projecting branches, now probing here, now there, one moment above a flower and bending down to it, the next hanging below it, and thrusting up its crimson beak to kiss its nectar-tube from beneath, the cloudy wings on each side vibrating with a noise like that of a factory wheel, and its entire throat, breast and belly clothed in scaly plumage of the richest green, contrasted finely with the velvety black of all beside. This scaly plumage would flash brilliantly back the sun's light, like a noble emerald in the crown of a king; then, by the slightest possible turn of the bird, it would become black, all the light being absorbed; then, on another movement, it would seem a dark rich olive, and in an instant flame forth again with emerald effulgence, over which olive and black clouds were momentarily passing and repassing. The phenomenon of this changing lustre is worthy of more careful attention 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 _Cicindelæ_ of America, and the Emerald Virgin Dragonfly (_Agrion Virginica_.)