To begin with it, we must think of all feathers first as exactly intermediate between the fur of animals and scales of fish. They are fur, made strong, and arranged in scales or plates, partly defensive armour, partly active instruments, of motion or action.† And there are definitely three textures of this
strengthened fur, variously pleasurable to the eye: the first, a dead texture like that of simple silk in its cocoon, or wool; receptant of pattern colours in definite stain, as in the thrush or partridge; secondly, a texture like that of lustrous shot silk, soft, but reflecting different colours in different lights, as in the dove, pheasant, and peacock; thirdly, a quite brilliant texture, flaming like metal,—nay, sometimes more brightly than any polished armour; and this also reflective of different colours in different lights, as in the humming-bird. Between these three typical kinds of lustre, there is every gradation; the tender lustre of the dove’s plumage being intermediate between the bloomy softness of the partridge, and the more than rainbow iridescence of the peacock; while the semi-metallic, unctuous, or pitchy lustre of the raven, is midway between the silken and metallic groups.
These different modes of lustre and colour depend entirely on the structure of the barbs and cilia. I do not often invite my readers to use a microscope; but for once and for a little while, we will take the tormenting aid of it.
In all feathers used for flight, the barbs are many and minute, for the purpose of locking the shafts well together. But in covering and decorative plumes, they themselves become principal, and the shafts subordinate. And, since of flying plumes we have first taken the seagull’s wing feather, of covering plumes we will first take one from the seagull’s breast.
John Ruskin, Laws of Fesole, 1877-9

Peacock feathers drawn by John Ruskin, and engraved for Love's Meinie (left) and Laws of Fesole (right)
Peacock feathers from the drawing collection
at the Ruskin Library. These came from one of the peacocks - Castor and Pollux - who for many years roamed freely in the University grounds.