be put on a dull 
 red fire, but we prefer a gas burner such as that made by 
 Fletcher, of Warrington, under the name of "radial" burner. 
 A coke stove makes a good heating appliance, and even an 
 apparatus with a very large wick, known as a "lamp stove" 
 may be employed. But the radial gas burner is probably the best. 
 If by sinking it in a suitable receptacle over the burner the heat 
 
PREPARATION OF OXYGEN AND HYDROGEN. 6 1 
 
 can be made to surround the retort, so much the better, especially 
 if there be a large quantity of " mixture " in proportion to the size 
 of the retort. 
 
 From the retort the gas passes along the neck-tube to a puri- 
 fying bottle. This may be any wide-mouthed bottle, having a 
 bung through which pass two tubes of metal. The retort neck is 
 connected by a rubber tube with one metal tube of the purifier, 
 and this dips below the surface of water to near the bottom of the 
 bottle. The second tube has one end inside the bottle but well 
 above the liquid and the other end outside, and connected by a 
 
 Fig. 38. — Purifying Bottle. 
 
 rubber tube, either with a second bottle or directly with the gas- 
 bag or tank. The first purifying bottle should be about two-thirds 
 filled with water, to which a small quantity of caustic soda or 
 caustic potash has been added. This and the water remove 
 nearly all the chlorine that comes over with the oxygen. If a 
 second bottle containing a similar solution be used the gas will, 
 after passing through it, be practically free from chlorine, as may 
 be verified by the absence of the peculiarly unpleasant smell 
 of the latter gas. Carbonate of soda or of potash will answer 
 almost as well as the caustic alkalis. A good indiarubber bung 
 is to be preferred to any metallic cap or fitting, as it acts as another 
 
62 
 
 THE OPTICAL LANTERN. 
 
 safety valve ; and a glass purifier is better than an opaque one, as 
 it is well to observe at what rate the gas is coming off as shown by 
 the bubbling inside the bottle. 
 
 It is well sometimes to dry the oxygen before putting into 
 the storage receptacle. For this purpose the second purifying 
 bottle may contain a small quantity of sulphuric acid, but 
 of course in this case both ingress and egress tubes are kept 
 above the surface of the acid. This process of drying is rarely 
 necessary; a second solution of alkali, or even a quantity of plain 
 water is, however, of considerable utility. 
 
 Having mixed the KCIO,, and MnO., as described, and 
 placed the mixture in the retort, and the retort on the heating 
 apparatus, the system is connected all along, the retort with the 
 first purifier, this purifier with the second if two are used. But 
 the last purifier is not connected with the gas-bag or gas-holder. 
 Before connecting any two parts of the apparatus it is desirable 
 to bloiv through each separate part to make sure that there is 
 no obstruction in the gas-way. The neck of the retort is to be 
 screwed pretty tightly on to the body, the safety-valve cork 
 pressed firmly but not violently home, and the retort is to be sup- 
 ported in such a way that it cannot be overturned, one fatal 
 accident is said to have occurred through the overthrow of a 
 retort, and consequent clogging up of the neck by the heated 
 mixture. 
 
 As the retort is gradually heated a slight crackling may be 
 heard, and probably an occasional bubble, will rise in No. i 
 purifier. Presently a slight rush of air may be perceived at the 
 free end of the arrangement — viz., at the end of the rubber tube 
 which, later, is to be connected with the bag. 
 
 A piece of brown paper or some such material is now lighted 
 by means of a match, the flame if any blown out, and the smoulder- 
 ing paper held in the current of air passing from the free end of the 
 tube. A lighted pipe or cigar is useful and often handy for this 
 test. The presence of oxygen in the current will easily be 
 verified by the paper or cigar bursting into a peculiarly bright 
 flame. The gas-bag — if such is to be used — should previously 
 have been as nearly as possible emptied of air ; to do this, fold 
 the bag up with the tap open and kneel on it or otherwise press 
 all the air out, shutting the tap instantly. The moment before 
 connecting the tube with the oxygen now issuing from it, the tap 
 
PREPARATION OF OXYGEN AND HYDROGEN, 
 
 63 
 
 of the gas-bag is to be opened ; this is sometimes forgotten, and 
 leads to a safety-valve being blown out. The bag should be on a 
 level higher than the purifiers, for sometimes when gas comes off 
 rather violently water gets blown into the tube leading out of a 
 purifier, and water in a bag would be rather out of place. The 
 oxygen is now allowed to come off steadily ; if ever the evolution 
 becomes violent the heat should be lessened either by lowering 
 the gas in the burner, or by taking the retort off the fire or stove. 
 The heating must not be violent at first, as above stated. Very 
 likely after a time the evolution of oxygen will stop and appear to 
 be finished, but if the proper quantity of oxygen has not been 
 obtained (see above) the retort should be shaken or receive a smart 
 knock, or may be turned partly on one side ; and often even if 
 it be left alone the oxygen will, of its own accord, begin 
 to come off again, though the pause may have extended over 
 several minutes. The bag may be filled "drum-tight," it will 
 probably be less tight after the oxygen in it has cooled. A little 
 deftness is needed if the bag be full before the oxygen has ceased 
 to come over. When the bag is nearly full the heat on the retort 
 should be lowered, but not entirely removed ; when it is desired 
 to stop operations, the gas-bag tap is closed, the tube connected 
 with it instantly removed ; the heat removed from the retort and 
 the tube below water in the first purifier hmnediately pulled out of 
 the water. If this be not attended to, the water from the purifier 
 will be sucked back into the retort, which will be burst in all 
 probability. Retorts should not be of cast-iron, but of wrought 
 metal, which will rip but not fly to pieces. 
 
 As soon as the retort is cold it should be well washed out with 
 water ; changes of water are to be put in until it comes out quite 
 clear. The manganese, as stated, may be collected by filtering; 
 in any case it is apt to make a very nasty mess if the contents of 
 the retort are poured out into a sink. The beginner should 
 examine the contents of the retort at this stage ; if he finds any 
 unaltered chlorate of potash he will know that his mixture was not 
 exhausted. Large cakes in the retort are a sign of over-violent or 
 over-rapid heating. 
 
 Hydrogen Making. 
 Pure hydrogen gas is stated to give with the lime a better 
 light than ordinary carburetted hydrogen, such as is used for 
 house illumination. Our own experience leaves us in doubt on 
 
64 
 
 THE OPTICAL LANTERN. 
 
 this point, but our trials were made with a kind of house gas not 
 common, as it was made directly from paraffin oil and merely 
 purified by water. 
 
 It is very easy to make hydrogen practically pure. Scraps of 
 zinc are placed, in the bottom of a Woolff bottle having two necks. 
 Into one neck is fitted a rubber bung bored to take a long thistle- 
 head funnel reaching well down into the bottle. The other neck 
 has also a bung fitted with a bent tube, reaching an inch or two 
 inside the bottle and connected outside with a water purifier as 
 for oxygen, which purifier is connected with the hydi ogen storage 
 receptacle. Into the thistle-head funnel dilute sulphuric acid is 
 poured (acid one part, water four parts), when the hydrogen will 
 be given off at the delivery tube. The funnel-point must, of 
 course, be below the surface of the liquid in the bottle. The 
 purifier contains plain water. 
 
 Fig. 39. 
 
 A flask with double-bored rubber bung may be used in place 
 of the Woolff bottle, as in Fig. 39. 
 
 With gas-bags " storage " is hardly a proper word to use, be- 
 cause the less time the gas is stored in a bag, the better for the 
 bag and for the light. But we may have to put the gas into a bag 
 for immediate or early use. For a person making his own gas 
 at home there is the alternative of a bag or a metal " holder" or 
 "tank," and in practice the bag will be found the more convenient, 
 if the " lantern display " is to take place away from home ; but if 
 the gas is to be used at home, then the metal holder has the 
 advantage. 
 
 Gas-bags are made of "indiarubber cloth," that is, of rubber 
 lined outside and inside with some kind of cloth. Generally the 
 
STORAGE IN BAGS. 
 
 65 
 
 inner lining is of canvas, the outer of a stuff known as " twill." 
 A gas-bag is usually of wedge shape, and in the middle of the 
 thin end of the wedge is let in a tap. The manner in which this 
 tap is let into the stuff of the bag, often makes all the difference 
 between a good bag and a bad one. The tap should screw into, 
 or be soldered into, a large, pretty thick plate inside the bag ; if 
 the plate is not thick but inclined to have sharp edges, the bag 
 will sooner or later be cut by it. Two qualities of bags are 
 usually sold, the better quality is invariably cheaper in the end. 
 But the better quality is not always dearer to purchase, for we 
 know second-rate bags made expensive by the quality of the 
 outer cover. The best bags the writer has ever used are plain 
 black outside, and have taps properly let in, and so made as to be 
 locked with a padlock when it is desired. 
 
 Bags are made in various sizes, a convenient one holds seven 
 cubic feet of ga.s, and may be thirty-six inches long, twenty-eight 
 wide, and have a wedge-depth of twenty-four inches. For a very 
 long lecture or for unusual pressure the bag may hold about ten 
 cubic feet, and be 40 x 32 x (wedge) 28 inches. Gas-bags used 
 to be made rectangular, and are sometimes so made still, but the 
 wedge is the usual and the better shape. 
 
 In order that the pressure may be applied to the bag or bags 
 they are placed between " pressure-boards." If the blow-through 
 jet is in use the oxygen bag is placed under pressure and hydro- 
 gen from the main is used ; so also with the oxy-calcium light, one 
 bag only is required. With ether-oxygen and benzine-oxygen one 
 gas only, oxygen, is used under pressure. In these cases a single 
 pair of pressure boards is required. But as double pressure- 
 boards answer also for single bags, we recommend the purchase of 
 double boards. 
 
 The bag or bags are placed in the jaws of this contrivance, the 
 taps projecting through the hole cut out for them at the apex of 
 the board-wedge. The bags are to be pushed well home into the 
 jaws of the pressure-boards, and the partition, usually of strong 
 sail-cloth, separates the two bags if two are used, or is allowed to 
 lie flat if only one bag is in use. After the bags are in position 
 the strap at back is to be tightly pulled and securely buckled, to 
 prevent the bags from springing backwards. The support in front 
 of the board will be found necessary when the bags are large and 
 full ; but it is hinged and folds out of the way when not re- 
 
66 
 
 THE OPTICAL LANTERN. 
 
 quired. Pressure-boards must be of strong wood, and the hinges 
 at the apex must be very strong. On the ledge near the top are 
 placed the weights. These may be twenty- eight pounds or fifty- 
 six pounds each, and should be fiat and not round. If any dan- 
 ger can be said to exist in working the mixed gas jet, it may be 
 said to lie in the possibility of the weights falling off the pressure- 
 boards, and so causing a suck-back of gas. Therefore the weights 
 should be tied on to the ledge in some secure way, more especially 
 if they are round weights. We recommend the worker at home to 
 have fifty-six pound weights made of lead or iron with a very 
 broad base. 
 
 In computing the pressure on the gas we have nothing to do 
 with the size of the bags, though it is very common to see calcu- 
 lations on this basis. It must be borne in mind that the actual 
 pressure varies with the position of the weights on the top board, 
 as they exert greater leverage the further they are placed from the 
 hinges In fact, with wedge-shaped bags, as commonly used, it is 
 next to impossible to establish any rule based upon pressure per 
 square inch. With the seven-feet bag suggested, and a top board 
 of, say, 40 X 28 inches, one hundredweight, if placed as far as 
 possible from the hinges, or on the thickest end of the wedge, ought, 
 with mixed gases, to give a very good light indeed. For a " blow- 
 through" jet, fifty-six pounds weight on the pressure-boards 
 (40 X 28 inches) ought to suflfice. 
 
 Gas must not be kept long in rubber bags; such prolonged stor- 
 age is bad for the bags and bad for the light. The sooner it is used 
 after it is made the better. Oxygen kept twenty-four hours in the 
 best bag will give a very poor light, as we have repeatedly found. 
 Even four hours has in our experience made a marked difference 
 in the light. Presumably hydrogen also deteriorates in the same 
 way, probably to a greater extent. Even such a septum as india- 
 rubber cannot prevent diffusion by osmosis, and, moreover the 
 bags themselves suffer seriously by having gas kept in them. 
 Immediately after the "display" or lecture is over the gases 
 ought to be forced out of the bags, the latter being folded and 
 kneeled upon after removal of the taps. If the inside of an old 
 bag be examined the effects of oxygen will easily be observed. 
 
 Oxygen may be stored for a considerable time over water in a 
 metal tank, but even under these conditions it deteriorates after 
 a time. Moreover, if the metal of the tank be not thickly tarred 
 
STORAGE IN BAGS. 
 
 67 
 
 or painted the oxygen will attack it. We need not figure a metal 
 tank which consists merely of a small edition of a gas-holder, or 
 what is for some reason unknown to us called a " gasometer," so 
 often seen at public gas works. 
 
 Before leaving the subject of bags we would say that if in 
 cold weather they become very stiff they should be warmed before 
 being filled with gas. The taps should be kept working " sweetly" 
 with vaseline, or some such lubricant. If a bag happens to get a 
 hole in it, the owner would do well to have it repaired by an 
 expert rather than at home. 
 
CHAPTER X. 
 
 STORAGE OF GASES IN CYLINDERS. REGU- 
 LATORS. GAUGES. 
 
 Since the manufacture of oxygen, and its storage in metal 
 cylinders at high pressure became in Great Britain an established 
 and successful industry, the use of gas-bags has to a great extent 
 fallen into desuetude, and, indeed, when we compare the advan- 
 tages and conveniences of the two systems of storage, there is not 
 much wonder that bags are rapidly dying out. It is long since 
 oxygen and other gases were first stored in cylinders, but when 
 the Erin's Oxygen Company started to make and trade in oxygen 
 at prices far below the previous quotations, the use of their gas 
 and metal cylinders rapidly gained the ascendancy, and "com- 
 pressed " gases will probably oust gas-bags entirely in a few years. 
 Another matter that has greatly popularized the cylinder system was 
 the invention by Mr. Beard of a regulator so effective, simple and 
 cheap, that we do not hesitate to stamp it as one of the most 
 satisfactory inventions ever made in connection with the optical 
 lantern. 
 
 Barium monoxide gently heated in air takes oxygen from the 
 air and becomes barium dioxide, thus : 
 
 BaO + O = BaOg. 
 If now the air supply be cut off and the temperature raised, 
 the BaO 2 gives up part of its oxygen and reverts to its original 
 state BaO, and these changes can of course be repeated time 
 after time by regulation of air and temperature. All this has long 
 been known, but nobody seems to have been able to devise 
 machinery for utilising these reactions until Brin's Oxygen Com- 
 pany came before the public with their patent machinery. On 
 Mr. C. H. Bothamley's authority the writer states that the follow- 
 ing are the requisites for success in this system of oxygen- 
 isolation, (i) Proper regulation of temperature. (2) Perfect 
 purification of the air fiom carbon dioxide, CO a- (3) Presence 
 of a certain definite amount of moisture in the air. (4) Mainte- 
 
STORAGE OF GASES IN CYLINDERS. 
 
 69 
 
 nance of a low pressure in the retorts. The furnace is ingeni- 
 ously made to regulate its own temperature by the expansion or 
 contraction of metal bars acting on "dampers." COo is elimina- 
 ted from the air by caustic soda and lime ; the air is then dried 
 or damped according to necessity, and passes over the hot BaO 
 in long convoluted cylinders; the oxygen is got from the now 
 BaO 2 by raising the heat, the nitrogen having been collected or 
 allowed to escape. The awkward part of the business was formerly 
 to get rid of the nitrogen ; but this difficulty has been overcome, 
 and the gas prepared by Brin's process may now be accepted 
 as practically free from nitrogen. 
 
 The gas is now forced at a very high pressure into metal 
 cylinders. These cylinders are of wrought iron or steel, and 
 should be tested to a point of pressure greater (say three times) 
 than what is to be actually used. In England the cylinders are 
 usually charged to a pressure of "120 atmospheres,'' or 1,800 
 pounds to the square inch. To give an idea of the result we may 
 state that in a cylinder thirty inches by five and three-eights, 
 weighing twenty-eight pounds, we can have forty cubic feet of 
 oxygen ; while ten feet of gas, ample for the most prolonged 
 lecture, can with ease be carried in one hand. 
 
 Hydrogen gas is also compressed into cylinders in the same 
 manner as oxygen by all the compressing companies, so that the 
 lanternist is now entirely independent of the supply of house 
 gas. 
 
 It will easily be understood that when we have gas under such 
 pressure as 1,800 pounds on the square inch, there is a certain 
 amount of difficulty in managing it, tubes are apt to be blown off 
 and there is a danger of leakage at all points not solid metal. The 
 valves of these cylinders, therefore, require to be very strongly and 
 accurately made, and a system for regulating the pressure of gas 
 outside the cylinder becomes necessary. The latter desideratum 
 is thoroughly well fulfilled by a small and simple apparatus, Beard's 
 patent Gas Regulator. By using this contrivance, which is fixed on 
 the valve tube of the cylinder, the gas can be turned on full 
 without opening the jet taps and without tying the rubber tubes 
 to jet tubes. This of itself is an enormous gain, for the gas can 
 be turned on at the cylinder or cylinders and then adjusted 
 in proportions as required at the jet. Provided there is sufficient 
 pressure of gas in the cylinder to keep the regulator open, the gas 
 
70 
 
 THE OPTICAL LANTERN. 
 
 comes in a steady stream. Fig. 40 shows the external and intenal 
 arrangements of Beard's regulator. 
 
 Fig. 40. — Beard's Regulator. 
 
 A pressure gauge is also frequently useful to show the rate at 
 which the gas is being used, or to indicate at any time what amount 
 of pressure, and consequently — the internal dimensions of the 
 cylinder being known — what amount of gas there is in the cylinder. 
 A cylinder fitted with these two contrivances may be put down as 
 perfectly suitable and convenient for all purposes. 
 
 It must be noted that in adding regulators 'and gauges to the 
 gas-storage system the chances of leakage are always increasing ; 
 every joint must be very tightly screwed up by a " spanner " or 
 other instrument of like nature nature, or hammered up. 
 
 A rule usually observed, and most useful, is that the two 
 cylinders for O and for H are painted in different colours. The 
 oxygen cylinder is gererally black or very dark, the hydrogen a 
 bright red. 
 
 As an additional security against filling the gases into wrong 
 cylinders the plan has been adopted by the Brin and other com- 
 panies of having the filling valve of the O and H cylinder fitted 
 with right and left handed screws respectively so that it is a 
 physical impossibility to fill the O cylinder with H or vice versa. 
 
 On next page we give a table showing capacity, size, and 
 pound-pressure of Brin's cylinders. 
 
GJ UGES. 
 
 71 
 
 D" I' 
 
 o 
 
 to 
 to 
 i_n 
 
 to 
 vj-i 
 
 
 00 
 
 0 
 
 4^ 
 
 lO 
 
 0 
 
 10 
 
 0 
 
 on 
 
 OJ 
 
 Capacity ot 
 
 cylinder 
 m cubic feet. 
 
 
 
 On 
 
 on 
 
 on 
 
 
 4^ 
 
 
 OJ 
 
 x- 
 w\ 
 
 OJ 
 
 Diameter 
 in 
 inches. 
 
 -P>. 
 
 00 
 
 0 
 
 ON 
 On 
 
 on 
 
 o« 
 
 10 
 
 ON 
 
 - 
 
 - 
 
 0 
 
 0 P 
 
 Length. 
 
 
 CUBIC 
 
 FEET 
 
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CHAPTER XI. 
 
 THE ELECTRIC LIGHT. 
 
 If the oil lantern be the most convenient form of light for 
 projection on a small scale and the limelight for general purposes, 
 the electric light is undoubtedly the ideal illuminant for the 
 lanternist, and at the same time the most theoretically perfect in 
 action, since it condenses the most brilliant light possible in the 
 smallest area, thus approaching more closely than any other 
 radiant to the ideal "point." 
 
 At the same time, it must be borne in mind that it is of com- 
 paratively limited application, since it is only where the conditions 
 admit of the supply being laid on for general illuminating purposes 
 that it can be applied with any advantage to the lantern. The 
 idea of generating the supply for the special purpose by means of 
 the battery, or even of accumulators, must be abandoned at the 
 outset, both on the score of convenience and portability and also 
 of economy, for under such circumstances the trouble and expense 
 involved in its use would place it quite beyond reach either for 
 home use or for travelling purposes. But a redeeming feature is 
 to be found in the fact that it is precisely under the conditions 
 where it proves of greater value that it is most likely to be available, 
 namely, for projection on a large scale in public halls. Here, 
 where the advantage of the powerful illuminant is most to be 
 appreciated, the electric installation is the more likely to be found, 
 for which reason the arc light must certainly be considered as 
 one oT the most valuable illuminants at the command of the 
 lecturer. In the remarks that follow, it is obviously impossible 
 to do more than very briefly touch upon the theory of electric 
 lighting, and it is, therefore, assumed that the would-be operator 
 either already possesses a knowledge of electrical science, or 
 will, at least, post himself up to a sufficient extent for the 
 purpose. 
 
 Of the various forms of lamp available, the incandescent or 
 "glow" lamp may be dismissed as useful only for small discs. 
 
THE ELECTRIC LIGHT. 
 
 73 
 
 under which conditions, when it happens to be available, it will 
 prove a convenient and cleanly source of light. But as usually 
 made for ordinary lighting purposes the area of incandescence is 
 far too large in proportion to its illuminating value to be of much 
 use. However, a special form of lamp is constructed by the Swan- 
 Edison Company for projection purposes. In this the filament 
 takes the form of a helical spring, or coil, which is presented 
 endways towards the optical system, by which device a compara- 
 tively small area of more intense light offers itself to the 
 condenser. 
 
 But for all really practical purposes we must fall back upon 
 the arc light, which can be obtained, under suitable condi- 
 tions, of a practically unlimited candle-power and of a whiteness 
 and brilliancy unapproached by even the best hme light. In this 
 form of light the illumination proceeds partly from the so-called 
 arc itself, which is formed by a stream of incandescent particles 
 from one of the carbon points to the other, but chiefly from the 
 carbons themselves, which are rendered incandescent under the 
 intense heat of the current. 
 
 Here it is necessary to call attention to the different forms of 
 arc light, one of which is formed by a continuous, the other by 
 an alternating current of electricity. In the first instance, the 
 current proceeds continuously in the same direction; in 
 the other it reverses or " alternates " its direction a very great 
 number of times in a second. The different kinds of current 
 exert a most important influence, not only on the character 
 of the light but also on the condition of using it, as we 
 shall endeavour to show. In the first place, as has been already 
 stated, the arc is formed by a stream or current of incandescent 
 particles of carbon passing from one point to the other, and it 
 
 will be readily understood that under such circumstances when 
 
 the electric current is a continuous one in the same direction 
 
 the stream of particles must partake of a similar character, with 
 the result that, though both carbons are consumed or burnt to some 
 extent, this does not occur equally; the positive carbon, or that 
 from which the current flows, suffers consumption twice as 
 rapidly as the negative one, towards which the incandescent 
 particles are borne by the current, and which is thus partly built 
 up again. In the case of the alternating current the consumption 
 of the two carbons goes on equally and symmetrically, since the 
 
 F 
 
74 
 
 THE OPTICAL LANTERN. 
 
 process is one of a give-and-take nature, and each point suffers 
 precisely similar treatment. It might be assumed that the latter 
 was the better form of light — though actually the reverse is the 
 case in practice, for reasons that will be shown. 
 
