196 ADAPTATIONS TO DIURNAL ACTIVITY 



gelatine, placed before the eye, will make the two books look unlike; for 

 certain wavelengths reflected by the pigment of one, and absorbed by the 

 filter so as to change the color seen through the latter, are not necessarily 

 emanating from the other pigment at all. By absorbing wavelengths com- 

 mon to the two unlike mixtures, the filter brings out the fact that they 

 are unlike, which is something the unaided eye cannot detect. 



A filter thus produces contrast between colored areas which otherwise 

 would look alike and would therefore be without a discernible boundary 

 between them. This fact was put to important use in World War I, when 

 colored goggles worn by reconnaissance aviators enabled them to detect 

 green camouflage produced by paints whose reflection-spectra were not at 

 all like those of the chlorophylls of actual foliage. Modern 'foliage' 

 camouflaging is more troublesome to both adversaries, for it has to con- 

 sist of actual foliage, which must be replaced frequently as it fades. 



A filter naturally tends to abolish just as many contrasts as it pro- 

 motes; but promotion is in advance of abolition when yellow filters 

 and natural colors are under consideration. By cutting out the different 

 amounts of blue in different but alike-looking green mixtures, the greens 

 are made to look unlike; and almost any other contrasts can be sacrificed 

 by the animal if only those between greens, so numerous in nature, can be 

 enhanced. The oil-droplet type of filter has a special advantage, since the 

 many colorless or other-colored droplets scattered among the yellow ones 

 in the whole mosaic will permit the perception of any contrasts which the 

 yellow droplets tend to iron out, and vice versa. By altering the propor- 

 tions of the different colors of droplets in different parts of the retina, 

 particular color-contrasts are enhanced in particular parts of the visual 

 field. Thus in the pigeon the ventronasal three-quarters of the retina have 

 the yellow droplets predominant, giving maximal contrast of objects seen 

 against the sky by eliminating the latter's blue color; while the dorso- 

 temporal quadrant, being especially rich in red droplets, affords maximal 

 visibility to objects seen against the green of the fields and trees over 

 which the bird is flying. In World War II, antiaircraft observers have 

 stumbled onto such tricks, and have learned to use filters when scanning 

 the sky for enemy planes. 



One thing which yellow filters might do— but don't— would be to absorb 

 harmful ultra-violet rays before these could reach the delicate cone outer 

 segments. Experiments have shown, however, that none of these rays sur- 

 vive absorption in the cornea and lens of a pigeon's eye, whose oil-drop- 

 lets consequently cannot possibly be purposed to protect against them. 



