300 THE STRUCTURE AND LIFE OF BIRDS chap. 



show some of the colours of the spectrum following 

 one another in their right order. Beginning with 

 bronze-red it will change to golden green, to green, 

 and thus, in some cases, on to blue and violet. There 

 is no brown or gray, but only the colours of the 

 spectrum ; another fact that makes it probable that 

 they are prismatic. The light when it falls on the 

 feather is broken up by a number of prisms in the 

 same way as drops of rain break up the light and 

 form a rainbow. 



" In the spring a livelier Iris changes on the burnished dove." 



These colours, then, are properly called iridescent. 



But it never happens that all the colours of the 

 spectrum are visible. Of this various explanations 

 have been suggested, for instance, that two prisms 

 overlap, and that the complementary colours produce 

 white light. Iridescent colours are seen also in 

 reptiles. The reticulated Python at the Zoological 

 Gardens shows wonderful changes when the sun falls 

 upon it. 



White stands by itself. It is due to the rays of 

 light being broken an infinite number of times and 

 reflected. It requires no pigment. To this breaking 

 of the rays is due the whiteness of powdered glass 

 and of snow. 



The lustre of feathers as distinguished from colour 

 is due to the reflection of the light from the polished 

 surfaces. 



