192 ADAPTATIONS TO DIURNAL ACTIVITY 



colored oil-droplets. As long as these held the stage, the mental myopia 

 of investigators prevented anyone's noticing the other types of filters 

 and using them to help explain the baffling oil-droplets. 



The Color-Vision Theory — The oil-droplets were formerly believed 

 to occur in a much greater variety of colors than is actually ever the case. 

 Those of birds seemingly ran the gamut of the visible spectrum; but 

 under modern apochromatic microscope lenses the violet, blue, and green 

 droplets lose their colors and are seen to be actually devoid of pigment. 

 They owe their chromatic appearance, under cruder lenses, to purely 

 optical phenomena. Only red, orange, and yellow droplets occur in birds 

 and turtles along with some colorless droplets. Most groups provided 

 with colored droplets contain nocturnal species whose droplets are all 

 colorless. The pigments involved are carotenoids, and those extractible 

 from chicken retinas have recently been tentatively identified as astacin, 

 sarcinene, and xanthophyll. 



When belief was current in a more complete spectral representation, 

 the theory of oil-droplet function first advanced by Krause in 1863 (and 

 based at first upon the supposition that lizards, as well as birds, had 'all' 

 colors) was most popular, and still has adherents. According to this 

 theory, each color of oil-droplet makes possible the independent sen- 

 sation of the corresponding color in the spectrum. The supposition was 

 that the bird has but one (not three) photochemical substances in its 

 cone outer segments (see p. 91), and that this undifferentiated sub- 

 stance would be affected equally by any and all visible wavelengths of 

 light. Discrimination of wavelengths on a qualitative basis — color vision, 

 in other words — would be possible only if certain cones were allowed 

 to be stimulated only by certain wavelengths, others by other wave- 

 lengths, and so on. The differently colored oil-droplets, standing in the 

 pathway of the light on its course toward the percipient outer segments, 

 were supposed to ensure this differential stimulation of different sets of 

 cones, which in turn connected with different sets of brain cells in which 

 the respective color sensations were registered. This mechanism of color 

 vision has seemed so simple and plausible that some students of human 

 visual physiology have fled to it as a refuge from the necessity of think- 

 ing through the state of affairs where, as in man, color-vision occurs with 

 all the cones alike, and have postulated that minute colored oil-droplets 

 occur in human cones— the while being careful not to look to see if they 

 are really there. 



