178 COLOE PHOTOGEAPHY. 



the colors of the spectram at their true color value in 

 monochrome. 



This is accomplished by combining with the emulsion 

 with which plates are coated certain dyes, which render 

 it more sensitive to the yellow, yellow green, and even 

 red rays, than they would be normally. 



In November, 1888, Mr. Ives communicated to the 

 Franklin Institute, of Philadelphia, that he had 

 demonstrated a procedure based upon the assumption 

 that although there are more than 3 or 5 or 7 primary 

 spectrum colors, all of them and in fact all the colors of 

 Nature could be counterfeited to the eye by three type 

 colors and mixtures thereof. 



He proved his process by photographing the spectrum 

 itself, employing compound color screens carefully ad- 

 justed to secure definite intensity curves in the spectrum 

 negatives, so that they would make color prints which 

 counterfeited the color effect of the spectrum when 

 superposed. 



Promising results, he says, were obtained by this pro- 

 cess, but he soon found that a process might reproduce 

 the color effect of the spectrum and yet not be able of 

 reproducing perfectly the compound colors. 



It was necessary to discover a new process by which 

 not only the spectrum would be reproduced but also all 

 the hues of Nature. 



This he discovered to be the making of sets of nega- 

 tives by the action of light rays in proportion as they 

 excite primary color sensations and images or prints 

 from such negatives with colors which represent primary 

 color sensations. 



In order to illustrate this principle he explains that 

 although the spectrum is not made up of three kinds of 

 color rays and mixtures thereof, the eye is only capable 

 of three primary color sensations. A distinction he 

 says of the utmost importance, for the reason that the 



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