 If after the light has been burning for some time under the 
 influence of a continuous current the carbon points be allowed 
 to cool, so as to admit of minuce examination, it will be noticed 
 that the negative carbon — usually the lower one — has had a small 
 rounded knob formed on its point, while the upper— or positive — 
 has been burnt into a small hollow or " crater," as it is 
 technically called; and it is from this crater that practically the 
 whole of the useful light proceeds, as will be seen on examining 
 the arc through a smoked or coloured glass while the current is 
 passing. A similar examination of the effect of the alternating 
 current will demonstrate the fact that each of the carbons is 
 equally hollowed, but very sHghtly, forming two imperfect craters, 
 which, besides giving an inferior light, throw it in a direction 
 in which it has no practical value, as will be shown later on in 
 describing the lamp. In addition to this, the arc itself assumes 
 a violet or reddish colouration, often varying rapidly with changes 
 in the strength of the current, and which detracts considerably 
 from the quality of the illumination. For projection purposes, 
 too, the alternating current has a special disadvantage, since it 
 throws a shadow on the centre of the screen, caused partly by this 
 coloured flame and also by the carbons themselves intervening to 
 cut off a portion of the light emanating from the craters. The 
 projected central shadow is, in fact, a shadow of the carbon 
 points. 
 
 Such is the rationale of the formation of the electric arc. We 
 will now proceed to describe the apparatus or lamp by which it is 
 produced. This consists, in its simplest form, of a standard or 
 upright carrying two holders or sockets, in which the carbons 
 are fixed so as to present their points to one another at a short 
 distance apart. But beyond this, some means must be provided 
 by which the distance can be varied so as not only to permit the 
 carbons being adjusted at the proper distance to give the best 
 light, but also to allow that distance to be maintained as the 
 carbons are consumed. Moreover, in order to start the light, it is 
 necessary that the carbon points actually touch one another for an 
 instant, since a current of the strength usually employed is incapable 
 
THE ELECTRIC LIGHT. 
 
 75 
 
 of leaping over any but the most infinitesimal gap until that space 
 is filled with heated particles of carbon, or carbon vapour. Thus 
 it becomes necessary to "strike the arc," as the operation of 
 momentarily bringing the carbons together is called, and then 
 recede them to the necessary distance. If this be insufficient, 
 light is wasted both by not getting the best result possible out 
 of the current and also by a portion of it being intercepted 
 by the negative carbon ; if the proper distance be exceeded the 
 arc flares up and becomes coloured, thus losing much of its 
 efficiency, besides spluttering and emitting an unpleasant hum- 
 ming noise. 
 
 Thus, it will be seen that there is room for the exercise of a 
 good deal of ingenuity in arranging for the due performance of 
 these adjustments, and a very great number of lamps have been 
 devised for the purpose. These are of two kinds —" automatic " 
 or " hand-feed " — in the former of which the distance is adjusted 
 or regulated automatically by the current itself, in the latter by 
 hand with suitably arranged rackwork. For ordinary illuminating 
 purposes the first description is practically a necessity, and the 
 lamp or " regulator " is, consequently, an elaborate, delicate, and 
 frequently cosdy piece of apparatus ; but for projection purposes, 
 although several simple forms of automatic lamp are in use, many 
 operators prefer to rely upon hand adjustment, which fulfils every 
 requirement and necessitates a less elaborate instrument. In the 
 Davenport-Steward Universal Arc Lamp (shown in Fig. 41), we 
 have an instrument of the latter class, consisting of a firm base 
 and standard carrying the carbon holders, which are adjustable by 
 means of separate milled pinions. The frame carrying the holders 
 is hinged to the upper portion of the standard, and this forms an 
 important feature in all lamps, since it permits the carbons to be 
 inclined at such an angle that the light proceeding from the crater 
 when using a continuous current is reflected or thrown right into 
 the condenser. The diagram shows the lamp adjusted for a con- 
 tinuous current ; but, in the case of an alternating current, the 
 carbons would occupy an upright or vertical position. From 
 what has previously been said in connection with other forms of 
 illumination, it will be remembered that the accurate centring 
 of the light is a matter of the utmost importance, and this is 
 no less the case with the electric arc. The standard is provided 
 with a rack and pinion for adjusting the height, and with a rotary 
 
 F 2 
 
76 
 
 THE OPTIC A I. LANTERN. 
 
 fitting and clamp at top for lateral centring. The hinge for 
 inclining the carbon is fixed in any desired position by means of 
 a clamp, and the further adjustment of the position of the crater 
 is quickly performed by means of the rackwork adjustment shown. 
 Another milled head, fitted with pinions of different sizes, separates 
 
 Fig. 41. — Davenport-Steward Universal Arc Lamp. 
 
 or brings together the carbons as may be necessary, and provides the 
 necessary adjustment to make good the consumption. A slight 
 turn every two or three minutes effects this purpose. 
 
 The Ross-Hepworth Arc Lamp (Fig. 42), another form of hand- 
 feed instrument, possesses special features. The pinion which 
 regulates the adjustment of the carbon is connected with a tan- 
 gent wheel and worm wheel, which impart a slow feeding motion 
 in contrary directions to the carbons. It is further ingeniously 
 adapted to perform another function — namely, the quick striking 
 of the arc ; for, if the feeding handle be simply pushed in, the 
 worm wheel acts the part of a rack, and imparts a rapid rotary 
 motion to the pinion, thus bringing the carbons quickly together. 
 A spiral spring on the feeding handle serves to instantly return 
 them to their original position. This permits the carbons to be 
 set at the proper distance before commencing an exhibition, and 
 
THE ELECTRIC LIGHT. 
 
 77 
 
 the arc can be quickly struck and the light set going without 
 any of the usual trouble. The lamp is also fitted with a vertical 
 rack and pinion, by which the point of light can be brought to 
 
 Fig. 42. — Ross-Hepwor ru Arc Lamp. 
 
 the optical centre if any irregularity in the rate of burning 
 should displace it. 
 
 The newest, and, in all respects, one of the most convenient 
 lamps on the hand-feed principle, is the New Model Projection 
 Arc Lamp of Messrs. Ross, Limited, shown in Fig. 43. The 
 
 Fig. 43.— New Model Projection Arc Lamp. 
 
 body in this is set at a fixed angle with the base, and serves the 
 dual purpose of supporting the working parts and of acting as a 
 screen for the back of the lantern. The large milled head in the 
 centre supplies the slow motion for feeding the carbons as they 
 
78 THE OPTICAL LANTERN. 
 
 consume, and that in the base controls the lateral adjustment, 
 the vertical being effected by the long thin one shown towards 
 the upper part. The arc is struck by means of the lever placed 
 just above the bottom holder. 
 
 If an automatic feed lamp be preferred, the pattern manu- 
 factured by Messrs. Newton & Co. — Major Holden's patent — can 
 
 Fig. 44.— Holden Automatic Feed Lamp. 
 
 be recommended. It is extremely simple in construction, and is 
 made to clamp on to the rod of the tray of an ordinary lime-light 
 lantern. It is essential, however, with this form of lamp, that the 
 current be steady and not liable to constant fluctuations ; other- 
 wise, and for alternating currents, it is preferable to use a hand- 
 feed lamp, which can be better regulated to the constantly chang- 
 ing current. 
 
 Very efficient lamps, both automatic and hand feed, are made 
 by Borland, of Leeds, under the name of the " Scissors " Arc 
 Lamp. 
 
 It is necessary now to say a few words on the subject of the 
 carbons themselves. These should be of the very best quaUty 
 obtainable, as the cheaper kinds, without effecting any very great 
 saving in first cost, give rise to a vast amount of trouble and dis- 
 satisfaction. The size and hardness of the carbons also should 
 
THE ELECTRIC LIGHT. 
 
 79 
 
 be selected with due consideration of the strength of the current, 
 the stronger the current the thicker and harder the carbon. When 
 using the continuous current, owing to the unequal consumption 
 of the two carbons, it is usual to employ them of different sizes, 
 the lower or negative being the smaller. For a current of ten 
 amperes, which gives a light about equivalent to a good mixed 
 limelight jet, the sizes generally employed are seven millimetres 
 for the lower, and ten millimetres for the upper ; while for currents 
 up to fifteen ampbres the sizes may be increased to ten and thirteen 
 millimetres. For alternating currents both carbons should be the 
 same size, and the larger dimensions given above should be 
 chosen. 
 
 With even the best carbons some trouble often arises through 
 the irregular formation of the crater, owing to slight variations in 
 the homogeneity of the material. To obviate this difficulty the 
 upper carbon is frequently " cored," that is, made with a central 
 core of softer carbon in order to encourage the formation of the 
 crater in a central position. With the alternating current, as 
 already mentioned, the light is always less satisfactory, by reason 
 of imperfect formation of the craters and their position. Here, 
 again, an improvement is effected by the use of eccentrically- 
 cored carbon, in which the core is placed at one side of the centre. 
 These form the subject of a recent patent granted to Mr. T. C. 
 Hepworth, and are intended to encourage the consumption of the 
 carbon in the proper place. If two of these pencils be placed in 
 the holders with their soft cores to the front, the carbon in those 
 portions will be consumed more rapidly, and the tendency will be 
 not only to keep the light on the front of the pencils, but also to 
 wear them away in such a manner that they do not interfere with 
 the proper direction of the light. It is claimed that by the use of 
 this form of carbon not only is the light produced by the alternat- 
 ing current doubled in value, but the violet band and shadow 
 already referred to are done away with. 
 
 It only now remains to say that in order to adapt the supply 
 to the amount of current required to produce a light of given 
 strength a " variable resistance," or rheostat, will be required. In 
 order to render the function of this intelligible it will be necessary 
 to refer briefly to the methods of measuring adopted in connection 
 with the electric current. The units adopted for this purpose are 
 the volt, the ampere, and the ohm. The volt is the unit of electro- 
 
8o 
 
 THE OPTICAL LANTERN. 
 
 motive force, or " pressure " of current supplied ; the ampere is 
 the unit of quantity of current employed ; while the ohm is the unit 
 of "resistance," or of work done. A volt is the amount of power 
 that will pass a current of one ampere through a resistance of one 
 ohm; I GO volts will send one ampere through loo ohms or lo 
 amperes through lo ohms. The usual voltage supplied is loo, 
 and, as has been mentioned, the strength of current employed from] 
 lo to 15 amperes. The resistance offered by an arc employing 
 10 amperes of current is about 3 ohms, so that in using 100 volts 
 it is necessary to provide an additional resistance of 7 ohms, 
 otherwise something will have to give way, and that something 
 will be the conducting wires, which will be overheated to fusion. 
 
 This is where the " resistance," or rheostat, comes in. It con- 
 sists of a frame carrying a quantity of coiled and insulated wire of 
 sufficient hardness and infusibility to withstand the great heat 
 generated— for the effect produced is similar to, though not per- 
 mitted to reach the same degree, as in the filament of an incandes- 
 cent lamp — and this is interposed somewhere in the circuit — i.e., 
 between the main wires and the carbons to absorb the surplus 
 voltage. Figs. 45 and 46 represent two forms of "resistance" 
 supplied by Messrs. Ross, Ltd., and Mr. J. H. Steward, and with 
 the description we have given will sufficiently explain themselves. 
 By altering the position of the connection with the rheostat, more 
 or less of the wire can be brought into circuit, and the resistance 
 varied instantly, when it is desired to lower or increase the power 
 of the light. 
 
 Fig. 45.— Ross's Rheostat. 
 
THE ELECTRIC LIGHT. 
 
 8l 
 
 In order to prevent accident from excess of current, it is usual 
 to provide safety fuses, placed somewhere in the circuit and in as 
 accessible a position as possible. These consist of short lengths 
 of fusible wire, usually of tin of such thickness as to melt under any 
 
 Fig. 46.- Steward's Rheostat. 
 
 abnormal increase of energy, and thus cut off the current alto- 
 gether and prevent further damage. The operator should be pro- 
 vided with a supply of suitable wire with which to make his con- 
 nection, and so ensure safety. 
 
 It is tolerably certain that in the future the application of 
 electric lighting to the lantern will undergo still greater develop- 
 ments than during the period that has elapsed since the earlier 
 editions of this work appeared. 
 
CHAPTER XII. 
 
 SCREENS AND FRAMES. 
 
 We have now to deal with a part of the system which the 
 practical worker will find much more important than the inex- 
 perienced or mere theorist might imagine. A good "screen " or 
 surface for receiving the image makes a great difference in the 
 results, a bad screen may easily absorb or disperse uselessly 25 per 
 cent, of the illumination. 
 
 A screen ought to be opaque, white, and matt, not translucent 
 (except for certain almost obsolete arrangements), nor " shiny," nor 
 yellow. At one time it was not uncommon, and even yet it may 
 at times be necessary to use a translucent screen; in this case a 
 very thin linen screen is used and is almost of necessity kept damp. 
 But such a system is comparatively poor, and ought to be resorted 
 to only when the other is not available. 
 
 If the same room is always to be used for the lantern work, 
 portability of the screen is of no moment, and in such a case we 
 can easily attain to a perfect receiving surface. A smooth, plastered 
 wall is as good a surface as can be got. A stout fabric such as 
 canvas, may be faced with paper, or it may be heavily coated with 
 sized white pigment. In mixing the pigment a little blue should 
 be added to kill the yellowness of the whitening. If the screen is 
 required to roll up like a school map, some care is required in 
 mixing the pigment to make it in such a way that there shall be no 
 cracking when the screen is rolled up. This is a matter for experi- 
 ment ; if when the screen is dry after painting the white can be 
 rubbed oft' easily by the finger, more glue is required ; if the surface 
 is at all glossy, there is probably too much glue. For painting, 
 the canvas is stretched on a frame, laid face upwards, and the pig- 
 ment laid on as evenly and as rapidly as possible. 
 
 Mr. Hepworth's directions are somewhat as follow : Tack good 
 unbleached calico to a frame so that the seams, if any, run horizon- 
 tally. Secure first the four corners with tacks to frame, then 
 nailing one side at a time to the frame. Give the sheet a good 
 
SCREENS AND FRAMES. 
 
 83 
 
 coat of the best size, melted by heat with its own weight of water. 
 The sized sheet is allowed to dry, and thereafter is painted with 
 whitening and melted size. The whitening, water, and size mixture 
 should, when first made, have the "consistency of cream"; when cool 
 it will be like a thin jelly. Mr. Hepworth then places the frame 
 upright, and proceeds to work the brush charged with the paint up 
 and down and sideways, so as to avoid leaving any lines upon the 
 surface. 
 
 We are informed that the best of all " pigments " to mix with the 
 size is zinc white; we have not ourselves tried this, but believe that 
 the claim for it is well founded. Mr. Bull, of Great Queen Street, 
 London, makes very fine screens with this pigment. 
 
 Facing a screen with paper requires, we fancy, an experienced 
 hand, the paper cannot be got in large enough sheets and the lay- 
 ing down of the edges where the sheets join is no easy matter. Of 
 course, if we only require a small screen, as for exhibitmg 
 to small audiences, single sheets of paper may be produced of 
 sufificient size, and only require at the most to be attached to suit- 
 able supports. 
 
 Up to (say) 10 feet square the advantages of a " faced " or sized 
 screen are so great as to quite balance the awkwardness of a 10 feet 
 roller for transportation from place to place. Up to 10 feet we 
 therefore recommend such a screen, even for travelling. But 
 beyond 10 feet the length of roller required becomes very incon- 
 venient, and we have to look for a screen material that may be 
 folded up, and a method of so stretching it when in use as to obviate 
 the creases iiaturally following the folding. To meet these 
 desiderata, we use rather thick linen or cotton screens, and we 
 stretch them on "screen-frames." Cotton seems to be less used 
 than linen, so we shall confine our remarks to the latter. We 
 believe that pieces of suitable Hnen can be got up to 10 feet square, 
 but, as we have said, we should prefer a faced or sized screen of that 
 size. If we have to join pieces of Hnen for a screen, say, of 18 feet, 
 we must avoid having our seam or joint near the centre. In such a 
 case it would be better to have a complete square of 10 feet in the 
 centre, with four feet tacked on all round, rather than simply to 
 make an irregular patch-work. As these screens are sold ready 
 made in any usual size, we need not do more than point out that 
 the seams should be as far from the centre as possible, and that 
 they should run perpendicularly rather than horizontally on the 
 
84 
 
 THE OPTICAL LANTERN. 
 
 picture. If a seam falls coincident with the horizon of a picture 
 the effect is apt to be very unpleasant. 
 
 In order to improve the surface and give a better lighted 
 picture, the screen has been prepared with a plain metallic surface, 
 or coated with a metal foil, but the objection to this style of screen 
 IS stated to have been that the projected image is only properly 
 seen from certain directions. To obviate this objection, Messrs. 
 Lewis Wright and John Anderton some two years ago took out a 
 patent for a metal-faced screen, in which the metallic surface 
 instead of being smooth has a finely-patterned rough surface. 
 The sheet is coated with "silver or other metal," or metallic 
 powder, and the surface is "covered over with fine grooves or 
 striations arranged perpendicularly or otherwise, or indentations, 
 corrugations, or other irregularities of any kind" . . . "for the 
 purpose when in use of causing the rays of light to be scattered 
 laterally, and thus increasing the lateral reflection of the light and 
 thereby causing the projected image or images to be seen with 
 equal brightness from any point of view." We have heard the 
 "silver screen" well spoken of, and no doubt for small discs it 
 may be an improvement. 
 
 There is no great variety in the designs of screen-frames, or 
 " elevators," as they are sometimes called. A screen-frame should 
 be as light as possible consistent with strength sufficient to keep a 
 screen quite taut ; it should take down into as many short pieces 
 as is consistent with strength; it should be capable of being 
 rapidly put together and taken down ; it should afford facility for 
 elevating the screen ; it should stand by itself, and it should be 
 capable of being tilted slightly without chance of falling down. 
 When the frame is tilted is the time when strength is required, 
 for a tilted screen will "sag" forward under such conditions unless 
 the frame be strong enough to counteract the tendency. We 
 have seen it stated in print that the advantage of being able to 
 tilt the screen is more theoretical than practical, but we must 
 dissent from this statement; a tilt is frequently a co?iditio sine qua 
 non of real success. In our own experience the screen is tilted 
 nine times out of ten. 
 
 We figure an "elevator" such as we find perfect for our pur- 
 poses up to a twenty-foot screen. The poles are two inches in 
 diameter, the length about five feet each, the junctions are strong 
 brass ferrules, the corners are solid metal. The lengths are so 
 
SCREEALS AND FRAMES. 
 
 85 
 
 made that we need not use all of them at a time unless we wish, so 
 that we may choose our size of screen up to twenty feet, increasing 
 or diminishing by about four feet at a time, or two feet if the 
 lengths are made on purpose for such a choice of size. (Fig. 47.) 
 
 At the foot will be seen strong cross-pieces, which form a base, 
 and with the addition of the four straps attached to the lower part 
 of the frame, help to prevent the screen and frame from toppling 
 over if tilted. The tilt is arranged and maintained by shortening 
 and lengthening the front and rear straps respectively, and the frame 
 works freely in the holes made for the purpose in the base cross- 
 pieces, so that any tilt can be obtained in an instant. It would 
 be better to attach the guys at a point higher on the uprights. 
 
 There are various methods of mounting a screen on its frame 
 in a hall preparatory to a lecture. Perhaps the most convenient 
 way is to put the poles forming the top of the frame together, 
 joining to these by the solid metal corners the first lengths of poles 
 below the top bar; to tie the screen by its tapes to these lengths, 
 and then to join on the next lower poles, tying the screen to these 
 in their turn till the whole is erected. By means of the bottom 
 tapes, and those within reach with or without the ladder, the whole 
 fabric is finally made taut. With a large screen we always fix guys 
 to the metal corners at the top, and fix the guys to anything that 
 comes handy, as a rafter or staple driven in by ourselves if the 
 authorities permit. We have a twenty-foot frame of four-inch 
 poles which always require guying, and in any case a couple of 
 guys when the sheet is tilted ease the strain on the pole ferrules 
 greatly. A very good adjunct to a screen is a pair of tasteful 
 
 Fig. 47. — " Elevato: 
 
 OR." 
 
S6 
 
 THE OPTICAL LANTERN. 
 
 curtains, which may be simply hung on the top of the frame and 
 drawn aside by simple means as the lecture is about to begin. 
 
 If an opaque or " faced " screen is to be used, nothing is 
 required but two side arrangements to support the ends of the 
 roller on which the screen is wound. 
 
CHAPTER XIII. 
 
 SUPPORTS. SLIDE CARRIERS. READING LAMPS. 
 
 SIGNALS. 
 
 We naturally require some kind of stand upon which to place 
 the lantern, and the larger and heavier the latter the stronger 
 must be the support. Sometimes stands are made with sliding 
 legs, so that the lantern ma)' be reared up very high, but we see 
 no use for any such height, in fact, the highest lantern, if we are 
 using a double or triple, should be as little above the level of our 
 eyes as possible. Even if we are with the lanterns in the middle 
 of our audience, the lowest lantern need only clear the head of 
 the sitter immediately in front, and if tilting the screen is incon- 
 venient, mounting a ladder to see into the lantern is ten times 
 worse. 
 
 But the kind of stand used in a photographic studio for carte 
 and cabinet cameras answers very well. Such a stand, however, can 
 hardly be called very portable, and if the box which carries the 
 lantern when travelling be placed on an ordinary table, more espe- 
 cially if a canting table be added, there will be no necessity for 
 further impedimenta. 
 
 "Carriers" may be described as guides by means of which 
 slides are passed through the part of the lantern in front of the 
 condenser. It is important that slides should pass smoothly into 
 and out of their position in the light-way; each one must find the 
 central position at once, without any fumbling on the part of the 
 operator, and when more than one lantern is being used, as for 
 dissolving effects, it is essential to success that the slides should 
 " register " — i.e., should have coincident discs on the screen. For 
 multiple lanterns a certain amount of intricacy in the carriers may 
 be necessary, but for a single lantern the simpler it is the 
 better. After using nearly every carrier known in the market, the 
 writer goes back to the simplest of all, the old " Chadwick," or, 
 failing that, one very nearly as simple, made by the late Mr. Place, 
 of Birmingham. 
 
THE OPTICAL LANTEKN. 
 
 Both these carriers are so made as to take and to centre at 
 once any ordinary size of slide. The " Chadwick » should be made 
 of thoroughly-seasoned, well-smoothed wood. The " steps " are 
 made to suit the three different lengths of slides in vogue at one 
 time ■ one slide pushes the one in front through, oni or other 
 "step" being used as a guide for the hand that pushes the slide. 
 In Mr. Place's carrier the slide is let in from above, a push on the 
 metal projection turned up in the apparatus at once centres 
 the slide, and on further pushing, the frame carrying the two 
 shdes shps along, and so one slide is put away as another comes 
 on. For dissolving lanterns "Beard's self-centring" is as -ood 
 as any. ° 
 
 Some people dislike an interval of darkness, however short 
 between the pictures; some dislike a period of brightness on the 
 screen. Many efforts have been made, and appliances contrived, 
 to simulate dissolving effects with a single lantern. So far as we 
 have seen, these contrivances are of no value ; if any such attempt 
 must be made, the operator may hold in front of the lens while 
 the slides are being changed a piece of very fine-ground glass. 
 J^or ourselves, we see no objection, but rather an advantage, in 
 the short space of semi-darkness produced by the use of a Chad- 
 wick or Place carrier. A blaze of light on a white screen for a 
 short time is, m our opinion, most objectionable ; it tries the eyes 
 and spoils the first effect of the pictures following. 
 _ We can speak highly of the "Eclipse" carrier of Mr. Beard • 
 It is very ingenious in construction, and the effect on the screen 
 is perhaps more nearly that of dissolving than is produced by any 
 other carrier. Mr. Beard has lately made a very simple carrier 
 on the push-through principle; the grooves are so bevelled that 
 the shdes always come to the same plane, they are all, there- 
 tore, at one focal plane and cannot be pushed one in front of 
 another. 
 
