PHOTOGRAPHY, SCIENTIFIC ASPECTS 



Mr. John W. Powrie in his color-screen has also 

 discarded ruling methods. He takes advantage of the 

 well-known hardening and protective property of 

 bichromate of potassium when exposed to light, to 

 stain a coated glass plate in lines of red, green, and 

 blue, one color after another. These lines are only 

 i/ 600 of an inch in thickness, and more than twice 

 as fine as those that can be obtained by ruling-machines. 



But the popularity of these geometrically made color- 

 screens (they are made in hexagonal and square patches, 

 as well as in lines) has been somewhat impaired by the 

 appearance of the "random grain" plates recently 

 invented by Messrs. Auguste and Louis Lumiere, 

 of Lyons, France. It is true that this idea is of some- 

 what earlier origin, for in the last decade of the nine- 

 teenth century, J. W. McDonough prepared plates 

 by scattering small flakes of colored shellac over the 

 surface and then fusing them, but he soon abandoned 

 this line of experiment for what he considered the 

 greater advantage of the ruled screen. The JLumiere 

 "autochrome" plate is made by placing on the glass a 

 layer of minute grains of potato starch colored red- 

 orange, green, and violet, so thoroughly mixed that 

 they present a neutral gray to the eye. The grains are 

 so small that five and a half million will go on a square 

 inch of surface, and any spaces between them through 

 which white light might filter are filled up with a black 

 carbon powder. This layer is rolled and pressed on 

 the glass, to which an adhesive coating has previously 

 been applied. The layer of colored grains then re- 

 ceives a coating of waterproof varnish and on this is 



[243] 



