304 ON THE COLOURS OF THE FEATHEUS OF BIRDS. 



the beak may be seen fore-shortened, the breast will shine with the 

 greatest brilliancy ; if, on the contrary, the bird be placed with the 

 breast uppermost, between the eye and the light, it will not shine at 

 all. This appears to me to be caused by the fringes being imbricated 

 (fig. 18), as may be seen with the aid of a microscope ; if the light 

 comes from a, it will fall on the barb b, and will be reflected ; and if 

 from c, finding no resistance, it will be absorbed in rf, and there will 

 be no reflection. There is another remarkable character in the golden 

 feathers of the colibri ; their beards are deeply crenate at their ex- 

 tremity, because the end of the quill is devoid of beards (fig. 13). 

 This extremity of the quill resembles a very loose hair, and is ter- 

 minated by a little swelling like the antennae of butterflies. 



This cause of the resplendency of the Trochilus mosquitus is found 

 in all feathers of extreme brilliancy, such as those on the throat of the 

 Trochilus auratus, T. pella, T. mango, &c. But some of these feathers 

 have not the beards crenate at the extremity. Those of the T. pella, 

 for example (fig. 19), are terminated in the form of a spear; at their 

 extremity is a beard which exceeds the others in length ; those of the 

 female (fig. 20), are also terminated in the form of a spear, but the 

 beards at the extremity are white, which contributes much to lessen 

 the brilliancy of this bird's breast. 



The metallic feathers of the humming birds do not all shine with 

 the radiance of which we have just spoken ; the beards of the feathers 

 on the back of these birds (fig. 21) are not channeled, but flat, and 

 terminate in a point, the lesser fringes being black at the base and 

 extremity, the middle only is coloured; from this cause the beards 

 have a longitudinal line of gold on each side of the quill : the feathers 

 on the back and breast are less brilliant in proportion as these lines are 

 narrower. 



From what has been said, it is evident that the metallic feathers 

 owe their brilliancy to their density, to the polish of their surface, and 

 to the great number of little concave mirrors which are perceptible on 

 the fringes; that the very brilliant feathers of the humming birds 

 differ from other golden feathers only in having their barbs hollowed 

 longitudinally, which produces an effect similar to that of a re- 

 verberator. 



