6 THE COLOURS OF ANIMALS 



bubbles of gas which are contained in their inter- 

 stices. Fat is also made use of to give a white 

 appearance ; and the same result may be obtained by 

 the presence of minute granules, probably akin to 

 pigment, but differing widely from it in optical pro- 

 perties, in that no absorption takes place. 



Colours due to thin plates 



It has been stated already that when light traverses 

 a sheet of glass surrounded by air, a certain pro- 

 portion of it is reflected back at the first surface and 

 a certain proportion at the second surface. The 

 light will be reflected in the same direction from 

 both surfaces. It is believed that the vibrations of 

 ether, some of which affect us as light, are in the form 

 of undulations of different lengths ; if, therefore, the 

 sheet of glass be sufficiently thin, some of the undula- 

 tions reflected from the second surface will interfere 

 with those started from the first surface. This will 

 happen when the sheet is of such a thickness that the 

 wave of light reflected from the second surface is half 

 an undulation behind that reflected from the first 

 surface ; for then the two sets of undulations will be in 

 opposite directions, and will therefore neutralise each 

 other. 



This will be quite clear if we apply the same 

 reasoning to those visible undulations from which 

 the name itself has been borrowed the waves on the 

 surface of water. If a set of ripples is started by the 



