lo COLOUR IN NATURE chap. 



Most people are familiar with the analogous process 

 of producing a white colour by pounding up colour- 

 less glass, or crystals of blue sulphate of copper, 

 while the whiteness of snow, which has furnished so 

 many metaphors, is produced in a precisely similar 

 manner. 



The fact that the whiteness of all these sub- 

 stances is due to an optical effect and not to a 

 pigment should be thoroughly grasped, otherwise 

 those not accustomed to dealing with colour pheno- 

 mena will find much difficulty in comprehending 

 structural colours in general. White sunlight is 

 produced by the combination of all the tints of the 

 rainbow. When objects permit light to pass com- 

 pletely through them, we call them transparent ; when 

 they reflect all the rays of the light uniformly, we call 

 them white. This whiteness may be produced in 

 one of two ways. A substance such as " Chinese 

 white " is white because it is a property of the 

 particles of which it is composed to reflect equally 

 all the rays of incident light ; it is further a familiar 

 fact that Chinese white can be employed to impart 

 its own colour to other objects, that is, it can be 

 employed as a pigment. Snow, on the other hand, 

 is white, not because its individual particles reflect 

 the light — on the contrary they are transparent — but 

 because these transparent particles are separated by 

 bubbles of air. The incident light in passing from 

 the one medium to the other is bent or refracted, and 

 the result is the appearance of whiteness. A white 

 colour in organisms, except in very few cases, is 

 similarly produced, and is not due to pigment. 



Other structural colours are to be accounted for 



