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- 



