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XXXII. An Application of the Diffraction- Grating to Colour- 

 Photography. By B. W. Wood*. 



IF a diffraction-grating of moderate dispersion and a lens 

 be placed in the path of a beam of light coming from a 

 linear source, and the eye be placed in 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 

 colour depending 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 displaced relatively to the first; and if the eye be placed 

 in the overlapping part of the two spectra, the corresponding- 

 portions of the grating will appear illuminated in different 

 colours. This principle I have made use of in the develop- 

 ment of a new method for producing photographs in natural 

 colour. I have eliminated the use of pigments and coloured 

 screens entirely in the finished picture, the photograph being 

 nothing more nor 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 colours when view r ed in the manner described. 



We will take at the start 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 colours of the Young-Helmholtz theory of colour- 

 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. If 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. 



The method that I first employed to produce photographs 

 showing natural colours on this principle is the following 5— 

 Three negatives were taken through red, green, and Mue 

 screens in the usual manner : from these, positives were made 

 on ordinary lantern-slides (albumen-slides are necessary for 

 reasons which I will speak of presently) . The positives, when 

 dry, were flowed with bichromated gelatine and dried in 

 subdued light. The three diffraction-gratings of proper 

 spacing, ruled or photographed on glass, w^ere placed over 

 these positives, and exposed to the sun or electric light for 

 * Communicated bv the Author. 



