34 THE BIOLOGY OF BIRDS 



or (c) by a combination of these. A high-coloured, ruddy 

 countenance is due to the pigment haemoglobin in the red 

 blood-corpuscles. The blood in the capillaries of the 

 dermis shines through the epidermis, and the skin is red, 

 as in a cock's comb or a turkey's wattles. The red colour 

 of a boiled lobster is likewise pigmentary. But the rainbow- 

 like colours seen on many a shell, when the mother-of-pearl 

 layer is exposed or when the inside of the shell is seen, are 

 due to the physical structure, to the thin lamellae of lime. 

 When the shell is pounded small, there is only white powder, 

 no pigment. A familiar characteristic of this sort of 

 coloration is that the colours change as the object is moved 

 about. The colours are " interference-colours," and 

 entirely due to the splitting up of the rays of white light. 

 The rainbow in a soap-bubble as well as in the sky is 

 altogether apart from pigmentation, as every one knows. 

 But, thirdly, some of the finest effects are due to a combina- 

 tion of pigmentary coloration and physical coloration, as 

 we see on a peacock's tail or on a butterfly's wing. There 

 is a sculpturing of the surface of the bird's feather or of the 

 scales on a butterfly's wing which enhances the value of the 

 pigment or pigments also present. 



The iridescent colours on feathers have been referred 

 to two causes, and it may be that the two interpretations hold 

 for different cases. According to Gadow and others, the 

 colours are due to dispersion phenomena caused by prisms 

 or fine lines. According to Mandoul (1902), the colours 

 are due to interference phenomena caused by delicate 

 lamellae in the superficial region of the feather. 



According to Michelson and Walter, the iridescence is 

 due to selective reflection from an intensely opaque material, 

 and, in a few cases, to diffraction from a finely striated 

 surface. According to Mallock (191 1), interference of some 

 sort is in most cases the active cause of the iridescence ; 

 although, in others, the possibility of selective reflection is 

 not excluded. The question turns on the size of the 

 " grain " of the colour-producing structure. Is it com- 

 parable with the wave-len^h of light or is it of molecular 



