YELLOW FILTERS AND CHROMATIC ABERRATION 195 



If a particular color-focus lies squarely in the visual-cell layer, the ho- 

 monomous color-value of the external visual field will be crisply focused 

 but all others will be represented, at the level of the visual cells, by sets 

 of blur-circles. Fortunately the ends of the spectrum are much less bright 

 than the yellow region; but even so, chromatic aberration results in a 

 considerable blurring of the image. 



In the fovea, chromatic aberration is partly compensated for (except in 

 birds) , by the greater length of the foveal cones (how many things we 

 find we can do with that greater length!), for a greater number of color- 

 foci can thus lie in the length of one cone. But foveal cones have their 

 limits in length, and fall far short of dealing adequately with chromatic 

 aberration by such means. Obviously, there would be no chromatic aber- 

 ration if a single wavelength of light were passing through the eye to the 

 receptor layer. To bring this about, however, would mean the elimination 

 of color vision — and of nearly all the light. A compromise must, then, be 

 made by which the spectrum is narrowed down enough to make a big 

 dent in chromatic aberration, without sacrificing much of the physiologi- 

 cally effective energy of whole sunlight, or many of the colors which 

 occur most commonly in nature. 



A yellow filter serves this purpose admirably. It cuts out much of the 

 violet light and some of the blue, which are the colors responsible for 

 most of the chromatic aberration, as Figure 83 demonstrates. At the same 

 time it lets through, unimpeded, most of nature's hues, and passes the 

 spectral regions which look brightest to both light- and dark-adapted eyes. 



Other Values — This reduction of the effects of chromatic aberration is 

 not the only performance of a yellow filter. Most scattered light is of 

 short wavelengths, and under bright-light conditions this scattered light 

 results in glare. Glare and dazzle are minimized by a yellow falter. Simi- 

 larly, the unfocusable shortwave light scattered in the atmosphere, and 

 responsible for the bluish cast of distant mountains and for the blue of 

 the sky, is cut out by a yellow filter which, as every photographer knows^ 

 creates a sharper image. 



Still another effect is the enhancement of contrast. It will be recalled 

 that the same color-sensation can be aroused by different mixtures of 

 wavelengths. One can easily find, say, two books on the shelf whose col- 

 ors appear to be identical blues or greens. Yet a spectral analysis of the 

 light reflected from them might show the like-seeming colors to be wholly 

 different in wavelength composition. Almost any filter of colored glass or 