 There are many kinds of reading lamps intended for use by 
 the person on the platform. Of course, the object is to throw 
 light on the book or paper without illuminating the screen. By 
 highly-ingenious contrivances of this kind we have never been 
 able to read comfortably, and we have repeatedly been left in 
 darkness during the performance. Since first we used a "candle 
 sfiade, kept at most good ironmongers, we have never used any 
 other reading lamp. 
 
S/GJVALS. 
 
 89 
 
 A signal between lecturer and lanternist is generally requisite; 
 an electric communication acting on a muffled bell is the neatest 
 device ; failing that, we know nothing better than a gentle tap with 
 a knife on a glass of water, or the click of a castanet d la 
 Muybridge. 
 
 All indiarubber tubing used for lantern purposes should be of 
 the very best quality, and a bore as large as may be consistent 
 with fitting the taps and jet-tubes. The larger the bore the less 
 the friction of the gases in the tubes, and diminution of friction 
 helps greatly towards brilliance and steadiness of light. Tubes 
 should not be unnecessarily long ; first, because redundant tubing 
 means extra friction ; secondly, because spare coils of tubing are 
 inconvenient and apt to be trodden on and get in the way; lastly, 
 rubber tubing can hardly be too thick in the walls ; very thick- 
 walled tubing is dear to buy, but cheap to use. Rubber tubes 
 should be pushed far up on the metal tubes to which they are 
 " sprung," and it is always a good precaution to tie on the rubber 
 tubes to the metal ones. 
 
 Various kinds of boxes are made to hold lantern slides; we 
 like a box for each set of slides — each lecture, for instance ; and 
 the box should have no grooves, but may have a spring arrange- 
 ment to push the slides up against each other in the box. 
 
 Slides frequently "sweat" in the lantern owing to moisture 
 being imprisoned between the glasses. If the slides can be 
 thoroughly heated before the lecture this very unpleasant defect 
 will be obviated. If slide-boxes are made of metal, say tin, they 
 can be put in a gentle oven, or on a hob, before they are put into 
 the lantern. We have even seen a metal box containing slides 
 heated by means of a spirit-lamp. 
 
 G 
 
CHAPTER XIV. 
 
 LANTERNS AND ARRANGEMENTS FOR 
 EXPERIMENTS. 
 
 There are many scientific experiments for full comprehension of 
 which we depend chiefly upon our eyesight, and it is easy to 
 understand that if we can utilise the lantern for showing on a large 
 scale the image of an experiment being made on a small scale, we 
 have found a further use for the lantern. As a matter of fact, 
 many chemical, magnetic, optical, and other experiments can very 
 easily be shown to a large audience, though conducted on a small 
 scale, and we may add to this the fact that it is possible to add 
 to an ordinary optical lantern a projection microscope, polar- 
 ising and spectroscopic apparatus, and various other optical 
 appliances. 
 
 Fig. 48.— Lantern Microscope. 
 
 It is long since attempts were first made to project images of 
 microscopic objects by means of a microscopical objective and 
 suitable condensing lenses. The animal and vegetable contents 
 of a drop of water have many times been projected with high 
 magnification on a screen ; fleas and other insects have many a 
 time figured on ten-feet discs ; but lately considerable advance has 
 been made in this line, and images of very minute structures may 
 now be projected by even immersion objectives. 
 
 Figure 48 shows a simple arrangement, worked out by Dr. 
 E. C. Bousfield and the writer, for the projection, with ordinary 
 
ARRANGEMENTS FOR EXPERIMENTS. 
 
 91 
 
 apparatus of the microscopist, of microscopic images when very 
 high magnification is not required. An ordinary " bull's-eye " 
 takes the place of the lantern condenser; in front of this is a 
 trough which holds water or alum solution ; there is the ordinary 
 fitting for a substage condenser and provision for a certain amount 
 of focussing of the condenser. The stage has a round plate with 
 apertures of various areas and clips for holding the objects. The^ 
 
 Fig. 49. — Wright and Newton's Combined Lantern and 
 Microscope. 
 
 microscope tube in front has a coarse and a fine adjustment, and 
 we find the whole arrangement works very well with objectives up 
 to ~/z and Y'z inch. The lens by Zeiss, of Jena, known as the 
 " aa," which has a focal length of about one inch, works very 
 well and without any substage condenser; but with a. j4 or 
 glass we find it better to use a low-angle condenser. So far as 
 this arrangement pretends to go it gives results as good as any 
 simple arrangement in the market, and its price is small. Mr. 
 Baker, of Holborn, made the one here figured. The entire 
 arrangement fits to the front of an ordinary lantern. A higher 
 class article is the outcome of Mr. Lewis Wright's experiment 
 and ingenuity. Here also (Fig. 49) we have power of using and 
 adjusting a microscope condenser, and there is a "fine adjustment" 
 for the micro-objective, as well as an alum cell to prevent the 
 slide being damaged by heat. Another excellent instrument for 
 
 G 2 
 
92 
 
 THE OPTICAL LANTERN. 
 
 the same purpose was worked out by Mr. E. M. Nelson, and is 
 made by Mr. Baker. 
 
 A special condenser of the triple type is recommended for this 
 projection arrangement. 
 
 In like manner, a polarising apparatus may be' attached to the 
 front of the lantern. The apparatus (Fig. 50) we have seen used 
 with great success. 
 
 Spectroscopic experiments may be demonstrated by means of 
 an ordinary lantern with a slit-diaphragm attached to the front, 
 prisms and collimator being placed on suitable stands between 
 lantern and screen. With an electric lamp and a primary battery, 
 and an apparatus designed by Mr. John Browning, the writer has 
 carried out many interesting experiments on the spectra of metallic 
 
 Fig. 50. — POLARISINC AlTARAirS. 
 
 and other substances. Demonstrations on such lines take a firm 
 grasp of the attention and interest of any audience. 
 
 For chemical experiments, and, indeed, for many others, an 
 " open stage" is almost a necessity. Fig. 51 shows a lantern well 
 adapted for such work. 
 
 A so-called tank or cell may be placed in the open stage, 
 and in this tank chemical reactions may be made to take place, 
 the action being finely displayed on the screen. The same 
 may be said of magnetic, electric, and very many optical 
 experiments. If a sheet of orange or yellow glass be placed 
 between the condenser and the tank, a photographic image may 
 easily be developed, coram populo, care being taken to use a 
 very " slow " plate, such as a chloride plate used for lantern slides. 
 For these and other experiments with liquid in the cell, a pipette 
 
ARRANGEMENTS FOR EXPERIMENTS. 
 
 93 
 
 should be used to drop in the reagents that determine the action, 
 the cell having been previously nearly filled with one of the liquids. 
 A very interesting series of experiments, capable of being well 
 shown by the lantern, will be found in the contributions of the 
 late W. B. Woodbury, to the English Mechanic some years ago, 
 and subsequently published in separate form under the title of 
 " Science at Home," by the Sciopticon Company. 
 
 Of late years considerable attention has been given, by opticians, 
 to lanterns of the highest class, adapted more particularly to scien- 
 
 FiG. 51. —Open-Stage Lantern. 
 
 tific purposes and to experimental demonstrations. In these pro- 
 vision is made for the introduction, between the condenser and the 
 objective, in the place of the ordinary slide carrier, of various 
 instruments and pieces of apparatus by means of which a great 
 variety of experiments may be demonstrated to large audiences by 
 projection on the screen. Arrangements are also made by which 
 special forms of microscope, polariscope, spectroscope, and other 
 instruments can be readily attached, and the various phenomena 
 exhibited to a crowded room, instead of being confined to a limited 
 number of observers. In Fig. 52 is shown one of the earlier, but 
 
94 
 
 THE OPTICAL LANTERN. 
 
 well-tried instruments of this class, constructed by Messrs. Newton 
 and Co. It is designed for use with limelight only, and in the 
 lower lantern such pieces of apparatus as the radiometer, the 
 electroscope, galvanometer, or Leyden jar may be introduced 
 between the condenser and projecting lens. The upper lantern 
 at the same time can be used for the projection of slides 
 in illustration of the experiments demonstrated by means of the 
 lower one ; or, when placed in the position shown, serves the 
 various purposes of vertical projection. This is useful, or indeed, 
 
 Fig. 52.— Newton's Scientists' Bi-uniai. Lantern. 
 
 essential, in a variety of physical experiments, such,' for instance, 
 as the crystallisation of salts, or it will enable sketches to be made 
 and exhibited on the screen while being drawn. In the ordinary 
 vertical arrangement the elements of the condenser are separated, 
 two reflecting mirrors being used, one of which is placed between 
 the separated elements. But in this instrument there is only one 
 reflecting surface, for which reason greater brilliancy is claimed ; 
 and as the condenser of the upper lantern is not interfered with, 
 
ARRANGEMENTS FOR EXPERIMENTS. 
 
 95 
 
 it is instantly available for either purpose by simply turning it on 
 its hinges. 
 
 Quite recently, in conjunction with Mr. Ives, Messrs. Newton 
 have patented an extremely handy and portable instrument, which 
 they call the Universal Science Lantern, and which is adapted for 
 use either with limelight or the electric arc. This is shown m 
 Fig. 53, and is fitted, as will be seen, with a triple front, any one 
 of the objectives of which can be brought into action in a couple 
 of seconds. The first objective consists of an ordinary 6-in. achro- 
 matic lens for the exhibition of slides or diagrams ; the second is 
 
 Yic,. 53.— Thr "Newtonian" Universal Science Lantern. 
 
 a microscopic attachment fitted with substage condenser and 
 various powers ; the third consists of a direct vision spectroscope 
 with achromatic focussing lens, and projects a brilliant spectrum 
 without the inconvenience attending the use of a single prism. A 
 second prism is provided for the comparison of different spectra. 
 These parts may be removed, and a vertical attachment, polari- 
 scope, or other apparatus attached in a few seconds. The appa- 
 ratus complete packs into a very small space, and is well 
 adapted for general lecture purposes. 
 
 Messrs. Ross & Co.'s New Patent Science Lantern is shown 
 in Fig. 54 as used in ordinary projection work, and in Fig. 55 
 when applied to vertical projection. It is double-fronted, each 
 front being fitted with a triple condenser of improved form. One 
 
96 
 
 THE OPTICAL LANTERN. 
 
 front takes ordinary slides, diagrams, or tanks for precipitation 
 experiments; the other is for parallel beam work and light 
 experiments generally, and is also arranged to take microscopic 
 attachment, polariscope, or any of the additions usually employed. 
 By raising the handle shown in front of the diagram (Fig. 54), 
 
 Fig. 54— Ross' New Patent S. iexce ].a.x,ekn (Ordinary Projection). 
 
 the stage can be raised into a horizontal position for use in vertical 
 work. By means of the handle projecting downwards from the 
 large tube of the opposite front, the whole instrument can be 
 swung round, the light remaining stationary, so as to bring either 
 front into a central position for use. The instrument as shown 
 
 Fig. 55.— New Patent SriicNCK Lantern (Vertical Projection). 
 
 is intended for use with limelight only, but a larger form is con- 
 structed for use with the electric arc. This forms a most com- 
 plete and efficient apparatus for the demonstration of every de- 
 scription of physical experiment. 
 
CHAPTER XV. 
 
 ANIMATED PHOTOGRAPHY AND PROJECTION. 
 
 Obviously a work like the present, which is intended to embrace 
 every phase of lantern projection, would be incomplete without 
 some reference, however brief, to the latest and most sensational 
 development of the science— a branch that only a few years back, 
 however much it may have stimulated the dreams of scientific 
 enthusiasts, would have been impossible without the successive 
 advances made in recent years in purely photographic processes. 
 The gradual increase in the rapidity of gelatine emulsions, im- 
 provements in lenses, and; above all, the production of suitable 
 flexible surfaces on which to support the pictures in such a 
 manner as to render them easily applicable to the purpose in 
 view, combined to bring the principle of animated photography 
 within the scope of practice, and after that the mechanical arrange- 
 ments were made with comparative ease. Although the apparatus 
 necessary for taking and showing a long series of photographs in 
 very rapid succession is necessarily somewhat complicated, it pre- 
 sents no serious difficulties to the expert mechanician, as is proved 
 by the large number of different instruments now in use; and at 
 the present day it is difficult to decide which to admire most— the 
 ingenuity displayed in the construction of, or in finding names for, 
 the different instruments. In the nomenclature advantage is taken 
 of prefixes referring to "motion," "change," " life," or " anima- 
 tion," as in the " cinematograph " or " kinematograph," " kineto- 
 scope," " mutograph," " biograph," " vitascope," and "animato- 
 graph," which may be selected as a few of the chief types that 
 may be etymologically classed. Of others, it can be said, as of 
 the " photo-rotoscope " and " motor-pictoroscope," that it is easier 
 to imagine what they are than to trace their etymology ; while 
 again, with others such as the " Birtac," a dive behind the scenes, 
 or a deep and intimate study of later nineteenth-century methods 
 of nomenclature, serves to show the hopelessness of attaching any 
 meaning to them at all. Where an attempt at etymology is made 
 
98 THE OPTICAL LANTERN. 
 
 at all, it would at least seem desirable to use the affixes " graph " 
 and " scope," to describe the taking and projecting instruments 
 respectively, and not to apply them indiscriminately, as now seems 
 to be the fashion. 
 
 At the time of writing these lines, it would certainly be incor- 
 rect to say that the practice of animated photography has taken 
 any great hold on amateurs, for whom this work is chiefly written, 
 though, when the expense of the necessary additional apparatus 
 and plant is not an obstacle, there is no reason why they should 
 not venture into this department of projection— at least, on a small 
 scale. But, up to the present time, the exhibition of animated 
 photographs remains, beyond doubt, a showman's business, and a 
 very popular and profitable one, while the production of the neces- 
 sary "films" constitutes a new and lucrative profession in itself. 
 Scarcely an event or ceremony of importance now takes place but 
 is reproduced by at least one of the numerous "graphs" or 
 " scopes," and, probably, the same evening is repeated in all its 
 details before the audience of one or other of the variety theatres. 
 In this manner public processions, funerals, weddings, launches, 
 horse races, and even prize fights, can be placed on record for the 
 benefit of future generations, and in the near future there is little 
 doubt but the kinematograph will be largely used as an aid in 
 astronomical observation. It is to be regretted that the first 
 attempt to so utilise it in connection with last year's solar eclipse 
 should have proved a failure from outside causes, but we may be 
 certain the experiment will be repeated on the first opportunity. 
 
 Theoretically, the production and exhibition of living and 
 moving photographs are simplicity itself, based as they are upon 
 the principle of persistence of vision. The usual image formed 
 upon the retina of the eye remains for a brief period — about one- 
 tenth of a second— after the object causing the sensation is re- 
 moved or hidden, and this explains why in winking the eyes we 
 experience no break or interval in the visual sensation. If, then, a 
 series of photographs be taken in very rapid succession of a moving 
 object or scene, and these be subsequently projected on the screen 
 with the same rapidity of succession and in such a manner that 
 the different pictures fall in exactly the same position, and if means 
 be adopted to cut off the light during the brief periods during 
 which the pictures are changing, we have the effect of a continu- 
 ously changing scene, the eye being unable, owing to the property 
 
ANIMATED PHOTOGRAPHY. 
 
 99 
 
 of persistence of the image, to recognise or take cognisance of the 
 extremely brief periods of echpse. Such is the simple theory, and 
 the perfection or otherwise of its realisation depends, of course, 
 upon the proper adjustment of the apparatus and its accuracy of 
 working. What that accuracy really amounts to may be gauged, 
 perhaps, by what follows. 
 
 It is customary in speaking of animated photographs, as at 
 present shown, to refer to the crude phenomena exhibited half a 
 century ago in such instruments, or toys, as the " praxinoscope," 
 the "zoetrope," or "wheel of life"; but beyond the fact that 
 these also were based upon the principle of persistence of vision, 
 there exists very little similarity. These earlier attempts were 
 simply made with more qr less inaccurate drawings of successive 
 phases of motion to depict the repetition of a single incident or 
 act as— a smith striking an anvil, a boy jumping, a girl skipping, or 
 the well-known example of our boyhood of the man swallowing 
 rats. These were amusing enough in their way, and interestmg 
 from a scientific point of view, but constituted crude, jerky, and 
 otherwise imperfect representations of the real incident. Even 
 when some twenty years ago in many opticians' windows were to 
 be seen at work zoetropes, in which the subjects were silhouette 
 copies of Muybridge's series of trotting and galloping horses, the 
 effect was scarcely any better, for, although the different phases of 
 motion, being from photographs, were accurate in themselves, 
 they were too few in number, and not accurately enough in 
 register. 
 
 Much nearer perfection, though still leaving much to be 
 desired, were the pictures from the original photographs pro- 
 jected on the screen by Muybridge himself for the benefit of such 
 fashionable audiences as those of the Royal Institution, the Royal 
 Academy, the Society of Arts, and, if we remember rightly, Eton 
 College, besides the principal scientific and artistic bodies in Pans. 
 These consisted, however, simply oi procession of the same horse 
 or animal across the screen, and in the more complete series, as, 
 for instance, a racehorse at full speed depicted in twenty-four 
 phases, representing something less than half a second of time, the 
 effect was very good. 
 
 Following the lead given by Muybridge, Professor Marey, of 
 Paris, turned his attention to the analysis of the flight of birds by 
 successive exposures made at extremely brief intervals. Muy- 
 
lOO 
 
 THE OPTICAL LANTERN. 
 
 bridge's exposures were mostly made with separate cameras placed 
 at stated distances apart, the object being made to pass before 
 them, and automatically make the exposures at fixed intervals. 
 Obviously, under such circumstances, the relative positions of the 
 moving object and the background or remainder of the subject 
 changed in each successive picture, but as these pictures were 
 nearly all taken against a perfectly white background, against which 
 the object appeared as a black silhouette, the inconvenience that 
 would otherwise arise from a constantly jumping and changing 
 scene was not experienced. In some taken against a natural 
 background the effect was not unhke what one might see from a 
 terribly jolting railway train. 
 
 Professor Marey's apparatus, we believe, took somewhat the 
 form of a gun, and was sighted in the same manner from the 
 shoulder, the pictures being taken upon a rapidly revolving 
 circular plate. They were exceedingly minute in size, and, 
 speaking from recollection, were taken as many as fifty or sixty 
 in a single second. These bore enlargement to considerable 
 dimensions, and gave a surprisingly accurate idea of the original 
 motions. 
 
 Lastly, as anterior to the era of modern animated photography, 
 came the pictures by Anschutzof subjects similar to those usually 
 selected for the old wheel of life, though many of them were much 
 more elaborate. These, being produced upon accurate principles, 
 and carefully registered, gave a much better representation of the 
 subjects than had hitherto been seen. 
 
 This brings us to the early " nineties," but before this others 
 had been busy upon the work of rendering moving scenes as 
 distinguished simply from single objects in motion. In 1890 Mr. 
 Friese Green exhibited and explained a camera devised for this 
 purpose, but we do not remember that he showed any of the 
 results produced. About this time, too, a patent was taken out 
 by another Englishman— Mr. Wordsworth Donisthorpe-— for a 
 camera for similar purposes, but, so far as we are aware, he never 
 went any further with it. In or about 1893 Edison took up the 
 idea in America, and worked out an apparatus to which he 
 gave the name of Kinetoscope. This, although inferior to the 
 instruments that followed, may be taken as the first successful one 
 of the kind. 
 
 In 1895, Messrs. Lumiere, in France, introduced their cine- 
 
ANIMATED PHOTOGRAPHY. 
 
 lOI 
 
 matograph, which, after numerous improvements, remains one of , 
 
 the leading instruments. Early in 1896, Mr. Birt Acres gave the iy^ 
 
 first public demonstration in this country of animated photography ^ / ^CcZ^, 
 
 before the Royal Photographic Society, and later on photographed 
 
 and exhibited at Marlborough House scenes at the wedding of 
 
 Prince Charles of Denmark and Princess Maud of Wales. Since 
 
 that period the growth in popularity of animated photographs, and 
 
 of machines for their manufacture, has been enormous, and there 
 
 is scarcely a music hall or variety theatre in any of our large towns 
 
 but has an instrument permanently installed for the exhibition of 
 
 current events and occurrences. 
 
 It would be quite impossible, even if we had closely examined 
 all the instruments in use— which we have not— to give a detailed 
 description of their working parts. We shall, therefore, content 
 ourselves with giving a general description of the principles on 
 which they are constructed, with diagrams of some of the leading 
 ones. The complete outfit for the taking and exhibition of 
 animated photographs may be divided into three parts — the 
 camera, the printing machine, and the projecting machine. Some 
 instruments combine all three; while others confine themselves to 
 t the task of taking only, or taking and printing ; while others, again, 
 only attempt the projection or exhibition. 
 
 Briefly, the method of procedure is as follows : — The pictures 
 are taken upon long strips of flexible celluloid film, such strips 
 being obtainable ready for exposure in various lengths according 
 to the duration of the scene to be depicted. The standard width 
 of film is T in., the individual pictures given by the Lumiere 
 machine being 25 by 20 millimetres, and the length of film 
 required for a series of exposures lasting one minute, is about 
 60 ft. The films as sent out for use are perforated at the edges 
 with holes to fit the sprocket or pin-wheels, by which they are 
 passed through the apparatus, and which are necessary in order to 
 ensure that the film travels at an absolutely uniform rate with the 
 rest of the apparatus, or, in other words, that the pictures are 
 accurately equi-distant. There are two different " gauges," to one 
 or other of which most other instruments conform — namely, the 
 Lumiere and the Edison, the former having one hole to each 
 picture, the latter four holes at either side. Greater lengths of 
 film can, of course, be obtained if needful, or several of the one- 
 minute lengths may be joined together, single films very often 
 
I02 
 
 THE OPTICAL LANTERN. 
 
 reaching to several hundred feet in length, and carrying some 
 thousands of exposures. 
 
 The unexposed film is placed in a receptacle provided for it 
 inside the camera, and one end being disengaged, is passed over a 
 series of rollers which take it in front of the lens, over the sprocket 
 cylinder, and finally it is clamped to the receiving spool, upon 
 which it is wound when exposed. With some forms of apparatus 
 it is necessary to effect this connection in the dark room, while 
 with others the daylight cartridge system is followed, and films 
 may be exchanged and replaced in the open air. The focus must 
 be obtained with the greatest accuracy, because not only does 
 the least departure from the correct plane — owing to the short 
 focus and large aperture of the lenses employed — produce a much 
 greater effect than under ordinary circumstances, but any such 
 want of sharpness will be rendered still more palpable by the 
 enormous degree of amplification the minute pictures have to 
 undergo. 
 
 The receiving spool is connected with a handle outside the 
 camera, by means of which, when all is ready, the film is wound 
 at a regular speed from its unexposed box on to the exposed 
 spool, and during the journey the whole of it passes in front of 
 the lens. But if the motion were a steady and continuous one, 
 the only effect would be to produce a continuous blur, to 
 <pbviate which is the function of the sprocket wheel and shutter. 
 By means of the sprockets which engage in the perforations of 
 the film it is pulled forward by a series of quick jerks, alter- 
 nating with comparatively longer periods of rest; the longer the 
 resting periods in proportion to the periods of motion the 
 greater the ef?iciency of the instrument so far as exposure is 
 concerned. The ratio between the two in the best instruments 
 is at least 2 : i. 
 
 But simultaneously with the jerk forward of the film, a rotating 
 or reciprocating shutter comes into action, and momentarily 
 closes the lens during the period the film is in motion, uncover- 
 ing it again the instant the period of rest commences, these 
 alternate motions taking place at the rate of from fifteen to thirty 
 times in a second, from which, as we have already said, it will be 
 evident that the very greatest precision of working and accuracy 
 of perforation are necessary in order to secure register of the 
 picture afterwards, upon which clearness and steadiness depend. 
 
