January 1, 1901.] 



KNOWLEDGE. 



tiu-er of the paper ou a substratum ruled with alternate 

 Hues of red, green aoid blue, as will be explained later. 

 The history of the process is iuterestiug luid even 

 romautic; for ouo of the e<u-ly workoi-s, a most fertile 

 iuventor, broke down entirely under the mental strain of 

 his work, while another, James McDouough, wreekcd 

 his coustitutiou by close and ceaseless application to 

 research, and died within a day or two of the making 

 of the hrst successful negatives by his process. The 

 tii-st published pai-ticulars of the process were given in 

 a little book (" Les Couleuis en Photographic : Paris, 

 Marion ') issued in 1S69, and written by Louis Ducos 

 du Hauron, who had previously made conununications 

 on various modifications of a triple process of colour 

 photography suggested by himself, on January 20th, 

 1S59, before the Society of Arts and Sciences of Agen. 

 This particular method was allowed to lie unused and 

 undeveloped, and nothing more was heard of it until 

 1S91, when it was patented, almost simultaneously, in 

 Britain and America, by Dr. John Joly, of Dublin, and 

 Mr. James McDonough, of Chicago. Both these workers 

 seem to have devised the method independently, and 

 without knowledge of the work of du Hauron. 



The process is based upon the well-known triple 

 negative process, in which, by means of plates sensitive 

 to the various colom-s of light, and by tlie use of lights 

 filters of colom-ed glass or coloured solutions, negatives 

 arc made separately of each of the {jrimaiy colour-sen- 

 sation effects. By the superposition of prints in corre- 

 sponding coloured inks or pigments from each of these 

 negatives, natural-colour prints may be made upon 

 paper, as in the well-known process of trichromatic 

 letterpress printing. Similaiiy, the superposition of 

 three transparencies or positives in stained gelatine, 

 gives us a natural colour' transparency or lanteni-slide. 



In the McDonough-Joly process, the three negatives 

 are made upon one plate by the simple expedient of 

 replacing the three successive light-filters by a " screen ' 

 ruled with fine lines of transparent stained gelatine 

 in successive rulings of red, gi'een, blue, red, green, 

 blue. Each of thesis lines is ^Aj, of an inch in width, and 

 they aa-e ruled so as to touch, edge to edge, thus entirely 

 covering the glass, of the screen. These iniled lines ai'e 

 pressed into contact with the sensitive dry-plate during 

 the exposure in the camera, with tlie result that the 

 negative so made consists of lines of silver deposited 

 under the lines of the screen. Thus, supposing a blue- 

 object, reflecting light of exactly the colom- of the blue 

 line of the screen, was photographed, the negative would 

 show two lines of clear glass (under the red and gi'een) 

 while under every blue line the silver would be strongly 

 deposited. Thus, every third line may be considered 

 as part of a distinct negative, recording one of the three 

 primary colour sensations. Where an object reflects 

 light of a comjK)site colour, both the colours that go 

 to make up the composite will be affected. Thus, 

 if the object is yellow, it will be represented under both 

 the red and the gi-een lines. 



Prom the colour-effect negative, which looks just like 

 any ordinary negative except for the fine lines, a trans- 

 pai-ency may be made by exposing a sensitive plate in 

 contact, and developing in the same way as in making 

 an ordinary lantern-slide. This is, of course, a reverse 

 of the negative, so that in the suppositious case of the 

 blue object the transparency would show silver deposit 

 in parts corresponding to the red and green lines, with 

 clear spaces corresponding to the blue. To view the 

 picture this positive transparency is placed in contact 

 with a lined colour-screen somewhat similar to the one 



used in making the original expo.siu'c. Looking through 

 the combined transparency and screen, one sees that 

 the silver deposit iu the transparency obscures the red 

 and green, allowing the blue to be plaiiiily seen. 



The same sort of thing happens with photogi-aphic 

 prints on jsaper. The raw paper is first ruled with the 

 fine red, green, and blue lines, after which it is coated 

 (say) with a sensitive gelatino-chloride emulsion, thus 

 converting it into P.O. P. or print-out paper. The 

 negative is plaecd upon a sheet of this, taking care to 

 register the lines of the negative exactly over their 

 proper colour-lines on the paper. The printing causies 

 the parts of the paper to which the light has access 

 to become dark-brown with the opaque reduced silver — 

 as under those lines of the negative which are trans- 

 pai-ent, because the light from the original object has 

 not acted upon them. The photographic image fomis 

 a stencil or mask to the coloured lines, just as it did 

 with tlie positive transparency. 



In photo-mechanical printing, a metal printing-block 

 takes the place of the negative, and a dense printing-ink 

 (black or a neutral grey) replaces the silver deposit. 



The lines, when ruled three hundred to the inch, 

 are so fine that, viewing a picture as a whole, from any 

 reasonable distance, they are quite inoffensive. 



Inttrrmcdiatc colours, made by the reflection of vary- 

 ing proportions of the primaries from lines so fine that 

 the eye does not sepai-ate them, arc purer and better 

 than those made by transmission of the light through 

 three supeqDosed films — always supposing, which is not 

 at present the case, that the original colours are equally 

 good and pure in each instance. On the other haiud, 

 the dark " stencil " image, occupying a large proportion 

 of the space, tends to lower the tones throughout the 

 joicture ; except in the case of a transpai'ency with ample 

 illumination. 



Many difficulties which beset the process, and which 

 rendered it practically impossible when first suggested, 

 have been removed in the general progress of photo- 

 graphy, without any effort on the part of those interested 

 in the process itself. These difficulties chiefly related to 

 the obtaining of plates sensitive enough to the red rays 

 of light to render short exposures possible; and plates 

 which should be fairly sensitive to the whole range of the 

 spectrum. The great attention given to orthochromatic 

 photography has led to the production of such plates, 

 and immensely helped all natural colour investigators. 



The difficulties peculiar to the McDonough-Joly pro- 

 ce.ss are chiefly connected with the manufacture of the 

 colour-screen. The problem was to rule screens that 

 should be cheap, transparent, permanent in colour, fine 

 enough to prevent the lines being offensive, yet coarse 

 enough to make registration on viewing screens or on 

 ruled paper, sufficiently easy. The people interested in 

 developing the process, headed by Mr. D. K. Tripp, of 

 Chicago, havo decided upon the standard gauge of 

 three hundred lines to the inch, and as the result) of a 

 few weeks' working with negatives and transparencies on 

 that scale I conclude that it is a vei7 practical ai-range- 

 ment. As to the permanency of the screens, I have 

 some of Dr. Joly's make, which seem to show no de- 

 terioration in a couple of years or so ; and I am 

 informed that some of Mr. McDonough's have been 

 exposed for about five years without apparent change. 



The question of price is, of course, most important, 



for if every transparency is to have its separate vicwing- 



screen the screens need to be cheap. At present, the 



demand for screens can not bo supplied at any price, 



1 because only the experimental ruling machineiy is 



