ntion than it has received. In such Humming-birds as I have
examined,--and possibly it may be a general rule,--the iridescence of
those portions of the plumage that are changeable is splendid in the
ratio of the acuteness of the angle formed by the incident ray and the
reflected one. Thus the scaly plumage of the neck of the Mango appears
to advantage in a room with a single window, only when the beholder
stands with his back to the light, and has the bird before him and
facing him. Then the perpendicular band down the throat and breast,
which seems composed of the richest black velvet, is bounded on each
side by a broad band of glowing crimson, mingled with violet. It is not
the _entire_ plumage of even a Humming-bird that displays these
refulgent gleams: some of the brilliant hues are permanent, not
changeable colours; such as the golden greens which adorn the back and
wing-coverts in so many species; in which the colour is subject to
little change, and the only effect produced by the alteration of the
angle of the light is the transforming the tips of the feathers into the
appearance of burnished gold.

Wilson[207] has remarked that the plumage of the Indigo finch
(_Fringilla cyanea_) in certain lights appears of a rich sky-blue and in
others of a vivid verdigris green, so that the same bird, in passing
from one place to another before your eyes, seems to undergo a total
change of colour. When the rays of light so fall on the plumage that the
angle of the incident and reflected ray is acute, the colour is green,
when obtuse, blue. I have myself noticed exactly the same thing in the
brilliant changeable colour of insects,--as, for instance, the
_Cicindelae_ of America, and the Emerald Virgin Dragonfly (_Agrion
Virginica_.)

To return, however, to our Humming-birds, of which my readers will like
to have one or two more described,--_la creme de la creme_, the very
_elite_ of this lovely little fairy population. If we were to cross the
Atlantic to Brazil, track up the mighty Amazon some thirty days' sail,
and a distance of a thousand miles, we should come to the mouth of the
Rio Negro, where a remarkable change in the appearance of the water
indicates a totally different region. Instead of the muddy water of the
Amazon, resembling pea-soup, that of the Negro is intensely dark, but
clear and limpid, every ripple sparkling like crystal. The land becomes
high, and the river, some four miles wide, passes between lofty cliff