the Diffraction- Grating to Colour- Photography, 371 



under a diffraction-grating placed in front of, and in contact 

 with, the film. On development, we obtain a negative the 

 dark portions of which are broken up into fine lines, corre- 

 sponding to the lines of the grating; and on viewing this in 

 the apparatus just described, the blue components of the picture 

 are seen, though not so brilliant as with the transparent 

 gelatine plate owing to the coarseness of the grain. 



I believe that by the use of a suitable photographic plate to 

 be exposed in succession in the camera, under red, green, and 

 blue screens, on the surfaces of which suitable diffraction- 

 gratings have been photographed, it will be possible to obtain 

 the colour-photograph directly. The screens can be swuno- 

 into position in succession by a suitable mechanical arrange- 

 ment operated outside of the camera. The plate, on deve- 

 lopment, will be a negative in the ordinary sense of the term, 

 though when seen in the viewing-apparatus it will appear as 

 a coloured positive, since on the transparent portions which 

 correspond to black in the original, no grating- lines have 

 been impressed : consequently these portions will appear dark. 

 The dark portions, however, where the lines are impressed will 

 light up in their appropriate colours. From this plate as 

 many copies as are desired can be made by contact-printino- 

 on bichromated gelatine. 



Of course it is a question whether superimposed gratino-s 

 can be impressed on a plate in this manner. Judging from 

 the experiments I have made, I imagine that the gratings on 

 the colour-screens would have to be made with the opaque 

 portions broad in proportion to the transparent. 



I have overcome the difficulty of obtaining large diffraction- 

 gratings by building up photographic copies in the followino- 

 manner. The original grating ruled on glass was mounted 

 against a rectangular aperture in a vertical screen, the lines 

 of the grating being horizontal. Immediately below this was 

 placed a long piece of heavy plate-glass, supported on a slab of 

 slate to avoid possible flexure. A strip of glass, a little wider 

 than the grating, sensitized with bichromated gelatine was 

 placed in contact with the lines of the grating, and held in 

 position by a brass spring. The lower edge of the strip rested 

 upon the glass plate so that it could be advanced parallel to 

 the lines of the grating, and successive impressions taken by 

 means of light coming through the rectangular aperture. In 

 this way I secured a long narrow grating; and by mounting 

 this against a vertical rectangular aperture, and advancing a 

 second sensitized plate across it in precisely the same manner, 

 1 obtained a square grating of twenty-five times the area of 

 the original. It was in this manner that I prepared the 



