462 TEE POPULAR SCIENCE MONTHLY 



light it may sparkle with brilliancy ; or there may be very little change 

 in the actual hues, but the portions of the picture which appeared to be 

 of greatest brightness in broad daylight may in dull light actually shift 

 to some other part. These changes are due to what is known as adapta- 

 tion of the retina. The most striking illustration of this is furnished 

 by observing the colors of a flower-border after sundown. Let us sup- 

 pose that the border contains geraniums (scarlet), lobelia (blue), and 

 coreopsis (orange). As darkness approaches it will be noticed that the 

 red geraniums become duller and duller until at last they turn black; 

 that the orange coreopsis also becomes more neutral, but that the blue 

 lobelia maintains the same color qualities as it possessed in daylight. 

 The most remarkable change of all occurs, however, not in the hues but 

 in the relative brightness of the colors, for it will be noticed that the 

 sensation of gi'eatest brightness has gradually shifted from the reds and 

 yellows to the blues and greens, so that the foliage and the lobelia may 

 actually come to appear brighter than the coreopsis and the geraniums. 

 It is needless to point out how important an appreciation of these 

 adaptations must be to the artist, how careful he must be to paint his 

 picture in the degree of illumination in which he expects it to be viewed. 

 The physiological explanation of this adaptation is that the outer por- 

 tions of the retina assume a much greater degree of sensitiveness in dull 

 light, indeed they come to be more sensitive than the central portion 

 itself. This curious change explains why without directly looking at it 

 we may be conscious of the presence of a small light in the darkness — 

 a star for example — which however disappears when we direct our gaze 

 to it. The ability of the thus sensitized outer portions of the retina to 

 judge colors differs from that of the central portion. 



When we come to apply many of the principles of chromatics in art, 

 we are met with difficulties which at first sight may appear to be insur- 

 mountable. In most instances, however, this is by no means the case, 

 and we shall now endeavor to show how certain of these difficulties can 

 be explained. First of all, with regard to the mixing of pigments as 

 compared with the mixing of colored lights, of course the two proc- 

 esses yield very different results: for example, mixing yellow and blue 

 lights, as we have seen, produces almost pure white, whereas mixing these 

 colors as pigments, as every artist knows, produces green. The entire 

 want of similarity in the results which follow the mixing of colors by 

 the two methods has had the effect of making some artists conclude that 

 the laws of chromatics are useless as guides in the practical use of pig- 

 ments. But this is wrong, the apparent difference being really due to a 

 very simple cause, namely to the fact that by mixing pigment we sub- 

 tract color rays from entering the eye, whereas wc add sucli rays when we 

 mix colored lights. To make this clear let us return to our example of 

 blue and yellow. "V^lien we use these as pigments, we must remember 



