ON THE COLOURS OF THE FEATHERS OF RIRDS. 303 



it is the same with the stalks of the beards, which are of two sorts : 

 those at the base of the feather are black, slender, terminating in a 

 point, and furnished the whole length with long and very fine fringes. 

 Those of the other half are also furnished with fringes on the part 

 adjoining the quill, but they are of the finest gold colour at the ex- 

 tremity. This coloured portion is much extended on the intermediate 

 beards ; it is much less so on* those at the sides, which, at the same 

 time are very long, this causes the edge of the extremity of these 

 feathers to describe a perfect half circle, and makes them when im- 

 bricated over each other resemble the scales of a fish. If we detach 

 one of these beards (fig. 13), we shall see, that, like the entire feather, 

 it consists of a long quill, and that the little fringes of the first half are 

 long and similar to very loose hair ; but the coloured part of this beard 

 is very different : at first the fringes of it are much larger, the material 

 extremely dense, and, consequently, the surface of an extremely fine 

 polish. I have weighed the feathers of this species, and have found 

 that a feather from the breast weighs as much as three feathers of a 

 dull colour and of equal size. But the principal cause of the extreme 

 brilliancy of these feathers consists in the coloured part of each beard 

 being deeply hollowed in channels, and presenting to the light a 

 concave surface similar to that of a reverberator. Hence it follows, 

 that if the ray of light fall horizontally on the beard (fig. 14), which 

 represents the section of it, there can be no reflection, consequently, 

 the throat of the bird will be dark: if it fall diagonally (tig. 15), the 

 part a b will be enlightened, and the plumage will shine ; and if it 

 fall perpendicularly (fig. 16), the light will be divided into a thousand 

 rays in this kind of focus, and reflect a dazzling light. This form of 

 the feathers in the Trochilus mosquitus, and the play of light which is 

 the result of it, explains very clearly in my opinion why, with the least 

 movement, the throat of this bird changes in an instant from dark to 

 the most dazzling brilliancy. If the beard of these feathers (fig. 17)> 

 be examined through a microscope, we shall see four or five very 

 brilliant spots ; they appeared to me to be of a golden red in the 

 middle, and of a golden green at the edges : they are, as I said before, 

 concave, and form so many little reverberators ; the intervals between 

 them are also sprinkled with very brilliant spots. The throat of this 

 species, however, does not shine in every position which permits the 

 light to reach the interior of the channel formed by the beards of its 

 feathers. If the eye be placed between the light and the bird, so that 



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