460 TEE POPULAR SCIENCE MONTHLY 



In general however it may be taken as a working hypothesis that good 

 combinations are always more than 80°-90° apart on the circle, that is, 

 they should be separated from one another by about one quarter of the 

 circumference. Even complementaries may form displeasing combina- 

 tions {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. When we are compelled to appose hues 

 having a hurtful influence on one another, the unpleasing impression 

 which they create may be lessened by certain tricks, such as by assign- 

 ing one of the hues to a much smaller field, or by decreasing the satura- 

 tion of one of them, or by adding a third hue whose position on the chro- 

 matic circle is as far as possible removed from the others : thus the dis- 

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

 addition of some violet, etc. 



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

 color, its hue, although in doing this it has been impossible entirely to 

 neglect the closely related qualities of brightness and saturation. 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 meas- 

 ured by finding a gray which appears of equal brightness to that of a 

 given color. Such measurements may be made with considerable accu- 

 racy by finding a gray background against which 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 grayness, 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 gradually move a small 

 piece of colored or of gray paper, mounted on a suitable handle, from 

 the periphery towards the center of the surface. At a certain position 

 the colored paper will be seen as gray because the rays of light from it 

 are striking the color-blind areas of the retina. Various grays are used 

 until one is found which matches exactly with that created by the 

 colored paper. A still simpler method consists in rotating the color on 

 a Maxwell disc along with a synthetic gray. In this case judgment of 

 equality may however be somewhat confused, on account of the gray 

 assuming the complementary hue. 



Brightness ])lays a most im]iortant part in the phenomenon of con- 



* The power to jiulgc hue depends on the presence in the retina of peculiar 

 nervo endings called cones. These are absent from the peripheral portions and 

 only gradually make their appearance towards 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 

 gray. 



