96 PROCEEDENQS OF THE AMERICAN ACADEMY. 



These are quantities as determined by slit width in the spectrum or 

 a like process. There is sufficient predominance of luminosity in the 

 green to avoid trouble f^om chromatic aberration, and the actual work- 

 ing of the combination in giving photographs in natural colors is such 

 as to indicate proper color vision. As yet, however, no means are avail- 

 able for producing all three primary colors efficiently, and for white arti- 

 ficial light we are compelled to rely on what is in effect building up a 

 nearly continuous spectrum from heterogeneous components, unless as 

 usual we employ the continuous spectrum of an incandescent solid. 