ANIMATED PHOTOGRAPHY. 
 
 103 
 
 In the case of a steady continuous motion the object would be 
 easy of attainment with ordinary gearing; but it is not quite so 
 simple a matter when the intermittent character of the movement 
 is considered, and it is here where the function of the sprockets 
 and perforations comes in. In some instruments the sprocket- 
 wheel itself communicates the intermittent action to the film; in 
 others it imparts to it a steady continuous motion, while other 
 means are taken to give the necessary jerk forward; but in all 
 cases the method by which the result is produced is much the same. 
 The jerk forward of just the length of one picture leaves that 
 amount of " slack " on the film, which, during the period of rest 
 of the remainder, is taken up by the steady revolution of the 
 receiving spool. The sprocket-wheel, or cylinder, also serves the 
 purpose, as will be evident, of a regulator of the amount of film 
 wound up by the receiving spool, as without its intervention the 
 increasing diameter of the spool would cause the film to travel at 
 a constantly accelerating speed. 
 
 We have dwelt at greater length upon the ])roduction of the 
 film negative than the objects of this work would seem to render 
 necessary ; but we have done so intentionally, because the 
 principles involved are precisely the same as those which rule in 
 connection with the projection of the pictures. We may pass 
 over the production of the positive film without saying more than 
 that a similar strip of sensitive film is passed through the machine 
 in contact with the negative strip, receiving exposure to the light 
 of a paraffin lamp or gas flame as it passes the lens aperture or 
 when special printing apparatus is used, as it passes the aperture 
 provided for the purpose, the respective films being re-wound 
 senarately, the positive on to the receiving spool, the negative on 
 to another specially provided. The development and further 
 treatment of the two films, moreover, forms no part of the object 
 of this work, so we may pass on to the projection. 
 
 The process consists essentially in a repetition of the operations 
 already described, with the difference that the apparatus is placed 
 in the axis of the cone of light emanating from a lantern of 
 ordinary or of special construction, but so far as the optical system 
 is concerned, the projection of animated photographs differs in no 
 respect from that of ordinary sUdes. We have, in fact, only to 
 deal with the mechanical arrangement employed to bring the 
 pictures into the beam of light. This consists of an apparatus 
 
I04 
 
 THE OPTICAL LANTERN. 
 
 similar to, or identical with, that used in the production 
 of the negative, and although, as we have said,. special arrange- 
 ments are provided for projection only, it would seem to us that 
 a more perfect result should accrue from the use for the latter 
 purpose of the identical apparatus employed in making the nega- 
 tive. Still, this is a matter rather of convenience, or one that will 
 depend, in great measure, upon circumstances, and until the 
 making of the negative films becomes a regular part of the 
 exhibitors' duty, the special projection apparatus will continue to 
 have its value for those who merely purchase or hire the negative 
 films ready made. 
 
 The special apparatus differs in no essential respect from that 
 used in making the negative, except in so far . as, for exhibiting 
 purposes, the shutter is not so necessary a portion of the 
 mechanism. Some of. the projection machines have shutters, 
 some are " shutterless "; while in others, again, that detail may be 
 employed or not at the option of the exhibitor. The points to 
 be observed in connection with the shutter are — first, that it 
 necessarily causes a certain loss of light, and by its intermittent 
 action, gives rise to an unpleasant '• flicker," or unsteadiness, which 
 in some of the earlier machines was most distressing. It will be 
 evident to all who are acquainted with lantern work that when 
 pictures of the size we have mentioned, representing as they do but 
 a quarter of the area of the ordinary lantern slide, are enlarged up 
 to anything like the dimensions usual with the latter, the necessity 
 for the very best light possible will be only too obvious; and 
 besides the greater degree of enlargement, it must also be borne 
 in mind that the celluloid films are by no means so clear and 
 transmissive as good glass slides. Taking these circumstances 
 into consideration, we can fully sympathise with the desire to do 
 away with the shutter when it still further decreases the available 
 light by at least one-third. 
 
 But, on the other hand, the suppression of the shutter gives 
 rise to another fault, a streaky appearance of the picture or 
 " raininess," as it has been termed, from a similarity to falling 
 rain. This is caused by the motion of the film, which takes place 
 in full view, and though each separate motion is extremely brief, it 
 naturally follows from what has already been said about persistence 
 of vision, that that property must exercise an effect in connection 
 with the moving periods precisely proportionate to the time occu- 
 
ANIMATED PHOTOGRAPHY. \0$ 
 
 pied by the separate phases. If the motion of the film were con- 
 tinuous, it would result in a complete blur, or a series of stripes 
 representing the salient features of the scene. If the motion 
 consist of alternate periods of two-thirds rest to one-third of 
 motion, the blur must still be visible in the form of "raininess.'^ 
 Clearly, the only remedy for this state of things, is to increase the 
 proportionate difference between the periods of movement and of 
 rest as much as possible. It has also been proposed to employ 
 a shutter of translucent material, which, while permitting a portion 
 at least of the light to pass, would obscure the details of the 
 moving picture, and obviate the streakiness by converting it into 
 an evenly diffused illumination. This is one of the points to be 
 considered in the selection of apparatus ; beyond that individual 
 perfection of mechanism is about the only matter to study. 
 
 Fig. 56. — LuMiKKE Cinematograph Camera. 
 
 Perhaps the most representative of the all-round instruments, 
 or those that perform the whole of the operation, is the Lumiere 
 Cinematographe, which forms camera, printing frame, and pro- 
 jection apparatus in one. The interior arrangements of the instru- 
 ment, as employed for taking the photographs, is shown in 
 Fig. 56, though essential portions of the mechanism are hidden, 
 for which reason we can attempt no detailed description of 
 it, merely referring our readers to the general remarks already 
 made. In this apparatus the intermittent progression of the 
 film is produced by the reciprocating action of an eccentric 
 crank operating alternately upon the sprockets which draw 
 the film forward, and the adjustable shutter that eclipses the 
 
 H 
 
io6 
 
 THE OPTICAL LANl'ERN. 
 
 picture during the periods of motion This eccentric motion 
 takes place eight times for each revolution of the crank, and the 
 relative periods of rest and of motion are about as 2:1, that is, 
 the light is actually thrown on the screen for two-thirds of the 
 total period, and obscured for one-third. The shutter is con- 
 structed in sections, in order to allow of adjustment in size when 
 necessary. Fig. 57 will give something of an idea of its arrange- 
 
 FiG. 57. — LuMiERE Camera axd Printing Slide. 
 
 nient when applied to printing purposes, though, of course, in 
 actual use it would be closed; and Fig. 58 gives a rough sketch 
 of a safety projection apparatus in which a globular flask of water 
 is made to take the place of the ordinary condenser. This 
 arrangement not only concentrates a more powerful beam of light 
 on the small picture, but absorbs a large proportion of the heat as 
 well. At the same time it will be noticed that a hinged shutter 
 
A A' /A/ A 7 ED PHO TOGRA PHY. i OJ 
 
 or cap is provided belween the source of light and the film to 
 obviate any danger from heat, and this shutter is kept closed 
 whenever the film is stationary, and only raised when the ma- 
 
 Fir,. 58. — Lu.MiiiRE Safety Lantern. 
 
 chinery is set in motion. The small object marked D is a piece 
 of coke suspended in the flask to prevent or minimise ebullition, 
 when from long use the water is raised to boiling-point. 
 
 The motor-pictoroscope of Mr. W. C. Hughes, of Kingsland, is 
 
 Fig. 59. — Hughes' Motor Pictoroscope 
 
 H 2 
 
io8 
 
 THE OPTICAL LANTERN. 
 
 one of the instruments intended for projection only, and is shown 
 attached to an ordinary projection lantern in Fig. 59. In this the 
 usual sprockets are replaced by a piston and plunger, by which it 
 is claimed that a smoother motion of the film is obtained. A 
 shutter is provided which is so arranged that it can be removed at 
 will even when the machine is in use. By increasing the rapidity 
 of the changing action, or, in other words, the proportion between 
 the periods of rest and motion, and dispensing with the shutter, 
 the "rainy" effect is said to be entirely obviated as well as 
 " flicker," and a gain of thirty per cent, in the illumination effected. 
 The shutter is to be employed only in the case of dark or dense 
 films. 
 
 The next diagram (Fig. 60) shows the method by which the 
 
 Fig. 60. — Warwick Tradinc; Co.'s Bioscope. 
 
 separate projection-mechanism is attached to an ordinary lantern. 
 The instrument figured in this case i3 the Bioscope of the 
 Warwick Trading Company, of London, which possesses some 
 good features. The frame, it will be noticed, consists of a 
 solid steel casting, which renders it impossible for the shaft and 
 bearings to get out of adjustment, and at the same time provides 
 a firm and steady base by which to attach the whole instrument to 
 the table or other support. In this instrument, too, the shutter 
 
ANIMATED PHOTOGRAPHY. 
 
 109 
 
 has been dispensed with, and a gain of at least one-third secured 
 in the illumination, while the flicker and "rain" effects are equally 
 avoided, the latter by increasing the proportion between the 
 periods of rest and motion to 8 : i. It will be noticed that the 
 base by which the instrument is attached to the table is provided 
 with a clamping-screw and fly-nut, shown in the diagram imme- 
 diately below the position of the ordinary slide-carrier. This is 
 of great importance, and enables the instrument to be swivelled 
 round out of position and replaced instantly without losing the 
 adjustment. Thus, if while changing films, or for any other reason, 
 it be desired in the course of an exhibition to substitute an 
 ordinary projection lens for the Bioscope, it can be done with- 
 out any trouble. 
 
 There are not wanting signs at the time these lines are 
 written that before long animated photography will be brought 
 
 F:c;. 61.— The Birtac. 
 
 within the range of the amateur. The first important step in that 
 direction has been taken by Mr. Eirt Acres, already referred to as 
 one of the earlier workers in this field. The idea has been to 
 supply at a reasonable price a complete apparatus by means of 
 which an amateur may take, develop, and show at home, or on a 
 small scale, whatever scenes or events may strike his fancy. The 
 Birtac—which is the name given to the instrument— takes a pic- 
 ture on a rather smaller scale than the usual instruments, which 
 it is capable of enlarging, by means of the special incandescent 
 gas burner suppUed with it, up to 3 feet or 4 feet diameter, which 
 is fully large enough for all home purposes. By utilising a special 
 method of securing higher pressure and better light, the pictures 
 may be increased to 6 feet or even larger, but the smaller dimen- 
 sion will suffice for most purposes. 
 
 The outward appearance of the instrument is shown in Fig. 61, 
 
I lO 
 
 I'HE OPTJCAL LANTERN. 
 
 from which it will be seen that it is no larger than an ordinary hand 
 camera; and Fig. 62 shows the arrangement in conjunction with 
 a specially devised lantern supplied with it, by which the pictures 
 are projected. The skeleton diagram (Fig. 63), shows in detail 
 the working of the apparatus, and by its means the course of the 
 film from the unexposed holder C to the receiving spool N, on 
 which It is wound after the picture is taken can be traced. The 
 essential point in which this instrument differs from most others 
 IS the manner in which the intermittent motion is communicated 
 to the film. This is eff-ected, not by the direct pulling or dragging 
 action of spasmodically-working sprockets or pins, which quickly 
 damage the perforations of the film and give rise to irregularity, but 
 by a projecting arm on the revolving plate, H. At each revolution 
 
 Fig. 62. 
 
 this arm, over which the film passes, in taking the position shown 
 in the diagram, draws the film into a sort of loop between E and 
 J ; as the projecting pins on the steadily-revolving cylinder, J, 
 prevent the film being drawn backwards, it follows, as a matter of 
 necessity, that at each revolution the film is drawn forward a cer- 
 tam distance by a rapid jerk, and during the short period of the 
 revolution that covers that jerk the lens is closed, and only re- 
 opened when the film has become stationary again. Most of our 
 readers are more or less familiar with the action of the sewing 
 machine, and we cannot do better than liken the action just 
 described to that of the needle and thread of the familiar house- 
 hold instrument. 
 
 We are informed by Mr. Eirt Acres that, with mechanism 
 similar to this he has taken as many as one hundred photographs 
 m a second, and he claims for it greater smoothness of working, 
 and less risk to the films than when direct-acting sprockets are 
 
ANIMATED PHOTOGRATHY. 
 
 I I I 
 
 employed. Besides this, it is possible to so adjust the position of 
 the projecting arm on H as to vary the length and duration of 
 the jerk forward to any extent, and, in fact, to make it occupy 
 almost any proportion of the total period of revolution. In the 
 Birtac the lilm is stationary for seven-eighths of the total period, 
 so that where, for projection purposes, the shutter is dispensed 
 with, the usual troubles are reduced to a minimum, if not abso- 
 lutely eliminated. 
 
 Fig. 63. 
 
 When used for projection purposes, the lens is removed from 
 Q to U, and the apparatus arranged as in Fig. 62. The whole 
 arrangements for development, printing, and finishing are so com- 
 plete,''and the apparatus generally so compact, that we have little 
 doubt but that in the coming season animated photography and 
 projection will begin to take a recognised position amongst 
 amateurs. 
 
I 12 
 
 THE OPTICAL LANTERN. 
 
 It would be impossible, without still further extending an 
 already lengthy description, to even mention a tithe of the instru- 
 ments and accessories the practical worker will soon become 
 familiar with. But we trust sufficient has been said to give our 
 readers an idea of the principles of animated projection. 
 
CHAPTER XVI. 
 
 PRACTICAL WORKING OF THE LANTERN, OIL 
 LAMPS, BLOW-THROUGH, AND MIXING JETS. 
 
 Little requires to be said with regard to lighting and using an 
 oil lamp in the lantern. The lamp itself must be thoroughly clean 
 outside and in, the wick or wicks accurately trimmed, and the full 
 flame turned up not all at once but gradually ; the wick ought at 
 no time to be turned so high as to cause a flare; if once this mistake 
 is made the lamp never seems to burn so well till all has been 
 cooled down and the wick retrimmed. With paraffin lamps it is 
 a common and commendable practice to put a lump of camphor 
 weighing (say) one-half ounce with half a pint of the oil. This 
 addition certainly whitens the flame. 
 
 As a rule there is no adjustment for raising and lowering an oil 
 lamp in a lantern, but we require by pushing the lamp forward or 
 pulling it backward to get the flame as nearly in the focal point of 
 the condenser as the size of our radiant will permit. This we judge 
 by watching the disc of light thrown on the screen; coloured edges 
 on the screen mean that the light is not in focus. With many- 
 wick lamps we are apt to have an ill-defined image of flame down 
 the centre of the disc ; we are informed that all lamps do not 
 necessarily show this defect, and we can say that with all the lamps 
 we have used of moderate pretensions to good quality, the defect 
 has been so slight as not to entail absolute failure. 
 
 Our chief business, however, is with the lime light, and to it 
 we now address ourselves. 
 
 The " hmes " are either turned from solid pieces of " hard " or 
 " so(t " limestone, or they are composed of a mixture of substances 
 the nature and proportions of which are kept secret. The best 
 limes, so far as light-giving qualities are concerned, are those turned 
 from the natural limestone, but for seme reason they are never 
 turned true, the hole up the centre is seldom true, and the cylinder- 
 shape is never accurately kept. The limes known as " Excelsior " 
 are turned accurately enough, but are slightly inferior as radiants. 
 
114 
 
 THE OPTICAL LANTERN. 
 
 As, however, the distance from nipple to lime is a factor of con- 
 siderable importance in producing the best light, and as of necessity 
 this distance varies as we turn the badly-shaped limes, the Excelsior 
 limes may actually give the better light. If we could get the 
 " Nottingham" limes truly turned we should never use any other; 
 as matters stand we use Excelsior or Nottingham as they come to 
 our hand, except for a very powerful light, when the Excelsiors are 
 not suitable. Probably the best lime for the blow-through jet is 
 the " Excelsior," which is too soft for the mixing jet. 
 
 The limes are packed in hermetically sealed tin boxes, with a 
 quantity of powdered quicklime, or in glass bottles with luted 
 corks and usually without the quicklime. The limes have a great 
 avidity for moisture, and will quickly go wrong if any moisture 
 reaches them ; so we must take every precaution to prevent the 
 access of damp. Various plans have been tried to secure the limes 
 from damp; our favourite system is to wTap a piece of paper round 
 each lime and dip it into "paraffin wax "just above melting point. 
 Some makers send out their limes already rolled in paper; this gave 
 us the idea of the paper, for without it the paraffin is Hkely to stick 
 to the lime cyHnder. Mr. Hepworth points this out, and suggests 
 ihe precaution of keeping the paraffin wax at a low temperature. 
 Opticians sell long brass tubes to hold one dozen, six, or three 
 limes end to end ; these tubes have well-fitting screw lids, and are 
 very useful. To one of these we added a little chamber in the lid, 
 fitted with fine gauze ; into this chamber is put calcic chloride, 
 which is not without use as a protection for the limes against damp. 
 In any case a tin of limes once opened is no longer a safe 
 receptacle ; more than once we have known a tin burst, and 
 ten or eleven limes lost under such conditions. At one time 
 we used a strong wide-mouthed glass bottle, into which we put 
 lime cylinders, quicklime and all, as soon as we opened a tin, luting 
 a tightly-fitting cork with melted paraffin ; this was fairly satisfac- 
 tory, but inferior to the brass cylinders mentioned. 
 
 Before " lighting up " for a display, two or three limes should 
 be baked in an oven or on a hob ; one is placed on the lime-pin 
 and the hydrogen at once lighted. The best distance from nipple 
 to lime is a matter to be decided by experiment, a mixing jet 
 usually has its nipple much closer to the lime than a blow-through ; 
 the distance for a mixing jet may vary from one-sixteenth to one- 
 quarter of an inch ; we have seen the approach so close as almost 
 
PRACriCAL WORKING OF THE LANTERN. 
 
 to amount to contact, and that in the hands of an excellent 
 operator. In all jets there is an arrangement for adjusting the 
 position of the lime with regard to the nipple ; of course the Hme 
 is moved, not the nipple. 
 
 We shall now suppose, first, that a blow-through jet is to be 
 used ; we have a bag full of oxygen between pressure-boards, or a 
 cylinder full of the gas, while the hydrogen tube is connected with 
 the gas supply of the hall or room. If we use a bag we must have 
 a weight of, say, fifty-six pounds, to which we may add one of 
 twenty-eight pounds in case of need. Before lighting anything we 
 open the bag-tap, and the O jet-tap connected with the bag by a 
 rubber tube, having previously put the weight on the boards, and 
 we allow the O to escape for about fifteen seconds to expel air 
 from the gas way. If we omit this, the lighting up may start with 
 a " pop," which is unpleasant. Then we shut off the oxygen and 
 light the hydrogen, keeping the flame about an inch and a half in 
 height or less. Next we gently and gradually turn on the oxygen, 
 till we get a fairly good light without much redness or scattering 
 of the gases. We may at this juncture turn the lime round a few 
 times to see that it keeps its distance from the nipple and also to 
 let it get pretty well heated all over. Next we turn up the H a 
 little, at once the flame will become red ; a little more O will once 
 more produce the blue and bright light ; and so on we may go H 
 and O alternately, till at last we get to a point of brilliance that 
 we cannot pass, and that may be taken as the best light the 
 apparatus will give. Extra weight on the bag may or may not 
 improve the light. 
 
 If we are using O in a cylinder with a regulator we proceed in 
 just the same way. We open the cylinder first, then the jet tap, 
 and here we go on increasing one gas after the other, H always 
 first, till we find that we cannot turn on enough of H to redden 
 the light, then the O tap will have to be manipulated till the best 
 effect is produced. During these steps there may be hissing of the 
 gases, but there is no danger, and when the light is right the his- 
 sing will stop, unless there is some defect in the jet. 
 
 Never put on or take off weights while the gas is burni^ig. 
 Never allow any persott to lean on, nor in any ivay to touch a gas- 
 bag while the jet burns. 
 
 Always light H first : always turn down O first. Always tie 
 weights on to the pressure-boards, and never use round weights. 
 
ii6 
 
 THE OPTICAL LANTERN. 
 
 Never use any such things as fenders or boxes of glass plates for 
 ivetght. Bags of heavy sand, made in suitable shape and tied on 
 to the pressure-boards, answer well. 
 
 Amount of weight, per se, is not the important factor for a good 
 light, but proportion of weight to diameter of gas-way is the factor 
 that makes or mars. 
 
 If anything goes wrong, turn off the O at once, the H as soon 
 after as possible. 
 
 A red light means too much H in proportion to O. A peculiar 
 lurid blue point of light means too much O for the H. 
 
 The use of a mixing-jet entails two bags or tw'o cylinders, with 
 a greater weight in the case of bags, but it enables us to get a light 
 certainly twice as brilliant as can be got through a blow-through 
 jet, and moreover the area of incandescent lime is with the mixing- 
 jet considerably smaller than with a blow-through, which, as pointed 
 out in an earlier chapter, is an advantage of great weight when we 
 desire the best optical effect. 
 
 When bags are used it is undeniable that there are possible 
 dangers, but the contingency of accident is indeed remote if any- 
 thing like common sense and common care be brought to bear on 
 the operations. A mixture of oxygen and hydrogen in certain 
 proportions will certainly on ignition explode^ and that with 
 violence if the gases are confined , in any chamber offering much 
 resistance. But in olden days the two gases used, as the custom, 
 to be mixed in one bag and buined at a nipple, yet we are not 
 aware of any accident having happened, nor was an accident at all 
 probable so long as there was a pressure on the containing bag. 
 Nowadays we put each gas in a separate bag, and we put the two 
 bags in the jaws of one pair of pressure-boards which we weight ; 
 unequal pressure on the bags is, therefore, out of the question, and 
 sucking back of one gas into the bag containing the other is 
 seemingly physically impossible. But even supposing that the 
 gases do get mixed in one bag, there is no likeHhood, still less any 
 certainty of an explosion, provided pressure is kept on the bag. 
 All the accidents that have ever come to our notice have arisen 
 from setting light to a mixture of the gases in a bag not under 
 pressure, and no reasonable person is likely to do such a thing if 
 he is sober, and aware of the effect of igniting such a mixture. 
 Even supposing a bag contains hydrogen alone it is a foohsh 
 thing to bring a light near, especially if there is no pressure on 
 
PRACTICAL WORKING OF THE LANTERN. 11/ 
 
 it. Hydrogen burns on being mixed with air, but hydrogen alone 
 is not combustible, nor a supporter of combustion. 
 
 A lighted candle thrust into pure hydrogen will be extinguished. 
 Even if hydrogen and oxygen are mixed, but one of the gases is m 
 large proportion to the other, say nine of H to one of O, or nine 
 of O to one of 11, there is no explosion or ignition, the gases ignite 
 slowly and there is steady combustion. But two parts of H to one 
 of O will explode with violence on ignition. We gather, then, that 
 even if a Httle of one gas gets into a bag containing a considerably 
 larger proportion of the other gas, there is no danger of violent 
 explosion, so that in order to produce an explosion we would 
 almost require deliberate intention. But each bag should be used 
 for one gas alone, and never for the other gas, and each bag shbuld 
 be conspicuously marked with "O" or "H," both on the side and 
 at the wedge. And if there is any doubt at any time as to the 
 contents of a bag, the test should never be made with a light 
 placed anywhere near an unweighted bag. Gas cylinders, in like 
 manner, should be conspicuously marked, preferably by being 
 painted in different colours. 
 
