. The structure and life of birds . our of its pigment. No blue pigment has been found; that which ablue feather contains is black-brown to yellow. Thecolour which it presents to the eye is due to thestructure of the horny feather coating which encasesthe pigment. It has been found that under a thinouter sheath, there are a number of small polygonalcones ; from the surfaces of these cones project ex-tremely fine ridges, and it is believed that to these1 See Beddard, A?iimal Coloration, p. 4. xi COLOUR AND SONG 299 ridges, with the very narrowest interstices betweenthem, the colour is due. No d
. The structure and life of birds . our of its pigment. No blue pigment has been found; that which ablue feather contains is black-brown to yellow. Thecolour which it presents to the eye is due to thestructure of the horny feather coating which encasesthe pigment. It has been found that under a thinouter sheath, there are a number of small polygonalcones ; from the surfaces of these cones project ex-tremely fine ridges, and it is believed that to these1 See Beddard, A?iimal Coloration, p. 4. xi COLOUR AND SONG 299 ridges, with the very narrowest interstices betweenthem, the colour is due. No doubt they somehowbreak up the light rays, but how they act in com-bination with the underlying pigment I am unable toexplain. Violet and green feathers have no coneslike these, but long thin ridges lying close togetherhave been found, so that in them too the cause of thecolours is the same. The underlying pigment in the case of the greenfeathers is yellow. There is one structural colourwhich seems to be produced without the help of. Fig. 73.—(After Gadow). Cone with fine ridges, found in blue feathers, s, thinsurface 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 AntThrush. (II.) Subjective Colours. Take a feather which has a metallic lustre. TheBronze-winged Pigeons will do fairly well ; HummingBirds, I think, supply the best. Hold it horizontallyon a level with the eye, and look along it from eitherend or across it, and it looks simply black ; but lookdown upon it, and move it to and fro, and it will 300 THE STRUCTURE AND LIFE OF BIRDS chap. show some of the colours of the spectrum followingone another in their right order. Beginning withbronze-red it will change to golden green, to green,and thus, in some cases, on to blue and violet. Thereis no brown or gray, but only the colours of thespectrum; another fact that makes it proba
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