THE PERCEPTION OF LIGHT 



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THE PERCEPTION OF COLOUR 



We have already seen that among Invertebrates the phototactic 

 reactions of some Crustaceans vary with the wave-length of the 

 stimulating light and that colour vision on the perceptual level has 

 been demonstrated in some Insects ; in the vertebrate j^hylum 

 its undoubted occurrence as a significant factor in behaviour has been 

 substantiated in only a few classes — ^teleostean Fishes, a few Reptiles, 

 Birds and the higher Primates. The subject, however, raises many 

 intriguing questions. So far as the evidence goes, the eyes of all 

 vertebrates including man are stimulated by approximately the same 

 range of the spectrum (760 ni/M-SOO m/x) with the highest sensitivity 

 at a band with a wave-length varying between 500 and 550 m/x ; it 

 is no coincidence that this corresponds roughly with the transmission 

 spectrum of water. The visual mechanism of Vertebrates was first 

 evolved in ^^■ater and their photo-pigments were presumably developed 

 as sensitizers to allow their possessors to leave the brightly-lit surface 

 and penetrate more deeply into the darker depths of the sea ; and it 

 would be surprising if their descendants discarded a mechanism which 

 their ancestors had found of such value. It is true that Hamilton and 

 Coleman (1933) demonstrated in the homing pigeon a capacity of hue- 

 discrimination slightly beyond the limits of human perception, a 

 faculty which may apply to the stickleback, Gasterosteus, in the ultra- 

 violet region of the spectrum (Merker, 1934-39) ; but in general, so 



