PHOTOGRAPHY IN THE COLORS OF NATURE. 159 



aud screens to secxiTe sjiectrtim negatives having definite intensity curves, 

 which I believe had never before been done, made all the difference 

 between an indefinite and uncertain method and one definite and pre- 

 cise. 



Promising results were obtained by this process, but I soon came to 

 the conclusion, already reached by Dr. Stolze, that a process miglit 

 re produce the color eft'ect of the spectrum, and yet not be capable of 

 re-producing- perfectly the compound colors. The solution of the prob- 

 lem was incomplete until I discovered a new principle, according to 

 which such a procedure can be made to re-produce not only the spectrum, 

 but also all the hues of nature. 



This new principle, first stated by mc in a communication to this 

 institution on November 21, 18S8,* is that of making sets of negatives 

 by the action of light rays in proportion as they excite primary color 

 sensations, and images or prints from such negatives with colors that 

 represent primary color sensations. 



In order to understand this principle, T must explain that although 

 the spectrum is not made uj* of three lands of color rays and mixtures 

 thereof, the eye is only capable of three primary color sensations, a dis- 

 tinction of the utmost importance, for the reason that the spectrum 

 rays, which most powerfully excite a ]irimary color sensation, are not 

 the ones which represent the character of that sensation. The primary 

 sensations are red, green, and blue (violet); but it is not the red, 

 green, and violet spectrum rays that most powerfully excite these sen- 

 sations. According to Cleric Maxwell, the orange spectrum rays excite 

 the red sensation more strongly than the brightest red rays, but also 

 excite the green sensation; the greenish -yellow rays excite the green 

 sensation more strongly than the purest green rays, but also excite the 

 red sensation; the yellow rays excite the red sensation as intensely as 

 the brightest red rays and the green sensation as intensely as the pur- 

 est green rays. 



The carrying out of my new ])rinciple, according to Maxwell's meas- 

 urements, therefore, involves the production of one negative by the 

 joint action of the red, orange, yellow, and yellow-green rays, in defi- 

 nite proportions, to represent the red sensation ; one by the joint action 

 of the orange, yellow, green, and green-blue rays, in definite ijropor- 

 tions, to represent the green sensation; and one by the joint action of 

 the blue green, blue, and violet rays, in definite proportions, to repre- 

 sent the blue sensation. 



Negatives of the required character can be made by exposing a cya- 

 nine-stained gelatine-bromide plate through a double screen of chryso- 

 idine-orange and aniline-yellow of suitable intensity for the red sensa- 

 tion, a cyanine-erythrosine gelatine-bromide ])late through a screen of 

 aniline yellow of suitable intensity for the green sensation, and an ordi- 

 nary gelatine-bromide plate through a double screen of crysophenine- 



Journal of llie Franktln InstUKic, January, 1889. 



