3eo 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. 
“Tn 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. 
