PROGRESS IN COLOR PHOTOGRAPH Y—SMILLIE. 233 
any one of the spectra formed to the right and left of the central image, the 
entire surface of the grating will appear illuminated with light of a color de- 
pending on the part of the spectrum in which the eye is placed. If one part of 
the grating has a different spacing from the rest, the spectrum formed by this 
part will be placed relatively to the first, and if the eye be placed in the over- 
lapping part of the two spectra, the corresponding portions of the grating will 
appear illuminated in different colors. This principle I made use of in the 
development of a new method for producing photographs in natural color. I 
have eliminated the use of pigments and colored screens entirely in the finished 
picture, the photograph being nothing more or less than a diffraction grating of 
variable spacing, the width between the lines in the different parts of the 
picture being such as to cause them to appear illuminated in their proper colors 
when viewed in the manner described. 
Take three diffraction gratings of such spacing that the deviation of the red 
of the first is the same as that of the green of the second, and the blue of the 
third (the red, green, and blue in question being of the tints of the primary 
colors of the Young-Helmhboltz theory of color vision). If these three gratings 
be mounted side by side in front of a lens, their spectra will overlap, and an 
eye placed in the proper position will see the first grating red, the second green, 
and the third blue. If the first and second be made to overlap, this portion will 
send both red and green light to the eye, and will in consequence appear yellow. 
Tf all three be made to overlap in any place, this place will send red, green, and 
blue light to the eye, and will appear white. 
Now if three negatives are taken through red, green, and blue screens in the 
usual manner, and from these positives are made on albumen lantern slides, 
and the positives when dry are flowed with bichromated gelatin, and dried in 
subdued light, and the diffraction gratings of proper spacing ruled or photo- 
graphed on glass are placed over these positives and exposed to the sun or 
electric light for thirty seconds, on washing these plates in warm water dif- 
fraction gratings of great brilliancy are formed directly on the surface of the 
film. Three sheets of thin glass sensitized with the bichromated gelatin are 
then placed under the three positives and prints taken from them. The por- 
tions of each plate on which the light has acted bears 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 
superposed and placed in front of a lens, and illuminated by a narrow source 
of light, appear as a correctly colored picture when viewed with the eye placed 
in the proper position. Perfect registration of the different parts of the pic- 
ture could not be obtained in this way, however, but if successive exposure of 
the same chrome-gelatin plate under the positives be made, registration being 
secured by marks on the plates, the desired result will be obtained. On wash- 
ing this plate in warm water and drying, it becomes the finished colored photo- 
graph. Where the reds occur in the original, the spacing of the first grating 
is present; where the yellows occur, the spacing of both the first and second 
are to be found superposed; where the blues occur are the lines of the third 
grating, while in the white parts of the picture all three spacings are present. 
Two new methods of producing photographs in color have been an- 
nounced the present year, the Autochrome and the Warner Powrie 
processes, and although they accomplish their ends by indirect 
methods, they are both thoroughly practicable. 
