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MANUAL OF ZOOLOGY 



SECT. 



The colours of feathers present great 

 variety. Black, brown, red, orange, 

 and yellow colours are due to the pres- 

 ence of definite pigments, i.e. are 

 absorption colpurs. White, and in some 

 cases yellow, is produced by the total 

 reflection of light from the spongy, air- 

 containing substance of the feather, 

 there being, as in nearly all other natu- 

 ral objects, no such thing as a white 

 pigment. Blue, violet, and in some 

 cases green, are produced by the light 

 from a brown pigment becoming bro- 

 ken up as it passes through the super- 

 ficial layer of the feathers in its passage 

 to the eye ; no blue or violet pigments 

 occur in feathers, and green pigments 

 are very rare. The beautiful metallic 

 tints of many birds are entirely the 

 result of structure, owing their existence 

 to a thin, transparent, superficial layer, 

 which acts as a prism : in such feathers 

 the colour changes according to the 

 relative position of the bird and of the 

 eye of the observer with regard to 

 the source of light. 



There is also infinite variety in the 

 general coloration of birds. In many 

 the colouring is distinctly protective, 

 harmonising with the environment, and 

 even changing with the latter, as in the 

 ptarmigan, which is greyish brown in 

 summer, white in winter, the former 



