PHYSIOLOGY OF COLOK VISION KELLER AND MACLEOD. 733 



the other hand, when a colored area is very small it iindergoes no 

 complementary change, but merely blends with the neighboring color, 

 (■i) To obtain full advantage of color apposition the colored patterns 

 should be very simple and of similar texture and their surfaces should 

 be broken up by detail to the least possible degree. (5) The most 

 marked complementarj^ effects are obtained when the opposing hues 

 are of equal brightness. 



When we attempt to employ the chromatic circle for another pur- 

 pose. name]^^ foi' determining what will be pleasing and what dis- 

 pleasing color combinations, we find that its use is somewhat limited. 

 This is because a psychological influence enters into our judgment in 

 such cases. In general, however, it may be taken as a working 

 hypothesis that good combinations are always more tlian 80-90*^ 

 apart on the circle; that is, they should be separated from one an- 

 other by about one-quarter of the circumference. Even complimen- 

 taries may form displeasing combinations (i. e., certain reds and 

 greens), in which case, as Rood has pointed out, the hues are usually 

 far removed from the line which separates those that are cold and 

 warm. A^^ien we are compelled to appose hues having a hurtful 

 mfluence on one another, the unpleasing unpression which they create 

 may be lessened by certain expedients, such as by assigning one of the 

 hues to a much smaller field, or hj decreasing the saturation of one of 

 them, or by adding a third hue whose position on the chromatic circle 

 is as far as possil>le removed from the others; thus tlie disagreeable 

 effect of a yellowish-green and yellow is much improved by the addi- 

 tion of some violet, etc. 



So far, for the sake of simplicity, we have regarded but one quality 

 of a color, its hue, although in doing this it has been impossible en- 

 tirely to neglect the closeh^ related qualities of brightness and satura- 

 tion. These we shall now proceed to consider. 



Brightness is most marked, under ordinary conditions of illumina- 

 tion, around the yellow portions of the spectrum. It is a property 

 which is exhibited in marked degree by different grays. Indeed, it 

 is measured by finding a gi*ay which appears of equal brightness to 

 that of a given color. Such measurements may be made with con- 

 siderable accuracy by finding a gray background against Avhich the 

 color becomes indistinguishable when viewed by the very outermost 

 portions of the retina which are color blind ; that is, which see no hue 

 in a color but only a gi'ayness, the degree of which is proportional to 

 the brightness of the color.^ To make such comparisons, the person 

 must regard a dot in the center of a plain black surface and must then 



■> The power to judge hue depends on the presence in the retina of peculiar nerve end- 

 ings called cones. These are absent from the peripheral portions and only gradually make 

 their appearance toward the center. There is, therefore, a region between the periphery 

 and the center of the retina which is partly color blind, blue and yellow being perceptible, 

 but red and green still appearing as gi-ay. 



