THE PHYSICAL CAUSE OF ANIMAL COLOUKS 9 



obliterating the chinks and excluding the air ; otherwise 

 the colours would be more brilliant than ever, because 

 the refractive power of air is even lower than that of 

 the liquids. The brilliant metallic appearance of 

 many chrysalides, especially in the genus Vaiiessa, 

 is caused by a large number of films of liquid enclosed 

 between the laminae of the dense outer layer. If the 

 pupa be kept in spirit or water the colour remains, 

 but disappears directly it is dry, although it can be 

 renewed any number of times by wetting. This may 

 even occur in a living animal, for Dr. Sharp has just 

 directed my attention to an interesting observation 

 made by Dr. Nickerl, who found that a brilliantly 

 golden beetle (Carabus auroyiitens) lost all its lustre 

 after hybernating in captivity, but entirely regained 

 it after drinking some water. 



Colours due to diffraction 



When white light falls upon a surface on which 

 there are a number of fine parallel grooves the re- 

 flected light appears coloured, the colour varying with 

 the angle at which the light falls on the surface, and 

 with the angle at which it is seen. This is due to the 

 light reflected from different portions of the surface 

 having different distances to travel before reaching the 

 observer : and when (as occurs when the grooves are 

 very close together) these differences amount to half 

 a complete undulation for any particular length of 

 vibration, interference is caused, and the vibration 



