IO 
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 
