PHYSIOLOGY OF COLOE VISION- — KELLER AND MACLEOD. 7S5 



When he is compelled to Icvver the saturation of one color he must 

 be careful to apply those neighboring on it in such a manner as to 

 give the impression that the whole of that portion of the picture is 

 of the same brightness. This he may do, either b}^ making his pig- 

 ments of similar saturation of by assorting the size of the colored 

 areas, so that they appear by contrast to be of similar saturation. 



It is a well-known fact that our judgment of the relative brightness 

 of colors, and to a certain extent of their hues, becomes altered when 

 the conditions of illumination are changed. A picture viewed in 

 broad daylight may create a very different impression from that 

 which it produces in dull illumination. For example, its hues may 

 be dull and muddy under the conditions of illumination that are 

 ordinarily present in a dwelling, or even in a gallery, whereas when 

 viewed in broad daylight 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 daj''- 

 light may in dull light actually shift to some other part. These 

 changes are due to what is known as adaptation of the retina. The 

 most striking illustration of this is furnished by observing the colors 

 of a flower border after sundown. Let us suppose that the border 

 contains geraniums (scarlet), lobelia (blue), and coreopsis (orange). 

 As darkness approaches it will be noticed that the red geraniums be- 

 come duller and duller until at last they turn black; that the orange 

 coreopsis also becomes more neutral, but that the blue lobelia main- 

 tains the same color qualities as it possessed in daylight. The most 

 remarkable change of all occurs, how^ever, not in the hues, but in the 

 relative brightnes of the colors, for it will be noticed that the sensa- 

 tion of greatest brightness has gradually shifted from the reds and 

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

 may actuallj' come to appear brighter than the coreopsis and the 

 geraniums. It is needless to point out how important an apprecia- 

 tion 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 ex- 

 pects it to be viewed. The physiological explanation of this adapta- 

 tion is that the outer portions of the retina assume a much greater 

 degree of sensitiveness in dull light, indeed, the}' come to be more 

 sensitive than the central portion itself. This curious change ex- 

 plains 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 Ave 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. 



Wheii we come to apply many of the principles of chromatics in 

 art, we are met with difficulties which at first sight may appear to 

 \>e insurmountable. In most instances, however, this is by no means 



