Diffraction- Grating and Colour-Photography . 369 



thirty seconds. On washing these plates in warm water, 

 diffraction-gratings of great brilliancy were formed directly 

 on the surface of the film. Albumen-plates must be used, 

 since the warm water softens and dissolves a gelatine film. 

 Three sheets of thin glass, sensitized with the bichromated 

 gelatine, were placed under the three positives, and prints 

 taken from them. The portions of each plate on which the 

 light had acted bore the impression of the corresponding 

 diffraction-grating, strongly or feebly impressed, according to 

 the density of the different parts of the positives. These three 

 plates, when superimposed and placed in front of a lens and 

 illuminated by a narrow source of light, appear as a correctly 

 coloured picture, when viewed with the eye placed in the 

 proper position. Perfect registration of the different parts 

 of the picture could not be obtained in this way, for obvious 

 reasons. I worked for awhile with the thin glass from which 

 covers for microscopical slides are made. This gave much 

 better results, but was too fragile for practical purposes. It 

 then occurred to me that if 1 could get the entire grating- 

 system on a single film, not only would the difficulty about 

 perfect registration vanish, but the pictures could be repro- 

 duced by simple contact-printing on chrom-gelatine plates as 

 easily as blue prints are made. I was surprised to find that 

 successive exposures of the same plate under the positives, 

 perfect registration being secured by marks on the plates, 

 produced the desired result. On washing this plate in warm 

 water and drying, it becomes the finished coloured photograph. 

 Where the reds occur in the original, the spacing of the first 

 grating is present; where the yellows occur the spacings of 

 both the first and second are to be found superimposed ; where 

 the blues occur are the lines of the third grating ; while in 

 the white parts of the picture all three spacings are present. 

 It seems almost incredible that, by exposing the plate in suc- 

 cession under two gratings the spacings, of both should be 

 impressed — superimposed — in such a manner as to give the 

 colours of each in equal intensity ; but such is the feet. Thus 

 far I have had at my disposal but two gratings of only ap- 

 proximately the right spacing, one giving the red, the other 

 the green : with these I have photographed stained- glass 

 windows, birds, and butterflies, and other still-life objects, 

 the finished pictures showing reds, yellows, and greens in a 

 most beautiful manner. By making a separate plate from 

 the blue positive, using the same spacing as with the green, 

 and setting this plate behind the other at an angle, 1 have 

 obtained the blues and whites, the grating-space being dimin- 

 ished by foreshortening, though, of course, perfect regis- 