 We suppose the two bags to be filled each with its own gas, 
 and placed between the pressure-boards. First we place the 
 H bag in position, taking care to push the tap well through the 
 hole in the pressure-boards intended for it, we then let down 
 the partition, and on this we place the O bag. Then the bags 
 are connected with the jet tubes by strong rubber tubes, taking 
 care to spring the rubbers well up on the taps and jet tubes. 
 Having put our weights on the boards we allow a little of each 
 gas to escape at the nipple of the jet and thereafter close the 
 taps. The lime cylinder being on its pin, and the surface of 
 the lime about one-eighth of an inch from the nipple, we light the 
 hydrogen. (If the "cut-off" arrangement (Fig. 32) is used, the 
 cut-off cross-piece is turned so that both gas-ways are full open.) 
 Now we turn on a little O ; when the flame is a brilliant point we 
 raise H a little, then the O again; and so on till we have the best 
 light we can get, or until the jet begins to " roar beyond endur- 
 ance. (Having got the best light, if the cut-off is in use, we turn 
 the latter completely off, the H will continue to burn gently, and 
 on turning the cut-off open again we shall find the light as we left 
 
 it at its best. Using the cut-off with cylinders alone does not 
 
 answer, but with Beard's Regulators on the cylinders, it will 
 
THE OPTICAL LANTERN. 
 
 work quite well.) For a mixing-jet where two bags are used 
 in one pair of pressure-boards, having a surface of top of 40 x 30 
 inches, 112 lb. wjU be found a good average pressure. In our 
 experience no "composite" Hme will stand the amount of heat 
 produced by this pressure and a good jet ; a "sohd" lime will be 
 required — i.e., one turned from limestone. 
 
 When we are dealing with multiple lanterns and dissolvers, we 
 have, of course, to follow the above course with each jet— getting 
 all the jets to burn as nearly as may be with equal brilliance, a 
 matter of no slight difficulty at times. But when once we have 
 got the jets to work equally well, a good dissolver will ensure the 
 changes being properly made, and if the slides are of equal density 
 we shall have equally brilliant images on the screen. 
 
 As has been already said, the light should be arranged and 
 everything centred and focussed before the audience begin to 
 enter the hall. This is all the more important when the lantern 
 is placed among the audience. The cut-off arrangement enables 
 us, having once regulated the light, to turn it down till we require 
 it for the lecture, and then to get it in an instant as we left it. If 
 the " cut-off," or an equivalent, be not available, we must at the 
 beginning of the lecture turn up the gases again as neatly and as 
 quickly as we can. 
 
 The cautions given in connection with the blow-through 
 apply equally for the mixing jet, and ought to be studied before- 
 hand, and committed to memory. 
 
CHAPTER XVII. 
 
 PREPARATIONS FOR LECTURE. 
 
 A PERSON lecturing for the first time cannot reasonably expect 
 to meet with such success as he may hope to achieve after some 
 practice. This remark holds good for an ordinary lecture or public 
 address of whatever kind, but it has much greater weight when 
 applied to lectures illustrated by means of the lantern, with all its 
 incidental cares. A few remarks suggested by experience may 
 therefore not impertinently nor inaptly be addressed to the young 
 or the intending lecturer, and these remarks naturally range them- 
 selves under two heads— viz.: points to be noted with regard to the 
 actual lantern operations ; and points to be attended to by the 
 lecturer during his actual occupation of the platform. 
 
 If any /lasco ever takes place in regard to a lantern display, it is 
 most likely to happen through some part of the apparatus being 
 left behind or not procured. And if anything is forgotten, it is 
 pretty sure to be a small article. One does not, as a rule, forget 
 to pack and carry the lantern, nor the pressure-boards, we are far 
 more likely to forget the key for opening the valve of the cylinder. 
 The limes are not seldom left at home, and, like the valve-key, are 
 almost impossible to get unless the lecture is to be given in a city 
 or large town. ^Ve propose, therefore, to give, as a sort of memoria 
 technica, a full list of all the articles required. This list will be 
 found on page 137. 
 
 It is always well to examine the hall where the lecture is to be 
 given, before it is actually time to start the erection of the screen. 
 Sometimes the hall or the platform is of such a construction as to 
 prevent the use of the ordinary screen or screen-frame; sometimes 
 the arrangements are such that we can advantageously dispense 
 with the screen-frame altogether; sometimes there is great 
 difficulty in finding a suitable site for the lantern ; and very often, 
 put the lantern where we may, there is a gas bracket or some such 
 obstacle between the lantern and the screen. If we see these 
 matters a day beforehand, we can frequently have them remedied 
 
I20 
 
 THE OPTICAL LANTERN. 
 
 in time ; if we do not see the difficulties till the last moment, we 
 may not have time to remove them. Gas brackets in particular 
 have to be seen to ; we have more than once had to cause the 
 removal of a rigid pendant. 
 
 It is by no means necessary to have the lantern on the middle 
 line of the hall; it is often very convenient both for lanternist and 
 audience to have the lantern quite at the side of the hall ; in such 
 a case, of course, the screen is not erected at right, angles to the 
 central line of the hall, but at a greater or less angle to it. If there 
 is no gallery or other eminence at the back of the hall and at a 
 suitable distance from the platform, this plan of " angling " the 
 screen and putting the lantern at one side of the auditorium is not 
 only passable but very desirable, for the lantern is not surrounded 
 by the audience and none of the audience can be directly behind 
 the lantern, and so be dazzled or have the view obstructed by it. 
 We strongly recommend the placing of the lantern at one side, 
 where no gallery is available right in front of the screen. Anything 
 is better than having the gas-bags or cylinders closely surrounded 
 by the audience. 
 
 Undoubtedly the best site for the lantern is the front of a 
 gallery straight in front of the screen. Sometimes, however, this 
 gallery front is too far distant from the screen for the disc we require. 
 Roundly speaking, the diameter of the disc should be about one- 
 third of the length from back to front of the hall. If we have a 
 hall forty-five feet from gallery to platform we shall get a fifteen feet 
 disc with a nine inch lens, about a seventeen feet disc with an eight 
 inch lens ; if our distance be sixty feet we shall get a twenty feet 
 disc with a nine-inch lens, a fifteen feet disc with a twelve-inch 
 lens. If we have only one projection lens, say of eight inches 
 focal length, we shall frequently have to go to the body of the hall 
 with our lantern. One thing we would try to impress upon our 
 reader ; a good small disc is vastly superior to a poor large one, 
 and the small bright disc is better seen than the larger poorly lighted 
 one. The foremost of the audience should be kept well back from 
 the screen, the larger the disc the further back the front row of 
 seats should be placed. In most cases the best place for seeing 
 is at about a distance from the screen of three times the diameter 
 of the disc, not less certainly than twice, but, of course, short- 
 sighted persons will see better if they are closer to the screen than 
 these distances. 
 
PREPARATIONS FOR LECTURE. 
 
 121 
 
 We will suppose the screen to have been erected, the site for 
 the lantern chosen, the lantern unpacked and on its table or stand 
 in the selected position. In choosing the spot for the lantern we 
 must take care that the optical axis of the system is perfectly 
 perpendicular to the screen ; in other words, that the lantern is 
 right opposite the centre of the screen. The tilt required for the 
 screen may be judged with fair accuracy by eye, but a safer way 
 is to centre the disc on the screen, place a slide in the carrier, 
 and tilt the screen until the foreground and top of the picture are 
 alike in focus. For centring the disc we use a blank slide, that is 
 to say, a circular mask mounted between two glasses of lantern 
 slide size, the centre of the circle being marked on one of the 
 glasses with ink or a piece of paper gummed to the glass. (To 
 make a good ink mark on glass lick the glass with the tongue, let 
 it dry, and then put on the ink. If soot is dusted over the ink 
 when dry the mark will be all the better.) It must not be for- 
 gotten that though the whole of the circular disc may be 
 shown, an oblong (or cushion-shaped) picture may overlap the 
 screen, so allowance must be made for this. In any case, the 
 picture must fall ivell within the limits of the screen, the half- 
 lighted margin round the real picture forms a nice mount- 
 ing for the latter. If it be seen that one side of the picture 
 or of the mask image, is more sharply focussed than the 
 other, the screen is not at right angles to the optical axis, and 
 either it or the lantern must be shifted. 
 
 The beginner should certainly see to all these matters some 
 hours before the show is to begin : the lantern ought to be in 
 position and the lenses focussed, so that when the time comes to 
 light up there may be no alteration of position required. Two or 
 three limes may also at this stage be put to bake in an oven, or on 
 a hob ; a well-baked lime is a considerable advantage. 
 
 Of course, if either or both of the gases are to be made by the 
 operator, it should be done a few hours before the display is to 
 begin, and we advise the tyro to make plenty of gas. A few pence 
 wasted will be well repaid by peace of mind. The oldest lecturer is 
 not without his qualms as to the gas supply, for one never knows 
 what accident may occur. Gas-bags should on no account be left 
 without surveillance in an open hall, even if they be locked ; 
 and if cylinders are used, the owner should not surrender charge 
 of the keys. There are always clever fellows knocking about who 
 
122 
 
 THE OPTICAL LANTERN. 
 
 are eager to show off their knowledge, and a cylinder of oxygen 
 yields many beautiful experiments. 
 
 Immediately before the final lighting up the front lens should 
 be well warmed. The condenser very quickly warms when the 
 gas is hghted, but the front lens does not get so much heat, as it 
 warms slowly, and is apt by " sweating " to spoil some of the first 
 pictures. But it may be removed only after the image is centred 
 and focussed, so that as soon as the lens is replaced the image is 
 found to be in statu quo antea. 
 
 During this afternoon visit to the scene of action the lecturer 
 should see his platform arranged ; his desk, chair, light, signal, 
 water-bottle, and glass. It is too late to do these things when the 
 audience begins to come in. The lanternist may see that his 
 slides are in order, but he must not leave them in the hall. Prac- 
 tical jokers are not extinct, and fools are numerous. 
 
CHAPTER XVIII. 
 
 MANAGEMENT OF LANTERN DURING LECTURE. 
 
 Having arranged all the apparatus so as to get a satisfactory 
 light, we proceed to centre the disc on the screen to " register " 
 the discs if we propose to dissolve, and to focus the light and the 
 projection lens. These operations are all to be done by experi- 
 ment and cannot usually be performed in any stated order, they 
 depend much on each other. Perhaps the easiest plan is to focus 
 the light first. We begin by projecting some kind of disc on the 
 screen, a blank slide being in the carrier. (The carrier itself is 
 usually centred by removing the projection lens, and looking down 
 the nozzle till by eye we get the aperture in the carrier central with 
 the nozzle. A very good plan is to put a " stop" or make a mark 
 on the carrier when centred, so that we may be able at any future 
 time to centre it at once.) Probably the disc at first has no 
 particular shape and is unevenly lighted ; by pushing and pulling, 
 and moving to one side and the other, and by raising and lowering 
 the jet we finally get a brilliant round disc evenly lighted, and 
 with sharp colourless edges, and if the lantern is properly placed, 
 the disc is in the centre of the screen. As already said, if the 
 disc-edge is not equally sharp all round the lantern is not " square 
 on " to the screen ; if the bottom or top of the disc is unsharp 
 the cant or tilt is wrong; if one side is out of focus the angle 
 of screen to lantern is wrong. If any of the pictures are to be of 
 shape other than circular we must try a mask of that other shape. 
 We finally remove the blank slide, and put in the first siide of the 
 lecture ; this we very carefully focus, and we are now ready to 
 begin. 
 
 If the lecture is to last, say, eighty minutes two limes will 
 assuredly be required, three will be better if we have a chance 
 of changing more than once. A pair of " pliers " should be at 
 hand to remove the used lime, in fact a pair of plumber's 
 pliers is what no lanternist should be without. The hole 
 up the middle of the limes should be cleared out before the 
 
 I 2 
 
124 
 
 THE OPTICAL LANTERN. 
 
 lecture ; sometimes the lime is not put on its pin without trouble, 
 and anything savouring in the smallest degree of a hitch must be 
 carefully avoided. The lime must be turned at intervals, greater 
 or shorter, according to the force of the jet. A blow-through jet 
 requires its hme turned, say, every four minutes ; a mixing-jet 
 under heavy pressure works best when the lime is almost constantly 
 on the move. Clockwork has often been used to drive the lime 
 slowly round. Anyhow, a deep pit on the lime must never be 
 allowed. Of course, a soft hme pits more readily than a hard one. 
 
 It is a sign of mismanagement when an operator has to keep 
 altering his jet-taps; if the light first attained be of proper quality, 
 and the jet properly made, no tampering with taps should be 
 needed at all. It is just possible that a few minutes after the 
 lighting up a slight alteration may be required (for some reason 
 when this happens it is the oxygen that requires to be slightly 
 increased, as a rule), but in a general way we do not require to 
 alter anything until the pressure in the cylinders or bags falls 
 materially lower, which is always near the end of a lecture and 
 often does not occur at all. But if the light becomes too red or 
 too large in extent, we are forced to reduce the hydrogen, which 
 is, perhaps, preferable to increasing the oxygen. Unless the jet 
 is in some way clamped in its position, care is necessary to avoid 
 knocking the end of the jet or the rubber tubes, and so uncentring 
 the light. Moreover, the slides should all slip sweetly into the 
 carrier; we rather object to a carrier into which slides are dropped, 
 as the carrier is apt to be knocked out of centre unless it is 
 clamped as suggested at page 26. 
 
 The slides must all be in order and convenient to the hand 
 of the lanternist, and they should be distinctly marked so that 
 the lanternist may know in the semi-obscurity of the lantern- 
 vicinity how the slides go into the carrier without having to hold 
 them up between his eye and the screen, or near the back of the 
 lantern. In England, if there is any standard slide-mark at all, 
 it is this : The slide is laid down as the picture actually appears 
 in nature or is intended to appear on the screen, and two 
 marks are affixed, one to each top corner. These two marks go 
 into the carrier next the light and downivards. 
 
 Slides that have passed through the lantern should be kept 
 quite apart from those still to pass. 
 
CHAPTER XIX. 
 
 DEPORTMENT ON TEE PLATFORM. 
 
 The young lecturer is sure to be more or less nervous, but in 
 different lecturers the nervousness shows itself in different ways. 
 One is full of diffidence and tremors, another is assertive and 
 inclined to " swagger." If the diffident one has himself super- 
 intended all the preparations, knows that his apparatus is good, 
 and his gas plentiful, he may keep his mind quite at rest, he has 
 done his " level best," and nervousness now may spoil the whole. 
 If the assertive one will only realise that a forward manner is 
 offensive to his hearers, and certain to vitiate his success in their 
 estimation, he may, perhaps, subdue for the time his conceit. 
 
 There are some who think that glitter of brass on the lantern 
 and gold lace curtains over the screen will make up for any short- 
 comings of the slides or of the lecture, while there are others who 
 fancy that disregard of external appearances looks business-like 
 and savours of the veteran lecturer. Probably both these parties 
 are wrong, and excellence lies midway between the two extremes. 
 All apparatus should be in thorough working order, and 
 scrupulously clean, and the screen is all the better for having 
 the frame-poles concealed by a tasteful, but simple pair of 
 curtains. On the other hand, acres of bright brass do not make 
 a good lantern any more than a dress suit constitutes a good 
 lecturer. There is a very good story told of a lecturer whose 
 lantern was so resplendent with bright brass work that he found 
 the audience all sitting with their backs to the screen and their 
 faces to the lantern; they could not believe that such a very grand 
 instrument was not the intended object of their attention. 
 
 There should be a thorough understanding between the 
 lecturer and the lanternist, certain signals should be arranged for 
 communication between them in case of mistake. For example, 
 if by some mismanagement a slide comes in the wrong order or 
 reversed as to right and left, it is most awkward for the lecturer 
 to hold a dialogue with the lanternist. It has already been stated 
 that the neatest kind of signal consists of an electric communica- 
 
126 
 
 THE OPTICAL LANTERN. 
 
 tion between speaker and lanternist, the bell at the lantern being 
 mufifled. The effect is exceedingl}^ good when the lantern can 
 be hidden from the audience altogether, as in a little room at the 
 back of the hall, the front of the projection lens coming close to 
 a hole made for it, and the lanternist having a small window 
 through which he can view the screen and platform. The writer 
 once lectured under the following pleasant conditions : — A screen 
 30 feet square, and quite opaque, a disc about 28 feet diameter, 
 the screen draped with red curtains at each side. The radiant, 
 an electric arc of 12,000 candles nominal power, worked by an 
 engine in another street. The lantern entirely hidden from view, 
 and electric communication with the lantern room. The pictures 
 came on the screen as if by incantation, for the lecturer designedly 
 concealed his " push " from the audience. 
 
 The lecturer ought always to make a few remarks to the 
 audience before turning down the gas or other light illuminating 
 the hall, and while he is making these introductory remarks he 
 should carefully study the faces in various parts of the hall, in 
 order to learn whether all can hear him well. Ears turned to the 
 front, and gaping mouths, are sure signs that the speaker is not 
 heard, and he must alter his voice and enunciation accordingly. 
 Shouting is never a good way of making oneself heard, a loud 
 conversational tone is the utmost amount of force likely to be 
 useful. Slow deliberate enunciation is what is wanted. Every 
 syllable must be distinctly pronounced, and rapid utterance must 
 never be practised. The nervous lecturer is almost certain to 
 speak far loo quickly, and consequently is very apt to stammer 
 and get mixed in his ideas, so the more nervous we are the more 
 carefully must we study slow, precise speech. 
 
 In a popular lecture jokes are valuable, in fact nearly 
 necessary. But the jokes must not be too stale, a jest familiar 
 to everybody is worse than useless. We must try to suit our 
 jokes to our audience, jokes that convulse a Scotch audience are 
 lost in England on the very same class of people, probably a 
 successful "quip" for a California lecture would fall flat in 
 Boston. We must not be vexed — or at least we must not show 
 vexation — if our jokes miss tire, we must try again with a different 
 projectile. The writer places considerable importance on a good 
 stock of jests for popular lectures ; if we cannot compose a sally 
 of wit ourselves, we may find what suits the purpose in books, 
 
DEPORTMENT ON THE PLATFORM. 
 
 127 
 
 and, as we said, provided the jokes are not really pre-Adamite, they 
 are sure to tell, and add to the success of the lecture. 
 
 Sometimes a foohsh audience is apt to become unruly when 
 the gas is turned down. This is almost always the fault of the 
 lecture or the lecturer. If the lecture is uninteresting, boys are 
 sure to lose patience, and indeed who does not sympathise with 
 them ? It is a fearful trial of patience to sit in the dark and see 
 poor slides and hear dismal inanities uttered. If such a case 
 should occur, and if any serious interruption to the lecture took 
 place we should simply cause the gas to be turned up, and 
 decline to proceed unless order should be maintained. But the 
 worst thing a lecturer can do in such cases is to lose his temper. 
 If he keeps calm, and even benignant, it will be a very low 
 audience that will not yield to his good nature. 
 
 In lantern lectures extending beyond forty minutes, there 
 should always be an interval, during which the hall is illuminated 
 with the usual lights. This affords a rest to all concerned, and 
 allows the lanternist to change his lime and see that all his 
 apparatus is in order. But the interval must not be long, five 
 minutes is perhaps the longest that will be safe. If the audience 
 is put into bad humour by being kept waiting the danger of a 
 disturbance is increased tenfold. 
 
 Unless the lecturer is very glib of speech, and has his subject 
 at his finger ends, the lecture ought to be written or printed. An 
 extempore lecture, when good, is a grand success, but when poor 
 is apt to be a dismal failure, and the lecturer is set down as a 
 conceited fool for attemping it. 
 
 To those' unaccustomed to prolonged stretches of public 
 speaking, especially when nervousness is felt, a lecture of, say, 
 eighty minutes, is a severe tax on the throat- Drinking large 
 quantities of water only makes matters worse sometimes, a tiny 
 tablet or pellet of chlorate of potash will be found as good as 
 anything to take. Borax is sometimes added to the potash, but 
 the advantage is very doubtful. Not much liquid of any kind 
 should be taken before lecturing, but hot drinks, as tea and coffee, 
 are the worst of all things. 
 
 No one but a very " old hand " can sing and speak m one 
 
 lecture. 
 
 Ninety minutes is the longest time advisable for the duration 
 of a lantern lecture. 
 
CHAPTER XX. 
 
 ARRANGEMENTS FOR THE LANTERN IN A 
 LECTURE ROOM. 
 
 As one of the chief aims of this book is to simpHfy and increase 
 the use of the optical lantern in lecture-rooms forming parts of 
 universities, colleges, schools, and other educational and recreative 
 institutions, we may do well to suggest a few of the steps that 
 may be taken to render the lantern a permanent instalment of the 
 place. In such a case it is indubitably the best plan to have an 
 opaque screen, and the best screen is, as previously stated, a 
 plastered wall, white and matt in surface. Almost equally good 
 in all respects, most convenient in many, is a strong opaque 
 faced" screen, such as was described in a previous chapter. 
 This may very well roll up like a map, to be let down at the 
 teacher's will. If it is convenient to set up the lantern straight 
 opposite and at right angles to the screen, the latter may be 
 allowed to hang down naturally ; if the lantern must go below the 
 centre of the screen, the roller may be brought a few inches out 
 from the wall at top, and the roller at foot of the screen may be 
 placed and held close against the wall. If the lantern goes more 
 conveniently on a higher level than the centre of the screen, as 
 will be the case in many lecture-rooms of the amphitheatre style, 
 then the roller at top may be as close to the wall as possible, while 
 the roller at foot of the screen may be pulled out and held a few 
 inches from the wainscot or lower part of the wall. 
 
 In nearly all cases, such as we are treating now, a ten feet disc 
 will be ample, and a blow-through jet sufficiently powerful; indeed, 
 a good acetylene jet will answer for such a disc. On the score of con- 
 venience the lime light will probably be found preferable, for the 
 requisite illumination can be got up more rapidly than with oil, 
 and the effect, especially with the ordinary run of slides and with 
 objects projected through a lantern microscope or a polariser, or a 
 prism, will certainly be better with lime than with oil or acetylene. 
 We recommend a good-sized cylinder of oxygen, say forty cubic feet, 
 
THE LANTERN IN A LECTURE-ROOM. 
 
 129 
 
 or else a metal tank of the gas, but not a bag if the cylinder or 
 tank can be obtained. Of course, the tank would not be large 
 enough to hold more than from six to ten cubic feet. Bags for such 
 a purpose would be wasteful even if they were not tampered with by 
 the students. While thus recommending the blow-through jet, we 
 must say that we have almost entirely given up its use, preferring 
 from every point of consideration the mixing jet with two cylinders 
 and two regulators, or a gas generator and saturator, such as 
 described elsewhere. For a lecture-room we should suggest one of 
 the open-stage lanterns, such as Figs. 51 to 55 ; this kind of lantern 
 is useful for every condition likely to arise. A part of the outfit 
 ought to be the arrangement for showing opaque objects. For 
 special illustrations such as spectrum analysis special accessories 
 must, of course, be added. 
 
 The matter that seems to give teachers the greatest amount of 
 perplexity is that of darkening the lecture-room during the day- 
 time, and even when the gas is lighted. The gas is easily arranged 
 by putting within reach of the teacher, the lanternist, or an atten- 
 dant, a bye pass tap, whereby the gas of the room can be lowered 
 "to the blue" without being extinguished. If the electric light 
 be used for the room it can easily be switched off, and here it may 
 be said that if electricity be used for the illumination of the room 
 or building, we should certainly utilise it for the optical lantern, 
 even were we restricted to an incandescent lamp. The electric 
 light has its disadvantages for the lantern, but its performance 
 is so good when in good order, and its convenience so great at 
 all times that, when available, it should certainly be utilised for 
 the purpose. 
 
