SCIENCE IN THE INDUSTRIAL WORLD 



to the eye by a proper blending of these three colors. 

 If three-color filters, made respectively of these three 

 colors, were used, and negatives made by means of them, 

 it would seem theoretically possible, at least, to reproduce 

 colored objects by staining the three negatives thus 

 made, red, yellow, and blue respectively, and super- 

 imposing them so that they are accurately registered. 

 As a matter of fact that is what is really done by many 

 of the most successful three-color photographic processes 

 practically the same method as used in three-color 

 process printing, referred to in detail in the chapter on 

 three-color printing. 



In the actual practice of three-color photography 

 many difficulties have to be overcome, and it is at best a 

 tedious process. Three exposures must be made, and in 

 making these the camera must be in exactly the same 

 position for each negative. For a long time the difficul- 

 ties to be overcome in this seemed insuperable, but Mr. 

 F. E. Ives, of Philadelphia, has invented a slide-carrier 

 for this purpose which works admirably; and Mr. 

 Sanger-Shepherd has invented a single-lens camera by 

 which all three negatives are taken at a single exposure. 



The length of time for exposures with the different 

 color-filters is important, and the time required is much 

 longer than for ordinary normal exposures. Generally 

 speaking, exposures through the blue filter are about a 

 hundred times more than normal length, the green or 

 yellow two hundred times, and the red three hundred 

 times. If the normal exposure were ten seconds, there- 

 fore, the total time required for exposures through the 

 color-filters alone, without deducting any time lost in 



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