Page:Popular Science Monthly Volume 83.djvu/466

462 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 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 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 greatest 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 portions 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 insurmountable. 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 processes 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 pigments. But this is wrong, the apparent difference being really due to a very simple cause, namely to the fact that by mixing pigment we subtract color rays from entering the eye, whereas we add such rays when we mix colored lights. To make this clear let us return to our example of blue and yellow. When we use these as pigments, we must remember