 For blocking out daylight the simplest and best contrivance is 
 a shutter on the " Louvre " principle. There is a shutter on 
 the market known as Clarke's patent, and it answers the pur- 
 pose admirably. It is made of slips of wood joined by strips of 
 strong cloth, and simply folds up into a coil which occupies, 
 when the shutter is open, a receptacle at top or bottom of the 
 window. This appliance, when well fitted, completely blocks out 
 light ; it is opened and shut in a few seconds. But even this is 
 not necessary to success, for a blind made of opaque stuff, fitted 
 in runners, for instance, close to the sides of the window, will shut 
 out light sufficiently for our purpose. Absolute darkness is not 
 ■essential to even a good image on the screen, while a powerful 
 
130 
 
 THE OPTICAL LANTERN. 
 
 lantern will project a useful picture even when the room is only in 
 semi-obscurity. We have seen a very successful series of lantern 
 illustrations in a room where every person present was visible to 
 the lecturer, though only dimly. But it must be understood that 
 where pictures, and not mere insttuction, are the object, the room 
 should be as dark as possible ; we only say that instruction of a 
 class is possible in a dimly-lighted room. And if the lantern 
 slides be dense, or the microscopic objects thick or heavily 
 stained, then the room must be really dark. 
 
 Throughout this book the writer has had in his mind the 
 instructor rather than the entertainer. The optical lantern, as a 
 means of imparting instruction to classes, is only beginning to 
 occupy the place it ought to fill on its merits. In the interest 
 of teacher and student alike, we venture to hope that the optical 
 lantern will soon take the place it deserves. 
 
CHAPTER XXI. 
 
 ENLARGING WITH THE OPTICAL LANTERN. 
 
 As those who possess an optical lantern frequently wish to use it 
 for enlarging from photographic negatives or positives, it may not 
 be out of place to offer a few remarks on the subject. 
 
 If the original negative be of lantern-slide size, or if the part 
 we wish to enlarge covers an area of not more than three and a- 
 quarter inches square, we shall not require any alteration of our 
 lantern with four-inch condenser provided the front lens racks out 
 far enough, and there is sufficient space in the carrier opening to 
 admit the larger negative. But if we propose to enlarge from entire 
 quarter-plate negatives, we shall require a larger condenser, and 
 so also for every increase of size. To find the size of condenser 
 necessary to illuminate properly any size of plate which we may 
 wish to enlarge, a simple rule is to take the diagonal of the plate; 
 the diameter of the condenser must be a little more than the dia- 
 gonal of the plate. Thus, to enlarge a "half-plate," x 4^ 
 inches (English size), a condenser of diameter not less than 8^^ 
 inches should be used. 
 
 We are less tied down in the choice of a projection lens, as 
 the focal work is practically immaterial for any such work as we 
 are likely to attempt. But lenses sold for the lantern only, and 
 not having their visual and actinic foci coincident, will not answer 
 for enlarging, or will not answer the purpose nearly so well as a 
 lens corrected for photography. But any photographic lens will 
 be available, if it give rectilinear images, and if we can rack the 
 lantern front sufficiently to give the lens play. 
 
 Manufacturers produce lanterns having the front so made 
 as to stretch outwards from the stage to a considerable 
 extent. Sometimes this motion is attained by bellows, sometimes 
 by telescope joints — one probably is as good as the other. The 
 reason for this extension of front Ues in the fact that in order to 
 get a focussed image twice the size of the original, the optical 
 centre of the lens must be one and a-half times the focal length 
 
132 
 
 THE OPTICAL LANTERN. 
 
 of the lens distant from the negative to be enlarged. Thus, with 
 a six-inch lens to get an enlargement twice the diameter of the 
 original, the optical centre of the lens — usually very near the stop 
 — must be nine inches distant from the negative. If a copy of 
 size equal to that of the original be required, the front must rack 
 out so that the centre of the lens can be placed at twice the focal 
 ■distance from the object being copied. At the end of this chapter 
 we give a table extracted from the almanac of the British Journal 
 of Photography. This table shows at a glance the distances from 
 ■centre of lens to object and to receiving surface for various degrees 
 •of enlargement or reduction. 
 
 Y\o. 64. — Patent Enlarging Lantern. 
 
 Although, with the limitations mentioned, the ordinary pro- 
 jection lantern may be utilised for enlarging purposes, it is only 
 with instruments specially constructed that complete efificiency, 
 and particularly comfort, are experienced. There are many such 
 special instruments on the market from which to choose, but one 
 of the most serviceable in every respect with which we are 
 acquainted is the Patent Enlarging Lantern issued by Messrs. 
 Ross, Limited. This, with the exception of extending front and 
 wood base is constructed entirely of metal, so that there is 
 
ENLARGING WITH THE OPTICAL LANTERN. I 3 3. 
 
 nothing in the working parts liable to suffer from the effects of 
 heat and lead to trouble in working. A feature in this instru- 
 ment is the rising body, shown in the illustration raised, for the 
 purpose of lighting; otherwise every adjustment even to filUng 
 the lamp can be performed from the outside. When closed, the 
 lantern is absolutely light tight. The space in front of the con- 
 denser for the insertion of the negative is open on three sides, 
 which renders it possible to work from portions of plates con- 
 siderably larger than the lantern is constructed to take— a matter 
 of some convenience, for instance, in enlarging a single figure 
 from a group; while by the use of a suitable carrier it may be 
 employed for projecting ordinary slides. The extending front, 
 which is on the bellows principle, slides smoothly but firmly on 
 telescopic tubes and provides ample range for any desirable focal 
 length of objective. The working of this portion is sufficiently 
 easy to admit of accurate focussing in case the lens is not fitted 
 with rack and pinion. The lantern here shown is provided with 
 an oil lamp ; but it can also be supplied, we believe, for use with the 
 limelight or electric arc lamp. Not the least important feature of 
 this lantern is its extreme compactness, 
 
 For those who find it more convenient to use ordinary gas, 
 Messrs. Butcher, of Blackheath, manufacture a special enlarging 
 lantern fitted with the Welsbach incandescent burner, which is a 
 distinctly useful instrument. Others of the comaiercial articles 
 can, with little trouble, be adapted for use with acetylene. 
 
 The sensitive material on which we project and finally develop 
 the enlarged image is, as a rule, the paper known to photographers 
 as bromide paper, but it may be a slow gelatine, a wet or a dry 
 collodion plate. The purely photographic part of the operations 
 is out of place here ; we may refer the reader to the " Processes 
 of Pure Photography," * or any other book of the same series 
 treating of photographic operations. 
 
 The substances used for the photography of enlarged images 
 are usually very sensitive to actinic light, and bromide paper, for 
 instance, will be damaged if any stray light from the lantern or 
 from the illumination of the apartment reaches it, unless such 
 stray hght be of a yellow or red colour. If there is much 
 light about the apartment, however non-actinic it may be, the 
 
 * " Processes of Pure Photography," by Burton and Pringle. 
 
134 
 
 THE OPTICAL LANTERN. 
 
 difficulty of focussing the image will be found great. The best 
 plan is to enclose the entire lantern in a box, letting the lens-front 
 alone protrude, and allowing the necessary draught to reach the 
 light either by " trapped " apertures, or by some arrangement of 
 thick cloth. Reflections, it must be remembered, must be kept 
 from the sensitive material as sedulously as direct stray light. 
 
 The paper, or other sensitive material, is to be placed in front 
 of the projection lens, and the receiving surface must be perpen- 
 dicular to the optical axis of the condensing and projection 
 
 Fig. 65.— Enlarginc. Easel. 
 
 systems. If this is not attended to some part of the enlarged 
 image will surely be blurred. Paper may be tacked with drawing 
 pins to a board or a wall, or it may be held on an easel in front 
 of the projection lens. The easel may be free on the floor, but 
 it is better when convenient to have it either running on a track 
 or sliding in slots or grooves along the optical axis of the system. 
 Fig. 65 shows a very useful easel made to hold a long roll of 
 bromide paper or cut sheets, as may be desired. 
 
ENLARGING WITH THE OTITCAL LANTERN. 135 
 
 The accurate focussing of the projected image may be adjusted 
 either by racking the projecting lens in and out, or by pushing 
 the easel backwards and forwards. The former is perhaps easier; 
 the latter always preferable. If accuracy in amount of enlarge- 
 ment is not of , vital importance, the former method may be 
 adopted. We may project the image and focus it either on a white 
 surface, as a sheet of paper, afterwards replacing the white paper 
 by the sensitive surface ; or we may even pin the bromide paper 
 over the plain paper on which we focussed ; again, we may place on 
 the lens a cap of non-actinic glass and focus directly on the sensi- 
 tive surface, removing the non-actinic cap when the focus is 
 adjusted. The question of exposure comes under the head of 
 photography, so we shall not here discuss it at length. The 
 actinic force of the radiant, the intensity ratio of the lens, the 
 density of the object to be enlarged, and the sensitiveness of the 
 receiving surface being all taken as fixed terms, the exposure varies 
 directly with the areas of enlargement and inversely with the 
 utilised area of the condenser. 
 
136 
 
 THE OPTICAL LANl'ERN. 
 
 TABLE FOR ENLARGEMENTS. 
 Copied from the ^''British Journal Almanac" 
 
 Focus of Lens. 
 
 Times of Enlargement and Reduction. 
 
 Inches. 
 
 I 
 
 In. 
 
 2 
 
 In. 
 
 3 
 
 In. 
 
 4 
 
 In. 
 
 5 
 
 In. 
 
 6 
 In. 
 
 7 
 In. 
 
 8 
 
 In. 
 
 2 
 
 4 
 4 
 
 6 
 3 
 
 8^ 
 
 -4 
 
 10 
 2I 
 
 12 
 
 2* 
 
 14 
 
 2* 
 
 16 
 
 2f 
 
 18 
 
 2\ 
 
 5 
 5 
 
 7* 
 3t 
 
 lO 
 
 il 
 J.) 
 
 12.3. 
 
 ~^5 
 3 
 
 17* 
 
 20 
 
 22f^ 
 
 3 
 
 6 
 
 6 
 
 9 
 
 4-J 
 
 12 
 
 4 
 
 15 
 
 3? 
 
 18 
 
 J 5 
 
 21 
 
 _3,i_ 
 
 24 
 
 3f 
 
 27 
 
 3f 
 
 I ,' 
 
 0/2 
 
 7 
 7 
 
 IO.\ 
 
 51 
 
 14 
 4§ 
 
 17* 
 4^ 
 
 21 
 
 4i 
 
 24* 
 
 __4:?tL 
 
 28 
 4 
 
 31* 
 
 _3ii. 
 
 4 
 
 o 
 
 6 
 8 
 
 1 2 
 
 6 
 
 ID 
 
 ■ 5i 
 
 20 
 5 
 
 24 
 4f 
 
 28 
 4§ 
 
 32 
 4f 
 
 36 
 4* 
 
 4* 
 
 9 
 9 
 
 I3i 
 
 6i 
 
 i8 
 6 
 
 2 2— 
 
 5I 
 
 27 
 
 5? 
 
 31* 
 
 56 
 5i 
 
 40* 
 5?8 
 
 5 
 
 lO 
 lO 
 
 15 
 
 20 
 
 6S 
 
 25 
 
 30 
 6 
 
 35 
 
 5-f 
 
 40 
 5f 
 
 45 
 5l 
 
 5J 
 
 1 1 
 1 1 
 
 \a 
 
 
 "61 
 
 33 
 61 
 
 38* 
 
 44 
 6-f 
 
 49* 
 6^ 
 
 6 
 
 12 
 12 
 
 i8 
 9 
 
 24 
 
 8 
 
 30 
 7| 
 
 36 
 
 42 
 
 ^ 
 
 48 
 6f 
 
 54 
 
 6f 
 
 7 
 
 14 
 14 
 
 21 
 
 lOjr 
 
 28 
 
 9j 
 
 35 
 8| 
 
 42 
 
 8f 
 
 49 
 8^ 
 
 S6 
 8 
 
 63 
 
 7J . 
 
 8 
 
 i6 
 i6 
 
 24 
 12 
 
 32 
 loS 
 
 40 
 10 
 
 48 
 9l 
 
 56 
 9j 
 
 64 
 9f 
 
 72 
 9 
 
 9 
 
 i8 
 i8 
 
 27 
 
 13^ 
 
 36 
 12 
 
 45 
 III 
 
 54 
 I of 
 
 63 
 loi 
 
 72 
 
 81 
 
 It is assumed that the photographer knows exactly what the focus of his lens 
 is, and that he is able to measure accurately from its optical centre. The use of 
 the table will be seen from the following illustration :— A photographer has a 
 carte to enlarge to four times its size, and the lens he intends employing is one 
 of six inches equivalent focus, lie must, therefore, look for 4 on the upper 
 horizontal line, and for 6 in the first vertical column, and carry his eye to where 
 those two join, which will be at 30 — 7j^. The greater of these is the distance 
 the sensitive plate must be from the centre of the lens and the lesser, the distance 
 of the picture to be copied. To reduce a picture any given number of times the 
 same method must be followed, but in this case the greater number will repre- 
 sent the distance between the lens and the picture to be copied ; the latter, that 
 between the lens and the sensitive plate. This explanation will be sufficient for 
 every case of enlargement or reduction. 
 
 If the focus of the lens be twelve inches, as this number is not in the col- 
 umn of focal lengths, look out for 6 in this column and multiply by 2 ; and so 
 on with any other numbers. 
 
THE OPTICAL LANTERN. 
 
 ^37 
 
 Memoranda for a Lantern Display. 
 {See Chapter XVI J.) 
 
 Lantern : Condenser, projection lenses, support. 
 
 Jet : Limes in box, tubing, needle for nipple (dissolver and 
 tubing). 
 
 Lamp : (Wicks trimmed, oil with camphor, scissors). 
 
 Pressure-boards : Weights. 
 
 Bags : Bag-taps (back-pressure valves). 
 
 Cylinders : Regulators, key, spanner. 
 
 Screen : Screen-frame, cord, tape (curtains), a few staples or brass 
 hooks with screw. 
 
 Reading-lamp : Signal, manuscript or book, list of slides in order 
 —for lecturer. 
 
 Slides in box : List of slides in order — for lanternist. 
 
 Miscellaneous : Gas pliers, hammer and nails, screw driver and 
 screws, gimlet and bradawl, instrument to bore out lime-hole, 
 file, extra tubing, matches, tape measure. Lubricate taps of 
 jet and bags (see that gas way of jet is clear). 
 
 For Oxygen Making : Retort, retort neck, tubing, oxygen mix- 
 ture, heating stove and tube if for gas, purifying bottles — 
 one or two — metal tubing for same, rubber tubing for same, 
 alkali. 
 
 Squeeze all air out of bag, lubricate tap, lock for bag, bag 
 marked O. 
 
 For Hydrogen : Generating bottle, scrap zinc, acid, purifier, metal 
 tubing, rubber tubing, or large tubing and metal cone or 
 "adapter" to join gas supply from main of building to the 
 H bag, gas pliers and wrench. 
 
 Squeeze air out of bag, bag marked H. 
 
 Note: Never put grease or oil on metal fittings of cylinders, nor 
 on regulators nor gauges. 
 
 K 
 
Appendix. 
 
 NE W OX YGEN GENERA TORS. 
 
 At various times attempts, more or less successful, have been 
 made to devise apparatus that will automatically produce oxygen 
 {luring the course of a display. There is no doubt that great as are 
 the advantages of storing gas under pressure in metal cylinders as 
 compared with the bag system, still there are very serious drawbacks 
 in the use of cyHnders. The weight of a moderate size cyUnder, 
 the dangers connected with very high pressures, the difficulty of 
 making absolutely gas-tight couplings under these pressures, the 
 expense of transport and the railway regulations attached thereto, 
 the doubt as to the quantity of gas in a cylinder unless a gauge be 
 used, the inevitable waste of gas when a cylinder is not quite 
 emptied : these and other imperfections are inseparable from this 
 system. It has long been felt that an automatic system of producing 
 at least the one gas, oxygen, in the quantity and at the time required, 
 would be a great boon to many, and we have lately seen apparatus 
 which appears to fulfil the desideratum admirably, and by a further 
 device connected with the lantern itself both gases are automati- 
 cally produced during the working of the lantern, and in quantity 
 commensurate with the requirements of the worker. Many years 
 ago, in 1867 and 1868, the late Mr. Michael Noton, of Man- 
 chester, used plugs, or blocks, of " oxygen mixture " in small tube 
 retorts, which were brought successively over a suitable Bunsen 
 burner, and the gas when given off could either be collected in 
 bags or used as manufactured. At a soiree of the Manchester 
 Photographic Society, in January or February of the latter year, a 
 lantern display was given, the gas for which was supplied as 
 required by Mr. Noton's apparatus, a small tank, or receiver, being 
 all that was necessary between the generator and the lantern. 
 Mr. W. I. Chad wick, of the same city, used blocks or cakes of the 
 same mixture in a specially constructed retort, and collected the 
 gas in a bag, which acted as a reservoir. Apparatus for the same 
 purpose was also patented in 1877 by Mr. Birrell and as well as 
 
APPENDIX. 1 39 
 
 by Mr. D. Young, and to a certain extent the generators to be de- 
 scribed are simply a further development of these earlier devices, 
 but probably improved in details. The first apparatus we shall 
 
 Fig. 66.— The "Star" Patent Oxygen Generator. 
 
 describe is known as the "Star" patent, and it is with advantage 
 used in connection with a lantern, described on pages 142-143. 
 
 B (Fig. 66) is a wooden box about 15 inches square by 9 inches 
 high, and contains a collapsible gas-bag, not seen in the figure, 
 
 K 2 
 
I40 
 
 THE OPTICAL LAN7ERN. 
 
 surmounted by a hollow platform P, which rises regulated by the 
 guides g, g, as the bag below is filled with gas. This part P is 
 filled with water at time of use, and thus furnishes pressure on the 
 gas-bag. If the platform P should rise too high from over-pro- 
 duction of gas the valve v impinges on the abutment a, and the 
 surplus gas is released. There are 5 or 6 short retorts, r, r, r, r, 
 open at their proximal ends to the hollow upright and closed 
 when in action at their distal ends by screw-plugs ; /is an ordinary 
 spirit lamp. These retorts can revolve on p, so that when the 
 "cartridge in one retort is exhausted, the whole " star" of retorts 
 is turned, after the manner of a capstan, till a fresh retort is 
 brought over the spirit lamp. When the platform descends below 
 a certain point, indicating that the cartridge in use is exhausted, a 
 signal is given to notify the fact. The retort-changing might be 
 made automatic, but as each cartridge lasts about half an hour there 
 is no necessity for such a complication. 
 
 The cartridges are of course moulded and compressed " oxygen 
 mixture," and they are made hollow and cylindrical, fitting loosely 
 into the retorts. They can be sent by post, and as is well known 
 oxygen produced from potass, chlorate has a peculiarly fine light- 
 giving quality in lime-light operations. 
 
 The oxygen begins to come off within five minutes of the heat 
 being applied, it passes from the retort down "the hollow pillar/ 
 into the water tank, thence to the lantern or saturator by the tube 
 t while the surplus goes into the bag at bottom of B. 
 
 The approximate weight of this entire apparatus — without the 
 water — is 15 lb., so that it can be easily carried in one hand, 
 and we have seen an example wherein a suitable lantern could be 
 packed in the box B, above the rest of the apparatus. 
 
 The second generator is manufactured by Messrs. Riley 
 Brothers, of Bradford, and has been christened the " Rilford." 
 In this a single retort of cylindrical form is employed, and this 
 is packed with cartridges of oxygen mixture at regular intervals 
 with air-spaces between them. A lamp travelling upon guides 
 beneath the retort is, by the rise and fall of the gas-holder, brought 
 automatically under each cartridge in succession until the whole 
 are exhausted, the manufacture of gas proceeding concurrently 
 with the display of pictures. The charging of the retort will 
 supply enough gas for a display lasting an hour and a-half if the 
 Lawson saturator be used, or a little less if a good blow-pipe jet is 
 
APPENDIX. 
 
 employed. The whole apparatus, which is shown erected for use, 
 packs into two cases — measuring 34 inches x 5 inches x 5 inches 
 and 14 inches x 14 inches x 16 inches respectively, and weigh- 
 ing, when packed, 46 lb. The diagram (Fig. 67) will serve to 
 explain the working arrangements, A and N representing the two 
 packing-cases, which serve to form a working stand when in use. 
 B is a cylindrical steel retort with a clamped cap at one end, in- 
 side which is the tube, made to hold eight cartridges, which form 
 a charge. The inner tube having been charged and placed in the 
 
 Fk;. 67. — The "Rilford" Generator. 
 
 retort, the cap is firmly closed and the retort placed in position on 
 the standard, as shown ; the lamp is lighted and pushed to the 
 extreme left of the carriage guides, and in about eight minutes the 
 gas will be given off and the container' begin to rise. When the 
 platform E rises sufficiently high it comes in contact with the 
 jointed lever D, the right-hand end of which is fixed, the other 
 free to slide, as it must, in the opposite direction, and this operates 
 the automatic arrangement by which the lamp is moved. As the 
 gas is used and the container sinks, the lever resumes its original 
 position, and the free end, engaging a paw! on the lamp carriage, 
 
142 
 
 777^ OPTICAL LANTERN. 
 
 moves it a certain distance to the right until it comes under the 
 second cartridge, and so on until the whole are exhausted. If the 
 gas is not used very rapidly, an escape-valve is brought into action 
 to relieve the pressure. H is a washing chamber containing water, 
 in which common washing soda has been dissolved, and the gas 
 passes through this before it can reach the open part of the con- 
 tainer. The weight of the washing-tank suffices to work the appa- 
 ratus, but a better light is obtained if an additional weight of about 
 28 lb. is placed at the bottom of the container. For long displays 
 a second retort is provided, which may be substituted for the 
 empty one while the gas is burning. We find on trial that this 
 apparatus works well. 
 
 NEW LANTERNS. 
 
 The lantern shown in Fig. 68 involves some new principles of 
 construction which coaimend themselves to us. It is the inven- 
 tion of Mr. A. Sweetser, who calls it the " Auto-hydrogen," and 
 we have been privileged to see it at work several times in con- 
 junction with the "Star" oxygen generator already described. 
 The lantern is extremely handy to work, and the light seemed to 
 us specially fine in quality. 
 
 The base ^ is a saturator of simple and efficient type, packed 
 with any suitable absorbent material damped with a light hydro- 
 carbon, such as gasoline or ether. The capacity of this saturator 
 is such that it will supply a strong light for several hours. The 
 saturator may or may not be used; in the latter case it simply acts 
 as a base to the lantern. The entire lantern, with the whole of 
 the optical system, is pivoted on two pillars, one seen at s, so that 
 when the lantern is tilted the whole of the optical system is tilted 
 with it, and each part of that system retains its parallelism with 
 the other parts. The advantage of this is specially obvious when, 
 for instance, we have to tilt the bodies of a biunial or triunial in 
 order to obtain " register " of the discs on the screen. These 
 pillars, moreover, are hollow, and through them are con- 
 ducted the gases to the jet by means of the rubber tubes shown, 
 and if a double lantern is to be used, the pillars of the upper one 
 
APPENDIX, 
 
 are simply screwed on to those of the lower, and all complicated 
 arrangements of rubber tubes are avoided. 
 
 Another constructive novelty is that the front lens 0 can be 
 adjusted along with, or independently of, the condenser from the 
 back of the lantern, by means of the telescopic tubes a and the 
 milled-headed screws seen below / The stretch of the front from 
 condenser c to front lens 0 is practically unlimited, and the " draws " 
 are so constructed that "sagging" is wholly obviated. 
 
 Fig. 68. — Sweetser's Auto-hydrogen Lantern. 
 
