4 Til OKI', Grating Films and Colour Photography. 



and bluc-vioIct in the spectrum of each, respectively, 

 coincide, and thus all colour combinations are rendered 

 possible, as is well known. 



The spectra produced by the rulings just mentioned 

 are, however, unfortunately very short, and the slightest 

 movement in observing the pictures brings other colour 

 combinations into view, and as the pictures could not be 

 produced in much closer lines by the contact method with 

 any degree of uniformity, I resorted to the following 

 means for lengthening the spectra, and so obtaining lines 

 of any degree of closeness. 



To the bichromated gelatine solution was added a 

 small percentage of glycerine, which had the effect of 

 rendering the surface " tacky " when the gelatine had set. 

 One of the celluloid films was then impressed, grating side 

 down, into the surface of the gelatine, which thus took the 

 form of the grooves. To do this without enclosing air 

 bubbles was no easy matter, and the plan adopted 

 eventually was to mount the film on a slightly convex 

 yielding surface, and perform the operation in vacuo. 



A transparency now being placed over the film side 

 of the plate, the whole was exposed to a strong light (the 

 length of time necessary varying from i y-z minutes with 

 a bright summer sun to a quarter of an hour when covered 

 with fleecy clouds), and the celluloid film dissolved off by 

 acetone, the whole plate was found to have a grating 

 surface, but more particularly decided in the portions 

 acted upon by the light. The plate was then washed in 

 warm water, when all parts but those affected were 

 rendered clear, the affected parts varying in their power 

 to produce colour effects in proportion to the degree of 

 opacity of the transparency through which they were 

 printed, whilst the sharpness of the diffracted picture was, 

 with the most successful experiment, all that could be 

 desired. 



