302 ON THE COLOURS OF THE FEATHERS OF BIRDS. 



the result of their density, which is found to be nearly equal to that of 

 the surrounding medium. 



However, it is as well to observe, that if the green cotinga were 

 plunged into a much denser medium, as water, the effect would be 

 absolutely the same as in the air. 



Metallic colours have a brilliancy exactly similar to that of metals. 

 All the beards of the feathers thus coloured which I have observed 

 are furnished with fringes, which have the appearance of hardness. 

 These fringes are equally wide the whole length, and appear lopped 

 at the extremity ; when seen through a microscope we observe on their 

 surface a row of very luminous spots which appear hollow ; for they 

 are the more brilliant as the light falls perpendicularly; and become 

 darker as it approaches horizontally. On the copper coloured starling 

 of Africa, the barbs of the feathers (rig. 7) have fringes entirely black 

 on the outer side, along two-thirds of the quill, parting at the base. 

 The fringes of the interior side are longer, and black towards the 

 stock ; but the tworthirds towards the extremity are of a very dark 

 blue colour. These beards are imbricated with respect to each other so 

 that the black part of the beards is entirely hid. 



The golden cuckoo of Africa has golden feathers on the back which 

 are very brilliant ; the beards of these feathers (tig. 8) have also fringes 

 which are entirely coloured. The beards of the peacock's feathers are 

 deeply coloured, of a golden green ; but in this the fringes are convex so 

 that the quill appears sunk (fig. 9 and a which represents the sec- 

 tion of this beard). In the jacamar the beards (fig. 10) are perfectly 

 flat ; the light however plays on these beards, so that in certain posi- 

 tions the quill appears to be raised, in others on the contrary it appears 

 to be depressed. But this is only an illusion ; the truth will be evident 

 on cutting the beard. On the Ccerebaflaveola the golden feathers are 

 at first black, coloured only at the extremity. The beards of these 

 feathers are furnished with very thick fringes of unequal length and 

 strongly marked with depressed or concave spots. 



But however brilliant the colours which adorn the birds of which 

 we have just spoken, they are far inferior to that on the breast of the 

 Trochilus mosquitus. Let us examine in detail a feather of this 

 bird (fig. 12), and we shall soon discover the cause of that dazzling 

 brilliancy which distinguishes the colibri and the humming-birds. To 

 the naked eye this feather is two lines and a half in length ; we first 

 remark the quill, which is white at the base, and dark at the extremity ; 



