I go I -2.] Photography in Natural Colours. 389 



three ordinary lantern slides are required. These must also be soft, 

 delicate, of a neutral tint, and of the same density. 



The three-colour transparencies, by negative synthesis, are com- 

 posed of three coloured positives, one on glass and two on celluloid. 

 From the negative taken through the red filter an ordinary lantern 

 slide in black and white is made, and the black silver image is changed 

 to a blue-green or minus red colour by an iron process. The positives 

 from the green and blue-violet record negatives are made, by a modi- 

 fication of the carbon process, on transparent celluloid coated with a 

 soluble emulsion of silver bromide in gelatine. These films, which are 

 quite similar to the regular Kodak film, are sensitised by soaking for 

 three minutes in a solution of bichromate of potash ; and when dry 

 are exposed under the negatives to daylight, the celluloid side being 

 next to the negatives. The image, being partially visible, forms a guide 

 to the length of exposure required, which averages about five minutes 

 in diffused light. The parts of the bichromated gelatine unexposed to 

 the light dissolve in warm water, and, when the silver bromide is also 

 dissolved out in a solution of hyposulphite of soda, there is left an 

 image in colourless gelatine in relief. 



The colour is given to these images by soaking in baths of aniline 

 dyes ; the print from the green-record negative in minus green or 

 magenta-pink, and from the blue-violet-record negative in minus blue 

 or yellow [Plate]. The gelatine being in relief, varying depths of 

 colour in the images are obtained, reproducing all the gradations. 

 Perhaps the most tedious part of the process is to obtain the correct 

 depth of colour in the prints. This can be tested by observing the 

 roughly superposed prints. A general blue, pink, or yellow tinge 

 throughout the picture shows that the corresponding prints are too 

 deeply stained. The colour given to black or grey objects also forms a 

 delicate test ; rusty brown blacks indicate that the pink print is too 

 strong, greenish blacks too weak ; violet blacks indicate that the yellow 

 print is too weak. Prints too strong are quickly reduced by soaking in 

 water, while if too weak are strengthened, to a certain limit, by a longer 

 immersion in the dye bath. This, of course, does not apply to the blue- 

 green image which when once made cannot be changed in intensity, 

 and, if not suitable, the only remedy is to make another. 



When the colours are correctly adjusted, the prints must be 

 mounted in register. The registration is not difficult if the prints are 

 all of the same size, and, when effected, a cover glass may be put on 

 and the slide bound in the ordinary way. The numerous internal 



