I 
THE COLOURS OF ORGANISMS 
n 
in a closely similar fashion. Glass is a colourless or 
transparent substance, when powdered it is white, 
when cut with prismatic edges it displays all the 
colours of the rainbow, and yet the qualities of the 
glass remain unaltered. These are of course very 
familiar facts, but it is important to realise also that 
many of the most brilliant colours of organisms are 
produced in a similar fashion—are adventitious and 
not due to the essential properties of the coloured 
substance. It is only in rare cases, however, that 
bright colours are produced in a way capable of 
simple physical explanation. There are usually 
complications arising from the presence of some 
amount of pigment, from the superposition of tissues, 
or from the complex nature of the individual tissues. 
Such brilliant optical colours occur apparently only 
in cuticular structures. It is a curious fact that, 
although such structures are of course not cellular, 
nor living, yet their colour frequently fades very 
rapidly after death ; that dragon-flies, for example, 
lose in a very short time all their gorgeous tints is a 
fact only too well known to collectors. This may be 
due to loss of water, or to changes in the underlying 
tissues. 
Structural colours are most brilliant and con¬ 
spicuous in birds and insects, but it is chiefly in the 
former that they have been studied. Dr. Gadow has 
especially studied the structural colours 1 of birds, 
and he divides them into two classes, according to 
their behaviour as regards incident light. Thus 
certain structural colours, such as green and blue, 
are unchanging in reflected light, and are then 
1 For a purely physical treatment see a pamphlet by B. Walter. 
