12 
COLOUR IN NATURE 
CHAP. 
not readily distinguishable from pigmental colours. 
These Gadow classifies as Objective Structural colours. 
Again many colours change in tint according to the 
angle at which they are viewed. Such metallic 
colours may be classified as Subjective Structural 
colours. 
2. Of Objective Structural colours, green and blue 
afford the best examples. Green seems to be usually 
produced by a combination of a yellow pigment and 
a structural modification, wherefore green feathers 
usually appear yellowish in transmitted light. The 
display of a blue colour again seems, at least in birds 
and probably in insects, to be always associated with 
the presence in the tissues of a dark-coloured pig¬ 
ment. A blue colour in the feathers of birds is always 
confined to certain parts of the feather, and its 
presence is associated with considerable modifications 
of feather structure. Blue is not however confined 
to the feathers, but may occur also on bare patches 
of skin, as in some of the Paradise birds, in the 
cassowary, etc. Curiously enough, a blue colour in 
the skin seems to be particularly unstable, fading as 
rapidly after death as does the blue colour in the 
abdomen of some dragon-flies. 
The exact physical cause of green and blue colours 
is still doubtful, but the fact that the colours are 
structural is readily perceived both by the strong 
surface gloss and by the disappearance of the colour 
in transmitted light. 
3. While unvarying blue and green tints in birds 
rather heighten the general effect than display in 
themselves great beauty, the Subjective Structural 
colours, on the other hand, display the most ex- 
