THE STRUCTURE AND LIFE OF BIRDS chap. 



(I.) Objective Colours. 



Colours due to pigment alone may be : (a) black ; 

 (b) brown ; (V) red ; (d) yellow or greenish-yellow ; 

 (e) very rarely green ; in fact, only in the feathers 

 of the Touracou. 



White pigment has been found in the pineal eye 

 of the Lamprey and in one butterfly. 1 But it does 

 not, as far as is known, exist elsewhere in nature. 

 Black, brown, and red are always due to pigment 

 alone, yellow sometimes. These are occasionally 

 modified by another layer of pigment overlying 

 them. 



Blue and violet belong to the second division of 

 objective colours ; pigment and structure combine 

 to produce them. Take any blue feather and hold it 

 up to the light, so that the rays pass through it. It 

 is no longer blue, but a dull black or gray. Hammer 

 it, and all the blue vanishes. Green, except in the 

 one case mentioned, is also a pigment-structural 

 colour. Hammer a green feather from a Parrot, 

 and it becomes yellow, the colour of its pigment. 



No blue pigment has been found; that which a 

 blue feather contains is black-brown to yellow. The 

 colour which it presents to the eye is due to the 

 structure of the horny feather coating which encases 

 the pigment. It has been found that under a thin 

 outer sheath, there are a number of small polygonal 

 cones ; from the surfaces of these cones project ex- 

 tremely fine ridges, and it is believed that to these 

 1 See Beddard, A?iimal Coloration, p. 4. 



