xi COLOUR AND SONG 299 



ridges, with the very narrowest interstices between 

 them, 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 



Fig. 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." 



(II.) 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 



