I THE COLOURS OF ORGANISMS ii 



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^ 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. 