 The usual "body" of the lantern is replaced by a hood,/, 
 made of fireclay or other refractory material, and perforated for 
 the dispersal of heat. This hood acts admirably in protecting the 
 lime from draughts of cold air, and by keeping the lime hot it 
 prevents cracking and probably increases its light-giving property. 
 A lime used in this lantern with gases from the generator and 
 saturator shows very slight " pitting," even after prolonged use, 
 and the condenser seems to be in no danger from deflection of 
 
144 
 
 THE OPTICAL LANTERN. 
 
 the flame from the Hme. Any rays of light escaping from the 
 fireclay hood are shut off from the audience by simple metal or 
 other shades. 
 
 In details of construction the designer claims several advantages 
 of simplicity and economy, but these matters are hardly suitable 
 for elaboration here. The instrument is not yet on the market, 
 so far as we know ; but as all the metal parts can be stamped, or 
 produced by ordinary press tools, the cost of the lantern will pro- 
 bably be small. 
 
 As evidence of the extreme activity amongst opticians at the 
 present in the perfection of lanterns for the highest class of scien- 
 tific and educational demonstration, we need only draw attention 
 to the latest efforts of Messrs. Ross, Limited, in that direction, the 
 various instruments to be described having been put on the market 
 since the earlier chapters of the work were in the press. 
 
 Figs. 69 and 70 represent two forms of their new Universal 
 Combination Lantern, designed for the combined purposes of 
 ordinary projection, science work, and enlarging. They are, in 
 fact, intended for the same classes of work as the more expensive 
 Science Lantern described in Chapter XIV. 
 
 Fig 69. — Ross' New UNiviiRSAi. Comijination Lantern (Arc Light). 
 
 Fig. 6g shows the lantern as arranged for use with the New 
 Model arc light described in Chapter XL, and for vertical projec- 
 tion. It will be noticed that it is fitted with a body of entirely 
 new design, adapted to withstand the intense heat of the most 
 powerful illuminants. This body is constructed of stout blackened 
 brass plates, surrounded by an outer casing of aluminium, by 
 which a most perfect system of ventilation is obtained, and the 
 
APPENDIX. 
 
 145 
 
 heating of the lantern reduced to a minimum. The vertical front 
 can be turned down in an instant when the instrument is required 
 for ordinary projection, or removed altogether and replaced by a 
 bellows extension front when wanted for enlarging purposes ; in 
 fact, its uses are universal. 
 
 Fig. 70 shows it as fitted for limelight, and with the bellows 
 extension in use. In this form it is built with the patent lime- 
 light body described in a former chapter. This entirely encloses 
 
 Fig. 70 — Ross' New Universal Combination Lantern (Limelight). 
 
 the mechanism of the jet, with the exception of the taps and 
 lime burner, and is provided with sight holes for examining 
 the light, but has no doors. The entire body can be raised on 
 sliding guides, as shown in the patent enlarging lantern figured in 
 Chapter XXI. As in the alternative form, the front here shown 
 can in a few seconds be removed and replaced by a vertical or 
 open front, and applied to all the purposes required in a science 
 lantern of the highest class with as much ease and comfort as to 
 ordinary projection and enlarging. 
 
 Fig. 71. ---Ross' New Limelight Lantern (No. i Model). 
 
 In the newest limelight lantern of the same firm, known as 
 Ross' New Model Limelight Lantern (Fig. 71) the very acme of 
 
146 
 
 THE OPTICAL LANTERN. 
 
 compactness would seem to have been reached, and a new depar- 
 ture made in lantern bodies. In this it will be seen that only the 
 light itself- — that is the jet-tip and the lime — is enclosed in the 
 metal body, and by this arrangement while the illumination is per- 
 fectly confined within the lantern, the small surface of heated 
 metal leads to far less heating of the rest of the apparatus than 
 with other forms of body. The extreme compactness and porta- 
 bility, combined with its moderate price, must recommend this 
 instrument to all who have to carry their apparatus from place to 
 place. 
 
 THE RADIANT LIMELIGHT JET 
 
 Messrs. Ross have also introduced a new lime-jet of very high 
 efficiency, to which the above title has been given, and for which 
 a greater intensity of light from a smaller area of lime is claimed 
 with a decreased consumption of gas. The principal feature in 
 this jet is found in the mixing chamber, which is of peculiar shape,, 
 and seems well calculated to bear out the claims made for it 
 
 Y\<c,, 72. — Ross " Radiant" Limelight Jet. 
 
 In addition, it is fitted with every motion possible, and exhibits 
 unusual solidity in its construction combined with elegance of 
 form ; and on actual trial we find it a very efficient jet, with a 
 facility and " sweetness " of vertical and horizontal moveraents- 
 not surpassed, if equalled, in any jet we have seen. 
 
INDEX. 
 
 PAGE 
 
 Abingdon Acetylene Generator . . 41 
 Acetylene and County Council . . 39 
 
 Acetylene Gas 37 — 43 
 
 Acetylene Generator, "Abingdon" 41 
 
 Acetylene Jets 42, 43 
 
 Animated Photography, Chapter XV. 97 
 
 Arc- Light, The 73— 81 
 
 Auto- hydrogen Lantern, Sweetser's 142 
 
 Back- pressure Valves 47 
 
 Bags for Gases 64 — 66 
 
 Baker's I^antern 26 
 
 Barium Oxides for Oxygen ... 68 
 Beard's Eclipse Carrier . . . . 8S 
 
 Beard's Regulator 69, 70 
 
 Best Light, How to Get .... 49 
 
 Bioscope, The 108 
 
 Birtac, Cinematograph, The . . . 109 
 
 Birt Acres loi 
 
 Biunial or Double Lanterns . . . 31 
 
 Blow-through Jets 45)46 
 
 Body, The Lantern, Chapter V. . . 24 
 
 Bore of Gas Tubes 49 
 
 Boxes for Slides 89 
 
 Brin's Blow-through Jet .... 46 
 
 Brin's Oxygen 68, 71 
 
 Broughton Tank, The 54 
 
 Butcher's Enlarging Lantern . . .133 
 
 Calibre of Tubes . 49 
 
 Carbons 78, 79 
 
 Carriers 87 
 
 Chadwick's Carrier 87, 88 
 
 Chad wick's Condenser 19 
 
 Chadwick's Table of Data . . .21 
 
 Chlorate of Potash 58, 59 
 
 Cinematographs, &c 97 
 
 Coal Gas for Lanterns .... 36, 37 
 
 Condenser, Area of 13) 15 
 
 Condenser, Baker's Nelson's ... 17 
 Condenser, Chadwick's Triple . . 19 
 Condenser, Construction of ... 16 
 Condenser, Focal Length 
 
 of II, 13) '5. 19 
 
 Condenser for General Work ... 18 
 Condenser, Functions of . . . 11, 13 
 
 PAGE 
 
 Condenser, Interchangeable . . . 19 
 Condenser, Nelson's . . . . . 17 
 
 Condenser, Position of 13 
 
 Condenser, Sweating 16 
 
 Condenser, The, Chapter IIL . . 13 
 
 Condensers, Angles of 18 
 
 Condensers, Care of 18 
 
 Condensers, Double . . . . 16, 18 
 
 Condensers, Glass for 18 
 
 Condensers, Triple 16, 17 
 
 Currents, Electric 73, 74 
 
 Cut off for Jets 51 
 
 Cylinders, Chapter X 68 — 71 
 
 Cylinder Pressures 69 — 71 
 
 Dallmeyer's Projection Lens ... 20 
 
 Danger with Gases 116 
 
 Davenport Electric Lamps ... 75 
 
 Dissolver, Sciopticon 30 
 
 Dissolving Lanterns 29 
 
 Dissolving Taps 32, 33 
 
 Drying Oxygen 62 
 
 Eastman Enlarging Easel . . . .134 
 Electric Light. Chapter XL . . 72 — 81 
 
 Elevators for Screens 84 
 
 Enlargements, Table of . . . .136 
 Enlarging Easel, Eastman's . . .134 
 Enlarging with Lantern, Chapter 
 
 XXL ....... 13T— 135 
 
 Essentials for Projection .... 10 
 
 Essentials for Sharpness on Screen 11 
 
 Ether Saturators 54 
 
 Experimental Lanterns, Chapter 
 
 XIV 90—96, 144 
 
 Focal Lengths of Condenser and 
 
 Lens II 
 
 Focal Length of Front Lens ... 21 
 
 Frames for Screens 84 — 86 
 
 Friese Green 100 
 
 Front Lens and Condenser ... 18 
 
 Front Lens, The 20 
 
 Gas, Acetylene 37 — 43 
 
 Gas Cylinders, Chapter X, , . 68 — 71 
 
148 
 
 THE OPTICAL LANTERN. 
 
 Gas-impact on Lime 
 Gas- pressure in Bags . 
 Gas Regulators . . 
 Gas Storage in Bags 
 Gas Storage in Water-tanl 
 Gas-Tubes, Bore of . 
 Gases, Preparation of . 
 Gasoline for Saturator . 
 General Optical System 
 Generators .... 
 Generators, Acetylene . 
 Generation of Gases, Chap 
 Gridiron Saturator . . 
 Gwyer's Jet .... 
 
 IX. 
 
 Hepworth Electric Lamp . . 
 Hepworth's Oxygen Mixture . 
 Hepworth's Patent for Carbons 
 Holden's Electric Lamp . . 
 Hughes' Miniature Lantern . 
 Hughes' Pictoroscope . 
 Hydrogen Making .... 
 
 . 66 
 69, 70 
 64-66 
 
 . 66 
 
 • 49 
 
 38- 64 
 
 • 56 
 . 10 
 
 . 138 
 
 39— 43 
 58-64 
 
 56 
 46 
 
 76 
 59 
 79 
 78 
 27 
 107 
 
 63 
 
 44 
 
 Illuminants, Chapters VH., and 
 
 VIII 34 
 
 Impact of Gas on Lime .... 48 
 Incandescence, Area of ....11 
 
 Incanto j^cetylene Generator ... 40 
 
 " Injector " Jet 53 
 
 Iron Sand in Oxygen-Making . . 59 
 
 Jet-Nipples 49 
 
 Jet, The " Injector " 53 
 
 Jet, with "Cut off" 51 
 
 Jets, Blow-through or Safety . . 45, 46 
 
 Jets for Acetylene 42, 43 
 
 jets, Interchangeable 50 
 
 Jets, Mixing 46 — 53 
 
 Jets, Noisy 49 
 
 Lamps, Oil, Various Makes . 24, 25, 35 
 Lantern Body, Functions of . . . 12 
 Lantern Body, Size of . . . .24, 25 
 Lantern Body, The, Chapter V. . 24 
 Lantern in Lecture-room, Chapter 
 
 XX. 128—130 
 
 Lantern Microscopes .... 90, 91 
 Lantern Slide Described .... 11 
 
 Lanterns, Biunial 31 
 
 Lanterns, Multiple, Chapter VI. . 29 
 
 Lanterns, Triunial 3i> 32 
 
 Lawson Saturator, The .... 57 
 Leakage of Light from Lantern . . 28 
 Lens, A Useful, for All-round Work 23 
 
 Lime Boxes 114 
 
 Lime, Impact of Gas on .... 48 
 Limelight . ., 44 — 71 
 
 PAGE 
 
 Limes 113 — 114 
 
 Lime-Shield, Wood's 51 
 
 Lime-Turning Arrangements . 50, 51 
 
 Liquids for Saturators 57 
 
 Lumiere 100 
 
 Manganese Dioxide . . . • 58, 59 
 Mantle of Welsbach Burner ... 37 
 
 Marey 99 
 
 Masks, Sizes and Shapes .... 14 
 Masks, With Regard to Condensers 14 
 
 Measures, Electric 79 
 
 Memoranda for Display . . . -137 
 
 Mixing Jets 46—53 
 
 Mixture for Oxygen 59 
 
 Moss Lantern Jet, Acetylene ... 43 
 Muybridge 99 
 
 Nelson's Condensers . . . . 17, 18 
 New Lanterns . (Appendix) 142 — 146 
 Newton's Blow-through Jets ... 45 
 
 Newton's Jets 47, 48, 51 
 
 Newton's Miniature Lantern ... 27 
 Newton's Refulgent Oil Lamp . . 25 
 
 Nipples of Jets 49 
 
 Noisy Jets 49 
 
 Oil for Lamps, Quality of .... 36 
 
 Oil Lamps, Stocks' 35 
 
 Oil Lanterns .... 24, 25, 34, 35 
 
 Oils, Flash Points of 36 
 
 Open Stages 92, 93 
 
 Optical System, General . . . . 10 
 
 " Optimus " Saturator 54 
 
 Ottway's Jet 46, 53 
 
 Ott way's jet and Fittings .... 52 
 
 Oxy-Calcium Jet 44 
 
 Oxygen Generators . . (Appendix) 138 
 Oxygen, Preparation of . . . 58 — 63 
 Oxyhydrogen Light 44—71 
 
 Pendant Saturator 55 
 
 Photo Lenses for Projection ... 20 
 Picture, Projected Size of . . . . 11 
 
 Pitting of Lime 48, 50 
 
 Place's Carrier 87, 88 
 
 Polarising Apparatus 92 
 
 Position for Lantern 120 
 
 Position of Lantern, To Find . . 21 
 Practical Working, Chapter XVI. . 113 
 Preparations for Lecture, Chapter 
 
 XVII 119 — 122 
 
 Pressure of Gas in Bags .... 66 
 Pringle-Beard Miniature Lantern 26, 27 
 Projecting Lens, The, Chapter IV. 20 
 Projection Lens, Focal Length of 21, 22 
 
INDEX. 
 
 149 
 
 PAGE 
 
 Pumphrey's Mechanical Stage . . 52 
 Purifying Oxygen 61 
 
 Quadruple Condenser 18 
 
 Radiant " Jet, Ross' .... 146 
 Radiant, Should be a Point ... 11 
 
 Reading Lamps 88 
 
 Retort for Oxygen 60, 63 
 
 Rheostat, Functions of .... 79 
 
 " Rilford " Generator 140 
 
 Ross Electric Lamp 77 
 
 Ross Enlarging Lantern . . . .132 
 
 Ross Radiant Jet 146 
 
 Ross Rheostat 80 
 
 Safety Jets 45. 46 
 
 Safety Valves 47 
 
 Salt in Oxygen making .... 59 
 Saturation, Principles of .... 54 
 
 Saturators 54 — 57 
 
 Saturators, Blowing Out .... 56 
 Science Lanterns, Newton's . . 94, 95 
 Science Lanterns, Ross .... 96 
 " Scientist's Biunial," Newton's . . 94 
 Sciopiicon, Marcy's . . .25, 34, 35 
 "Scissors" Arc Lamp, Borland's 78 
 
 Scott's Saturator 54 
 
 Screen Elevators 84 
 
 Screens, Chapter XII 82—86 
 
 Sharpness on Screen 11 
 
 Shutters for Lecture-Room . . . 129 
 
 Signals 89 
 
 Sizes of Slides 11 
 
 Slide-boxes 8y 
 
 Spirit Jets 44, 45 
 
 PAGK 
 
 " Star" Dissolver 32, 33 
 
 •'Star" Patent Generator . . . 139 
 Steward's " Premier " Jet and Tray. 52 
 
 Steward's Rheostat 81 
 
 Stocks' Oil Lamp 35 
 
 Storage of Ciases in Bags . . . b\ — 66 
 
 Supports for Jets 52, 53 
 
 Supports for Lantern 87 
 
 Swan Edison Incandescent Lamp . 73 
 
 Sweating of Condenser 16 
 
 Sweating of Slides 89 
 
 Sweetser's Auto-hydrogen Lantern . 142 
 
 Swift's Condenser 18 
 
 Swift's Front Lens 20 
 
 Table of Enlargements 136 
 
 Table of Pressures, Brin's . . . . 71 
 
 Tank for Gas-Storage 66 
 
 Taps 50 
 
 Taylor's Condenser ...... 16 
 
 Thorne & Hoddle, Acetylene . . 40 
 
 Triunial Lanterns 31) 32 
 
 Tubing, Rubber 89 
 
 TurnbuU's Oxy-calcium Jet ... 45 
 
 Valves, Safety 47 
 
 Weights for Pressure-Boards . . .115 
 Welsbach Burner for Radiant. . . 37 
 Wicks for Oil Lamps .... 25, 35 
 Willway's Pendant Saturator ... 53 
 
 Wray's Front Lens 20 
 
 Wright, Lewis, His Jet .... 48 
 Wright-Newton Projection Micro- 
 scope ,91 
 
 Wright's Metal-faced Screen ... 84 
 
Advertisements. 
 
 TU LECTURERS AND OTHERS USING THE OXY-HYDRIC LIGHT. 
 
 BRIFS OXYGEN COMPANY, LIMITED. 
 
 Telegraphic Addj-ess — "Erin's Oxygen, London." 
 Telephone No. — Westminster, hi. 
 
 Price List. KeX^ 
 
 OXYGEN AND COAL GAS. 
 
 CYLINDERS. 
 
 Oxygen or Coal Gas. 
 
 Cubic 
 Contents 
 in 
 Feet. 
 
 Approxi- 
 mate 
 Diameter 
 in inches. 
 
 Approxi- 
 mate 
 Length 
 over all 
 
 in Inches. 
 
 Approxi- 
 mate 
 Weight 
 in lb. 
 
 Prices of 
 Cylinders 
 complete 
 withValve. 
 
 Rent 
 per week 
 after first 
 14 days. 
 
 Prices per 
 cubic foot in 
 the Company's 
 Cylinders. 
 
 Prices in 
 Customers' 
 
 own 
 Cylinders. 
 
 lO 
 12 
 
 15 
 20 
 40 
 60 
 60 
 80 
 80 
 100 
 100 
 
 4 
 4 
 4 
 4 
 
 S% 
 7 
 
 ^% 
 7 
 
 ^% 
 7 
 
 19 
 23 
 25 
 32 
 
 33 
 49 
 29 
 64 
 38 
 
 79 
 48 
 
 15 
 17 
 
 20 
 26 
 41 
 52 
 54 
 
 67 
 69 
 87 
 
 93 
 
 39/- 
 
 40/. 
 
 41/ 
 
 44/6 
 
 56/6 
 
 62/6 
 
 70/ 
 
 69/- 
 
 89/- 
 
 75/- 
 100/- 
 
 1 1/3 
 
 1 1/6 
 }2/- 
 } 2/6 
 
 Quantities 
 ot less lhan 
 2C feet, 4d. 
 
 Quantities 
 of less than 
 20 feet, 3d. 
 
 Quantities of 
 20 feet and less 
 than 60ft., 3d. 
 
 Quantities of 
 20 ft. and less 
 than 60, 2Xd- 
 
 60 feet 
 and upwards, 
 2^d. 
 
 60 feet 
 and upwards, 
 2d. 
 
 Larger Cylinders than those enumerated above 
 can be supplied, if retjiiired, up to a 
 ctibic capacity of 22^ feet. 
 
 For larger quantities than 
 the above the Company 
 are prepared to quote 
 Special Prices. 
 
 FITTINGS :— Nipple and Union, 2/- ; Key, 1/6. 
 REGULATORS (with or without differential Screw), 30/-. 
 GAUGES, 30/- ; with Special Safety Check, 35/-. 
 CONNECTORS for old and new style valve fittings, 3/-. 
 SPECIAL CONNECTIONS for Gauge and Regulator, 7/ . 
 GUN METAL PLUGS for Valves, 2/-. 
 
 TERMS :— Net, cash with order. 
 
 Deposits. — Customers ordering gas in the Company's cylinders, must deposit the value of the 
 cylinder on giving an order ; the deposit will be refunded on return of the cylinder, carriage paid, and 
 in good condition. 
 
 Orders and remittances to he sent to tlie Offices, 34, Victoria Street, Westminster, S, W. 
 Cylinders, to he filled, to he sent to the Works, 6q, Horse/erry Road, S. W. 
 
 BRIN'S OXYGEN COMPANY, LIMITED, 
 
 34, Victoria Street, Westminster, S.W. 
 Works : 69, HORSEFERRY ROAB, WESTMINSTER, S.W. 
 
VI. 
 
 The Optical Lantern. 
 
 FOR FINE DEFINITION. 
 
 Fine definition in the lantern slide is the 
 
 first essential to getting fine definition on the 
 screen. 
 
 The best way to get fine definitions is to use 
 
 Some leadlnor makers of lantern slides have 
 discarded all their old lenses, and use only Cooke 
 lenses for this purpose. 
 
 Ask for the Cooke Bookie". 
 
 Jaylor, Jayloi^^ [^op5orl. 
 
 Slate Street Works, LEICESTER, 
 
Advertisements. 
 
 vu. 
 
 DALLMEYER'S LANTERN LENSES 
 
 (patent) 
 
 1 And CONDENSERS. 
 
 THESE Lenses are specially con- 
 structed for use with the Optical 
 Lantern and are not intended for 
 Photographic purposes. They 
 will be found to give a perfectly 
 achromatic image, combined with an 
 absolutely flat field, and great bril- 
 liaricy of definition. The enlarge- 
 ments upon t"he Screen are perfectly 
 true to the original, free from all dis- 
 tortion, and well defined throughout, 
 whilst the equality of illumination 
 obtained is a remarkable feature. 
 
 Each Lens supplied with a 
 rack and pinion move- 
 ment. 
 
 Diameter 
 of front 
 com- 
 bination. 
 
 Diameter 
 of back 
 
 com- 
 bination. 
 
 Back 
 focus. 
 
 Equiv. 
 focus. 
 
 Price. 
 
 
 Inches. 
 
 Inches. 
 
 Inches. 
 
 Inches. 
 
 £ s. d. 
 
 No. 1. LANTERN LENS 
 
 V-A 
 
 m 
 
 
 5 
 
 4 0 0 
 
 No. 2. Do 
 
 IK 
 
 2 
 
 4 
 
 6 
 
 5 0 0 
 
 No. 3. Do 
 
 2% 
 
 
 5K 
 
 8 
 
 7 5 0 
 
 No. 4. Do. 
 
 
 3 
 
 
 10 
 
 9 10 0 
 
 No. 5. Do 
 
 3 
 
 
 9A 
 
 12 
 
 12 0 0 
 
 Diameter 
 
 £3 10 0 
 4 12 6 
 
 4 111. 
 
 £4 0 0 
 
 5 5 0 
 
 4/4 in. 
 
 £5 0 0 
 6 7 6 
 
 5 in. 
 £6 10 0 
 8 5 0 
 
 CONDENSERS 
 
 Double 
 Triple 
 
 For use with first class Objectives. Made to formula 
 concentration of the greatest possible amount of light, and carefully corrected. 
 The glass employed is the finest obtainable, and free from any strise or similar 
 defects. It is specially annealed and — with ordinary care — wil/ not crack. 
 
 :alculated for the 
 
 Cinematograph Lenses. 
 
 SPECIAL LANTERN LENS, //2-2, = in. eq. focus covers, I X K in. (with 
 
 rack and pinion) .. .. .. , . 3 15 
 
 MEDALLION LENS, //a'a, ij^ in. eq. focus covers, K X K i"- • • 2 7 6 
 
 MINIATURE LENS, /2-2, 3in. ,, „ 2X 2 in. (with rack 
 
 and pinion) .. .. 5 10 0 
 
 Special Mountings adapted to any machine to order. 
 
 A Yarieiy of Special Lenses for Cinematographic Cameras are kept in Stock. 
 
 J. H. DALLMEYER, LTD , 
 
 Optical hw.c,„,> : Newman Street, LONDON, W. 
 
Vlll. 
 
 The Optical Lantern. 
 
 Tb€ "loj€Ctor" MU€d J^t. 
 
 PATENT 10554/93. Price 30s. 
 
