2 THE COLOURS OF ANIMALS 



from the surface of the paper, re-traverses the glass, 

 and emerges. Similarly, in painting, bright effects 

 are produced by covering a surface of Chinese white 

 with the desired colour. The light passing twice 

 through the thickness of the colour, absorption is 

 far more complete than when only one thickness is 

 traversed, as in a piece of red glass held up to the 

 light. Absorption being more complete, the red colour 

 is deeper. Animal pigments are nearly always twice 

 traversed by the light, and therefore a very thin layer 

 produces a considerable effect. 



Animal colours are therefore generally due to 

 precisely the same optical principle which causes the 

 colour of a wall-paper, a carpet, or a picture. Certain 

 transparent animals are, however, for the most part 

 coloured by light which passes but once through them, 

 upon the same principle as the colours of a stained- 

 glass window. The beautiful transparent blue of 

 many pelagic animals, such as the Portuguese Man- 

 of-war (Physalia), is caused in this way. 



It would be out of place to discuss the details of 

 the causes of colour by absorption. I may, however, 

 mention that vibrations of very different rates are 

 started in the luminiferous ether by the sun, the 

 electric light, &c. A certain series of these vibrations 

 causes the effect of white hght when it falls on our 

 retina ; but there are vibrations above and below 

 this visible series — vibrations which we cannot see. 

 We can, however, prove their existence in other ways ; 



