RI (inches ter Memoirs, Vol. xliv. (1900), No. \% 3 



experienced and reliable results are even now not always 

 obtainable. 



Monnted on concave spherical surfaces and silvered, 

 the films ^ivc brilliant s[)ectra suitable for photographic 

 work, but owing to the nature of the ruling, as is well 

 known, the aberration is too great to admit of their being 

 used for other work. 



Whilst engaged in making and mounting these films 

 it had often occurcd to me that they might in some way 

 be utilised for exhibiting photographs in colours, but not 

 until Professor Wood, of Wisconsin, U.S.A., had published 

 his ingenious plan, did any practical method of doing so 

 suggest itself Briefly, Professor Wood's process consists 

 in using gelatine plates rendered sensitive to light by 

 means of bichromate of potash, placing a transparent 

 grating of some 2,000 to 3,000 lines to the inch in contact 

 with the film, and printing through a transparency made 

 from a negative taken through a coloured screen (as a 

 matter of fact Professor Wood used slides prepared for the 

 Kromscop ; they were also adopted by me in my experi- 

 ments as being taken through the proper screens and 

 ready to hand). 



When a plate so exposed to light is washed in warm 

 water the parts affected are insoluble, and a slight ridginess 

 due to the action of the grating is produced, whilst those 

 parts unaffected by the light are washed away to some 

 extent and no effect is produced upon them. 



By superposing three such plates acted upon through 

 transparencies, properly illuminating and viewing them, 

 one picture in natural colours, or very nearly approaching 

 them, is seen, the effect being produced by diffraction from 

 the grooves or ridges of the gelatine films. 



To obtain this effect Professor Wood uses gratings of 

 2,000, 2,400, and 2,750 lines to the inch, as the red, green, 