 ■yHis is the only perfect Mixed Jet which can be worked with coal gas taken 
 » direct from the town supply, and oxygen at high pressure from a cylinder. 
 It combines the full efficiency of a mixed jet with the safety, economy, and con- 
 venience of a blow-through. Blow-through jets are quite 
 
 superseded by it. An ordinary mixed jet 
 which will produce any given candle- 
 power when supplied with both gases 
 under pressure will, when fitted with our Injector, give the same Power when 
 taking its coal gas supply from the house service-pipe. When required the jet can 
 be worked with both gases taken from cylinders just as an ordinary mixed jet. 
 
 ALL FURTHER PARTICULARS FROM 
 
 THE MANCHESTER OXYGEN (brin s patent) CO., LTD., 
 
 Great Marlborough Street, MANCHESTER. 
 
 THE 
 
 Sted man-Brown Automatic 
 Oxygen Generator. 
 
 IS THE ONLY PERFECT OXYGEN 
 IPRODUCING OUTFIT THAT IS 
 COMPLETE IN ITSELF. 
 
 It will maintain a continuous supply for 
 two hours with absolutely no attention, the 
 oxygen being produced as required at a 
 steady and even pressure entirely auto- 
 matically. If the gas is not being used the 
 apparatus will stop generating. 
 
 There is no danger with this apparatus, 
 and nothing to get out of order. 
 
 For laboratories, private use, for travel- 
 lers and lecturers, both at home and 
 abroad, there is nothing so compact or so 
 efficient. 
 
 The whole apparatus packs in its own 
 box for travelling, and can be fitted up and 
 gas produced in less than fifteen minutes. 
 
 Further particulars and Lists of every description of Limelight Apparatus, 
 Acetylene Jets, Saturators, and other Apparatus, post free from— 
 
 F. BROWN, 13, Gate St., London, W.C. 
 
 Sole Agents for the Continent : Clement & Gilmer, 8 and 10, Rue de Make, Paris. 
 
Advertisements. 
 
 IX, 
 
 THE 
 
 KING OF CINEMATOGRAPHS. 
 
 CAMERA & PROJECTOR. 
 
 (PATENTED). 
 
 2,5oo Latest Film Subjects 
 
 (COPYRIGHTED). 
 
 Best Makes, including LUMIERE, MELIES (Robert Houdin) 
 and the famous "W.T.O." subjects. 
 
 OPTICAL LAMPS. 
 
 Latest Models- HEPWORTH ELECTRIC ARC LAMPS, 
 
 RHEOSTATS, GWYER LIME LIGHT JETS, CARBONS, 
 
 LIMES, OBJECTIVES, etc., etc. 
 
 All Accessories of Best Quality pertaining to the 
 Cinematograph Trade. 
 
 SENlD FOR CT^XT^LiOGUES. 
 THE WARWICK TRADING CO., Ltd., 
 
 5, Warwick Court, High Holborn, London, W.C. 
 Telegrams : " Cousinhood, London," Telephone ; "1135, Holtoorn." 
 
The Optical Lantern. 
 
 RILEY BROTHERS, Limited, 
 
 THE LARGEST LANTERN DEALERS IN 
 THE WORLD. 
 
 The "RILFORD" 
 
 A(iton)atic Oxygeo ^eocrator, 
 
 Price, £5 10s. 
 
 Oxygen generated as the Lecture proceeds. Exceedingly efficient 
 witli single Lanterns and Blow-through Jets or Saturators. Very- 
 economical in working. 
 
 SEND FOR DESCRIPTIVE PAMPHLET. 
 
 SOLE MAKERS OF THE 
 
 ''LAWSON PATENT SATURATORS," 
 " PRAESTANTIA " 4-guinea Lantern, etc., etc. 
 
 Catalogues, post free, Gd. Hire Lists, free. 
 
 55 & 57, Godwin Street, BRADFORD 
 
Advertisements. 
 
 XI. 
 
 fHORNTON-PlCKARD 
 
 CAMERAS -° 
 SHUTTERS 
 
 MAKE PHOTOGRAPHY A PLEASURE. 
 
 Time and Instantaneous 
 Shutter. 
 
 Simplest, Best, and Cheapest. 
 
 Largest Sale in the World. 
 One Shutter will fit two or more Lenses. 
 
 Works absolutely without vibration. 
 
 Price from 12/6. 
 SNAP SHOT from 8/-. 
 
 Focal Plane Shutter. 
 
 For High-Speed Instantaneous Work. 
 From 35/-. 
 
 'Amber' and 'Ruby' 
 Cameras 
 
 For Hand or Stand. 
 
 The " AMBER " competes with the most expensive 
 in quality and the cheaptst in price. 
 
 Quarter Plate from £2 13s. 6d. 
 
 The "RUBY" is Three Cameras in One. 
 
 Perfect as a Stand Camera ! 
 
 Perfect as a Hand Camera ! 
 
 Perfect as a Stereoscopic Camera ! 
 
 Half-Plate from £4 2s. 6(1. 
 
 ILLUSTRATED CATALOGUE POST FREE FROM— 
 
 The Tliornton-Pickard Manufacturing Co., Ld., Altrinchani. 
 
The Optical Lantern. 
 
 NOHKES & NORMAN 
 
 (Late D. NOAKES & SON), 
 
 MANUFACTURERS OF 
 
 Cinematographs, 
 Lanterns, 
 
 AND EVERY DESCRIPTION Ol'^ 
 
 Accessory Apparatus. 
 
 South London Optical Works : Nelson St., Greenwich, London, S.E. 
 
 Sl'ECIALi riES : 
 
 Dissolving View Apparatus and Patent Triple Dissolver. 
 
Advertisements. 
 
 xiu. 
 
 The Magic Lantern : Its Construction and Use— Contains complete instructions (cloth 
 
 covers), PriCC 6d. 
 
 i 
 
 OPTIMUS 
 
 MAGIC LANTERNS. 
 
 Each Magic Lantern is efficient for exhibitions. The Lens gives crisp definition, being a 
 superior Achromatic Photographic Combination, with rack and pinion. It is fitted to a telescopic 
 lengthening tube, so gaining increased focal accommodation. The Condenser is composed of two 
 plano-convex lenses of four inches diameter. The refulgent lamp has three wicks (or, four wicks. 
 
 2S. extra). Each is complete in box. 
 
 Superior Japanned Metal Student's Lantern (to take tlemonstratmg 
 Body. 2nd Quality, 21S. ., tank), with Brass Slidmg Tubes. 
 
 Russian Iron ISody, 
 Brass SlidingTubes. 
 
 Perforated Russian 
 Iron Body, Brass 
 Sliding Tubes. 
 
 Educational Lantern, Russian Iron 
 Body, Brass Sliding Tubes. 
 
 Mahogany Body, Bras ^ 
 Stage & Sliding Tubes. 
 
 LANTERNS FOR LIMELIGHT. 
 
 BIUNIAL. 
 
 Seasoned Mahogany Body, 4 
 Panelled Doors, Moulded Foot, 
 picked out with black. Brass 
 Stages and Tubes, Achromatic 
 Front Lens, Compound Con- 
 densers. 
 
 £10 10s. Od. 
 
 TRIPLE. 
 
 Seasoned Mahogany Body, 6 
 Panelled Doors, Moulded Foot, 
 picked out with black. Brass 
 Stages and Tubes, Achromatic 
 Front Lens, Compound Con- 
 densers. 
 
 £17 10s. Od. 
 
 T/u' Top Lantern may he used separately with Oil Lamp. 
 Magic L.anterx Transparent Photografhs : 
 Plain, 12s.; Coloured, 18s. 6d. per Dozen. 
 
 "OPTIMUS" CANTILEVER PHOTOGRAPHIC 
 ENLARGER. 
 
 --■ We are now offering this already popular model, finished 
 in a very superior and practical style. The lamp gives a 
 brilliant and even light. The optical system is throughout 
 
 of the best. The Lens will toe found to give a very- 
 fiat field and good marginal definition. ' Suited 
 FOR I'DCKET Camera (Kodak) negatives. g 
 Diam. of Condenser . *4-in. 5-in. 6-in. 8-in lo-in. 
 Price without Front Lens 62S. 70s. 80s. IIOS. 200S. 
 Extra for Front Lens . 27S. 37s. 63s. 63s. 105S. 
 
 INCANDESCENT GAS, 7s. 6d. EXTRA. 
 
 Catalogue of Apparatus Post Free. 
 
 PERKEN, SON & RAYMENT, %5ffl%?f5fl? LONDON. 
 
 established 1852. 
 
xiv. 
 
 The Optical Lantern. 
 
 Second Edition. Crown 8vo, cloth. Price 5s., poSt free. 
 
 Photo-Mechanical Processes: 
 
 A Practical Guide to Photo-Zincography, Photo-Lithography, and 
 
 Collotype. 
 
 By W. T. WILKINSON. 
 
 CONTENTS.— Part I. The Negative for Photo-Zincography in Line and 
 Half- Tone, and Photo- Lithography in Line. — Part IL Photo-Zincography 
 in Line.— Part IIL Photo- Zincography in Plalf-Tone. — Part IV. Photo- 
 Lithography in Line. — PartV. Cr>llotype. — Part VI. Trichromatic Photo- 
 graphy. — Appendix. Hints on all the latest Photo Process Methods. 
 
 Crown 8vo, cloth, gilt-lettered. Price 3s. 6d. Post free 3s. 9d. 
 
 Photographic Reproduction Processes 
 
 A Practical Treatise of the Photo-Impressions without Silver Salts. 
 
 By P. C. DUCHOCHOIS. 
 
 Edited with additional matter, l)y E. J. Wall, Author of "The Dictionary of Photography." 
 In this work the fullest details are given, and the clearest instructions and hints for 
 practical working. 
 
 CONTENTS. — The Designs (How to Make Negative Drawings for the Pro- 
 cesses, giving Positive Impressions from Negative Cliches) ; Choice of the 
 Paper ; Sizing with Arrowroot and Gelatine. The Cyanotype, or Blue 
 Process. The Cyanofer. The Black, cr Ink Process. The Cuprotype and 
 Uranium Black Process. The Aniline Process. The Primuline, or Diazo- 
 type Process. Feer Process — Printing on Wood, Canvas, Opal, and Glass ; 
 Transparencies. Tracing Process on Metal. Graphotypy. The Uranotype. 
 The Platinotype. Artigue's Process. The Carbon Process. The Powder 
 Process. Various Processes by Burnett. Obernetter. Liesegang, &c. 
 
 Crown 8vo, cloth. With Ilbtstrations. Price Is. 
 
 Lig^hting^ in Photogfraphic Studios: 
 
 By P. C. DUCHOCHOIS. 
 
 Revised and Enlarged by W. Ethelbert Henry, C.E. 
 CONTENTS. — General Principles to be Observed in Making Portraits. The 
 Lighting. Different Modes of Distributing the Light. Rules and Effects of 
 Lighting. The Studio. Backgrounds. Lighting the Model. Treatment 
 of the Eyes. Development in Relation to Lighting. The Use of Ortho- 
 chromatic Plates in Portraiture. Formulfe for Lighting. 
 
 IN THE PRESS. Crown 8vo, cloth. Illustrated. Price 2s. 6d, 
 
 Practical Eng^raving: on iVIetai : 
 
 By C. A. BANNER. 
 
 CONTENTS. — A short History of Engraving and Engravers, with a description 
 of the various Processes. Engraving on Gold and Silver for Surface Decora- 
 tion. Description of Tools and How to Use Them. Lettering on Gold, 
 Silver, Brass, etc. Heraldic Engraving. Outlines of Heraldry, Monograms, 
 etc. Saw Piercing and Metal Carving. Tools and Appliances. Metal 
 Inlaying on Wood, etc. Gun Engraving. Copperplate Engraving. Stencil 
 Cutting, and Dry Point Etching. Etching on Copper. Various Appliances 
 used in connection with Engraving, etc. 
 
 W^RITE FOR CATALOGUE. 
 
 London: HAMPTON & Co., 13, Cursitor Street, E.G. 
 
Advertisements. 
 
 XV. 
 
 TO LANTERNISTS, LECTURERS, AND 
 USERS OF Cinematographs. 
 
 THE NEW li.C.C. TUBING. 
 
 This tubing embodies all the requirements of the London 
 County Council, and can now be had in large or small quantities. 
 
 Beard's EclipseCarfier, 10/6, 
 
 It produces an effect similar to dissolving. 
 
 The Slides are Inserted and 
 Withdrawn by a Single 
 Movement. 
 
 BEARD'S REGULATORS 
 
 Are the most efficient and perfect for 
 
 Controlling Coninressed Gas. 
 
 By using the Gauge attached the quan- 
 tity of Gas in Cylinder is shown. 
 
 Limelight Jets of all descriptions a Speciality. 
 
 The best Hard Limes, 2S. 6d. per Tin of i Dozen. 
 
 R. R. BEARD. 10, Tralfagar Road, 65s. 
 Old Kent Road, LONDON, S.E. 
 
 J. OTTWAY & SON, 
 
 OPTICAL WORKS. 
 178, St. John Street Road, Clerkenwell, E.C. 
 
 TelegraDis : " 0/tway, London." 
 
 Improved High-Pressure Jet - - - - . - £1 10 0 
 Ditto, with Screw Regulating Valves - - - - 1 19 0 
 Ditto, with improved Cut, fitted with Screw Adjustment 
 (for the amount of hydrogen left burning) and 
 Screw Regulating Valves - - - - - 2 9 0 
 
xvi. 
 
 The Optical Lantern. 
 
 Pbotoarapbp 
 
 The Journal of the Amateur, The Pro- 
 fession, AND The Trade. 
 
 This is the journal especially for the 
 photographer who is fond of . . 
 
 LANTERN 
 SLIDE WORK. 
 
 It has a column specially devoted to 
 giving . . 
 
 INFORflATION 
 TO 
 
 TOURISTS 
 
 and describing interesting and pretty 
 places. 
 
 It has also a " Worktable " column, in 
 which appears the 
 
 EXPERIENCES OF 
 
 PRACTICAL 
 
 WORKERS 
 
 in every section of photographic practice, 
 and queries in connection with . 
 
 LANTERN 
 SLIDE WORK 
 
 will always evoke the best expert 
 opinion. 
 
 Weekly: ONE PENNY. 
 
 3, ^t5. Bride ^i, London, E.C. 
 
 PUBLICATIONS FOR -x- 
 
 PHOTOGRAPHERS & LANTERNISTS. 
 
 s. d. 
 
 A Guide to Modern Photography. 
 
 —By Harold Baker. Strongly bound m 
 cloth and illustrated (postage i%d.) ..06 
 
 The Photographic Colourist. - A manu- 
 al for the use of amateurs. Giving every 
 particular required for painting lantern 
 slides and other transparencies, colouring 
 paper prints to imitate oil paintings, 
 crystoleum paintings, etc., etc. By J. W. 
 Neville^ CI. bound, i2mo. (postage id.) o 6 
 
 Practical Enlarging. -By J. A. Hodges, 
 
 vice-president West London Photo So- 
 ciety. Ulus. CI. bound (postage 2 J^d). . 20 
 
 Photography in a Nutshell.— By ' ' The 
 
 Kernel." Crown 8vo. Attractive paper 
 covers (postage 2d.) .. .. .. i a 
 
 The Magic Lantern.— By Dr. J. A. 
 
 Manton, iVi.R.C.S.Eng., L.R.C.P.Lond. 
 
 A most compreher sive and handy little 
 volume, replete with information for the 
 lanternist. Just pub. Bound in cloth . . o 
 
 The Hand Camera and How to Use it. 
 
 — By Walter D. Welford. Revised edi- 
 tion. With illustration in half-tone. 
 Crown 8vo. Bound in cloth . . ..10 
 
 Short Lessons in Photography.— By 
 
 G. Ardaseer, Member of Koyal Photo- 
 graphic Society of Great Britain. 
 Crown 8vo. Well illustrated, and bound 
 in neat paper covers (postage 2%d.) ..10 
 
 An Introduction to the Science and 
 Practice of Photography. — Third 
 
 edition. Revised and enlarged. By 
 Chapman Jones, F.I.C., F.C.S. With 
 numerous ilhis. Crown Svo. Pap. cover 2 6 
 Ditto cloth bound (postage 4d. and 3d.) 3 S 
 
 A Treatise on Photogravure.— In In- 
 taglio, by the Talbot-Klic process. By 
 Herbert Denison, F.R.P.S. Strongly 
 bound in buckram (postage, 3d.) . . ... 4 6 
 
 The Photographic Reference Book. 
 
 — Hints, information, and methods con- 
 cerning all kind^ of photographic work 
 and recreation. Compiled by W. A. 
 Watts, M.A., under the direction of 
 Henry Sturmey, editor of Plwtography. 
 A comprehensive work of over 400 pages. 
 Copiously illustrated, and well bound in 
 
 cloth (postage 4d.) . . _ 60 
 
 "Of infinite value to professional & amateur." 
 
 The Encyclopaedia of Photography. 
 
 — By Walter E. Woodbury. Containing 
 over 1,500 references, and illustrated with 
 above 200 explanatory sketches and 
 diagrams. Cloth bound (postage 6d.) ..76 
 
 The Gum-bichromate Process.- A 
 
 book of instruction for obtaining a per- 
 manent photograph in pigment by photo- 
 graphic means withotit transfer. With 
 illustrations. By W. J. Warren. Cloth 
 covered (postage 2d.) . . . . ..10 
 
 The First Principles of Photo- 
 graphy. — By Clement J. Leaper. With 
 numerous diagrams and illustrations. 
 Crown Svo. Cloth bound (postage 3d.). . 5 o 
 
 lUFFE, SONS & STURMEY, LTD., 
 
 3, St. Bride Street, London, E C. 
 
Advertisements. 
 
 xvii. 
 
 ARCHER & SONS' ■-"p™"''' Optical Lanterns 
 
 ANIMATED PHOTOGRAPHS and all Apparatus. 
 
 TWO PRIZE MEDALS— Liverpool liUernational and Photographic Exhibitions. 
 Will show to perfection any distance, i to loo feet from Screen. 
 
 "THE3 IDBAIi" LANTERN. 
 
 THE LANTERN OF THE FUTURE. See enthusiastic Testimonials from Paul Lange, 
 Esq. ; F. Anyon, Esq. ; G. E. Thompson, Esq. ; The Manchester Camera Club, etc., etc. 
 
 Our New Patent "IDEAL" Dissolver 
 
 For Single Lantkkns. Improved Blow-through Safety Jet, 500 c.p. .. 16s. 
 The "PHOTINUS" (Reihstered). ^ Price, 
 
 The most powerful Oil Lantern in the World . . . . . . £4 4S. 
 
 Full-Sized LANTERNS from £1 Is. to £100 
 
 Illustrated Price Lists, many Novelties, post free, id. 
 One of the finest and largest stocks of SLIDES in the Kingdom at most Reasonable PriceS, 
 
 ARCHER & SONS, '^^'''''tzXc^n^ s^^i^"' Liverpool. 
 
 Established 1848. Telephone No. 5615. 
 
 Dark Room for trying Lanterns, etc., on the premises. Workshops on the premises. 
 
 PHOTOGRAPHIC CAMERAS and all Accessories. 
 
 ^ Photographic Catalogue, 130 Pages, nost free, ild. 
 
 SLIDE PAI/NTI/NG COLOUHS, 
 
 Specially Prepared for Painting Magic Lantern Slides. 
 
 New Lists just Readv. Lists Post Free. 
 
 J. BARNARD & SON, Manufacturing Artists' Colourmen, 
 
 RETAIL, WHOLESALE, AND EXPORT, 
 
 19, BERNERS STREET, LONDON, W., ENGLAND. 
 
XVUI. 
 
 The Optical Lantern. 
 
 W. I. CHADWICK, 
 
 26, King Street, MANCHESTER. 
 
 LANTERNS, MICROSCOPES, 
 POLARISCOPES, etc. 
 
 For PROJECTION. 
 
 High-class only; indeed, the most perfect 
 Instruments for the purposes for which 
 they are intended, but without the super- 
 fluous elaboration which contributes only 
 to cost. 
 
 My Catalogue will give more information than 
 I have space for here. 
 
 Again, my address is . . . 
 
 26, KING St., MANCHESTER. 
 
Advertisements. 
 
 xix. 
 
 GWYER'S 
 HKliH-POWER LIMC-LI^HT JETS. 
 
 These jets are constructed on a new principle, and 
 are capable of giving more light ; are more econo- 
 mical and more easy of adjustment than any other. 
 They are perfectly silent when giving the most 
 powerful light. 
 
 TH€ PENPANT SATURATOR. 
 
 The recent improvements have made this Saturator 
 the best means of supplying hydrogen for oxyhydrogen 
 light. It is absolutely safe and requires no special 
 experience. 
 
 FREE TRIAL ALLOWED. WRITE FOR PAMPHLET. 
 
 MANUFACTURERS— 
 
 J. S, Willway & Sons, st, Augostine's, Bristol. 
 
XX. 
 
 The Optical Lantern. 
 
 W. WATSON & SONS, 
 
 Manufacturers of HIGHEST CLASS 
 
 OPTICAL LANTERNS. 
 
 The 
 
 For use with eitner OIL or LIMELIGHT. 
 
 As Supplied generally to Akt Cluhs, 
 Science Teachers, &c., .... 
 
 FOR Illusi katinc, LeC'I^UKES 
 
 PRICE Complete in Case, f%A -flnA 
 
 Metal Body, I US. 
 
 Ditto, Mahogany Body, nc i K«» 
 
 As :illustrated. *»w i fSi 
 
 WRITE FOR 
 CATALOGUE 
 
 No. 4, post free. 
 Contains . . . 
 particulars of 
 every description 
 of Lantern and 
 Apparatus . . 
 
 WATSON & 
 SONS' New 
 Model 'U.E.' 
 Lantern with 
 
 tilting base and 
 fly open doors 
 allowing satura- 
 tor, arc lamp, 
 oil lamp, or jet 
 to be fitted and 
 worked with ease. 
 
 THE MOTORGR APH. a Compact Machine for projecting Animated Photographs- 
 
 .Si.Mi i.E. S rnoNG. Efficient. Can be fitted to front of any Lantern, Price £12 12s. 
 
 NEW SEASON'S LANTERN SLIDES: " Captive Wild Animals." 
 
 The Finest Series of Animal Studies extant. Negatives by GAMHIER BOLTON, f.g.s. 
 
 T\. T\a\SOXV % SOXIS, 313. High Holborn, 
 
 ( 78, Swanston St., Melbourne. 1 r^Xir^rkXT 
 
 \ 16, Forrest Road, Edinburgh. L-U1ML>U1M. 
 
 Branches 
 
Advertisements. xxi. 
 
 Newman 
 
 MANUFACTURER 
 
 OF EVERY ARTICLE FOR 
 
 THE PHOTOGRAPHIC COLOURIST, 
 THE ARTIST IN WATER COLOURS. 
 THE ARTIST IN OIL COLOURS. 
 
 OF SUPERIOR QUALITY. 
 
 24 SOHO SQUARE, LONDON. 
 
 Catalogues and Circulars Post Free^ 
 
 C- BAKER, 
 
 * 244, 
 
 CATALOGUES HIGH HOLBORN, 
 
 ON 
 
 APPLICATION. 
 
 '''' LONDON. 
 
 SPECIALITIES: 
 
 Hig:h-Class Projection Lanterns. 
 . ^ Microscopes. 
 
 ogrraphic Apparatus. 
 
 3 ^ 2- I 4" Etc., Etc. 
 
 E. LEITZ, Wetzlar; 0. REICHERT, Vienna. 
 
 of Second -Hand Instruments. 
 
 NT£R LIBRARY