THE PHYSICAL CAUSE OF ANIMAL COLOURS 5 



between the refractive powers (viz. the power of 

 changing the direction of rays of light) possessed by 

 the grains of glass and the substance, such as air or 

 water, which lies between them. Thus the refractive 

 powers of glass and water are much nearer than 

 those of glass and air : hence a dry powder will 

 reflect far more than a wet one, and will appear 

 much whiter. 



To take a few familiar examples : snow is white, 

 because of the minute globules of air which refract 

 very differently from the crystals between which they 

 are entangled ; ice, on the other hand, is transparent. 

 If snow be compressed the air is driven out, and the 

 mass becomes transparent ; if ice be powdered it be- 

 comes white like snow. The froth of a coloured liquid 

 is not coloured like the latter, but is white. Milk and 

 fat are white because light is scattered from the 

 surfaces of the countless oil globules, which refract 

 very differently from the substance which lies between 

 them. The surface of well-polished glass is almost 

 invisible, because it reflects regularly, but a scratched 

 surface is very visible, because there are surfaces in 

 many different directions, which therefore scatter the 

 light, while the far more numerous surfaces of ground 

 glass scatter the light far more effectually and produce 

 a white appearance. 



The white markings of animals are produced in 

 various ways. White hairs and feathers owe their 

 appearance, like snow, to the number of minute 

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