8 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 



