XI COLOUR AND SONG 299 
ridges, with the very narrowest interstices between 
thenl, the colour is due. No doubt they somehow 
break up the light rays, but how they act in com- 
bination with the underlying pigment I am unable to 
explain. Violet and green feathers have no cones 
like these, but long thin ridges lying close together 
have been found, so that in them too the cause of the 
colours is the same. : 
The underlying pigment in the case of the green 
feathers is yellow. There is one structural colour 
which seems to be produced without the help of 
Fic. 73.—(After Gadow). Cone with fine ridges, found in blue feathers. s, thin 
surface layer overlying cone ; R, R, rays of light. 
pigment, or possibly the pigment has not been dis- 
covered. However this may be, the resulting colour, 
yellow, is found in the feathers of the Pitta or “Ant 
Thrush.” 
CII.) Subjective Colours. 
Take a feather which has a metallic lustre. The 
Bronze-winged Pigeon’s will do fairly well ; Humming 
Birds, I think, supply the best. Hold it horizontally 
on a level with the eye, and look along it from either 
end or across it, and it looks simply black ; but look 
down upon it, and move it to and fro, and it will 
