PROCEEDINGS OF THE SOCIETY. 119 



sented by three photographs, then by projecting these three photographs 

 each through a colour-screen similar to that used in obtaining the photo- 

 graphs, and superposing the images, an exact reproduction of the various 

 colours of the original would be obtained. 



Instead of projecting the images by means of a lantern, photographs 

 in the form of prints or transparencies may be obtained by a modifica- 

 tion of the same idea. Ducos du Harmon was the first to obtain a 

 picture in pigments by this method, but his effects were not satisfactory. 

 In 1890, Mr. F. E. Ives produced some good results in Philadelphia, and 

 in 1894 exhibited them in London ; but in his lecture before the Society 

 of Arts, April 22, 189 G, he described the process as almost impracticable. 

 The two specimens shown, which were made by Mr. Ives about this date, 

 are, however, very good. 



Briefly, the details of this process are as follows : — Three negatives 

 are taken of any given subject by means of an ordinary camera, three 

 colour-screens, as suggested by Maxwell, being employed in order to 

 produce the requisite colour records, these colour-screens being respec- 

 tively red, green, and blue, and the plates employed being sensitive to 

 these colours. Then three pieces of mica, split to the thickness of 

 thin cover-glass, which have been coated on one side with bichromated 

 gelatin, are exposed, one under each negative, to daylight. The par- 

 ticular coloured light, which was absorbed by the colour-screen when 

 taking the negative, will be represented by clear glass, and the light 

 will render this exposed portion insoluble, so that, upon washing away 

 that portion of the gelatin which remains soluble by means of warm 

 water, an image in clear insoluble gelatin is obtained, which in its 

 varying thickness is an exact record of the coloured light absorbed by 

 the colour-screen ; thus the red screen absorbed the green and blue 

 light, and the resulting gelatin print is stained in a greenish-blue dye : 

 the green screen absorbs red and blue light, and the gelatin print from 

 this negative is stained with a dye which transmits red and blue ; the 

 print from the negative taken through the blue screen is stained yellow. 



When making the exposure of the bichromated gelatin under the 

 negative, the mica is placed next the negative, so that the insoluble 

 image which results from the exposure to daylight may have the mica 

 on which to adhere ; if printed on the gelatin side, a layer of soluble 

 gelatin will remain between the insoluble image and the support. The 

 necessity of printing through the support at once demonstrates the 

 superiority of mica over celluloid, the former being so thin that the 

 finest details will print sharp. A sheet of celluloid, on the other hand, 

 is of quite an appreciable thickness, and microscopic detail cannot be 

 secured. 



The special advantages of using Lumiere's process for scientific 

 work are, that, in the first place, by the employment of mica in the 

 place of celluloid, very fine detail can be secured. As the mica has no 

 tendency to become distorted, there is no difficulty in obtaining accurate 

 registration of the three monochromes. Thirdly, as there is no necessity 

 to balsam the three images together, the resulting picture will not be 

 affected by prolonged exposure to the heat of the projection lantern. 

 By the employment of exactly the same process to obtain all three 

 monochromes the scale of gradation is preserved, and the most delicate 

 tints may be accurately reproduced. 



