THE CONSTANTS OF COLOR. 



647 



duction of a set of standard colored disks with known constants, which 

 can afterward be combined with each other, as well as with standard 

 black or white disks, so as to generate at will, with ease and certainty, 

 an immense number of tints whose constants will be known. If we 

 make a record of the constants involved in such experiments, we can 

 afterward reproduce the tints just as they originally were, or alter 

 them to any desirable extent. To carry out the letter of this it will 

 of course be necessary to view the standard disks under similar illu- 

 minations at different times, a point which can be secured with the 



Fig. 6. Ete-Piece for isolating the Tints of the Spectrum. 



aid of the photometer above referred to. The standard disks can 

 also be used for building up a set of standard charts, containing a 

 vast variety of tints of known composition, arranged methodically 

 with regard to purity, luminosity, and tone. These matters will be 

 considered at some length in a separate chapter, and are now only 

 hinted at as a justification for the trouble we have been at in defining 

 the constants of color. 



There is another point to be touched on in this connection. One 

 of the most noticeable things about colors is their difference in in- 

 tensity. Colors are intense when they excel both in purity and bright- 

 ness ; for it is quite evident that, however pure the colored light may 

 be, it still will produce very little effect on the eye if its total quan- 

 tity be small ; and, on the other hand, it is plain that its action on 

 the same organ will not be considerable if it is diluted with much 

 white light. Purity and brightness, or luminosity, are, then, the fac- 

 tors on which intensity depends. We shall see hereafter that this is 

 strictly true only within certain limits, and that an inordinate in- 

 crease of luminosity is attended with a loss of intensity of hue. 



Having defined the three constants of color, it will be interesting 

 to inquire into the sensitiveness of the eye in these directions. This 

 subject has lately been studied with care by Aubert, who made an 

 extensive set of observations with the aid of colored disks. 1 It was 



1 Aubert, " Physiologie der Netzhaut," Breslau, 1865. 



