COLOUR IN NATURE 
CHAP 
for their full manifestation. Many of the most 
brilliant tints of animals are similarly optical colours, 
but as they are produced by the special structure 
of the coloured parts, they are more frequently 
known as structural colours. 
Structural or optical colours in organisms can 
be recognised by the following tests. They usually 
vary either with the angle of incidence of the light, 
or with the change from reflected to transmitted 
light; tissues or organisms showing these colours 
have a very marked surface gloss, and the colour 
is usually destroyed by injury to this surface; 
immersion in a neutral medium whose refractive 
index is different from that of air, also usually 
destroys the colour. Optical colours may further 
be recognised by the negative character that the 
coloured tissues do not yield to any reagent a 
pigment of the same tint as that which they them¬ 
selves possess. A peacock’s feather affords an 
excellent example of a type of structural coloration 
which is widely spread in birds. If the reader stand 
in front of a window and hold a peacock’s feather in 
his hand nearly on a level with the eye, and then, 
still with the feather at arm’s length, slowly describe 
a semicircle on his own axis, he will note that the 
colours of the feather undergo a complete cycle of 
changes. In this case, therefore, the colours change 
with the change of the angle of incidence of reflected 
light. If the feather be now held up to the light 
it will be seen that the colours disappear, to be 
replaced by a dull brown or black tint. Thus the 
feather changes colour according as it is viewed by 
reflected or transmitted light. The presence of a 
