Feathers have been studied from the earliest days of the microscope, indeed long before the modern microscope came into existence. Malpighei, Hooke and Leeuwenhoek all wrote on the subject, and not a little of our knowledge dates from their time. Since then authors have constantly written on feathers and their colors, until the papers on the subject may be counted by hundreds. Accordingly little that is new can be expected from this short article, nor even a history of the literature of the subject. My only object is to give an idea, so far as is known, how the colors of feathers are produced, the literature of the subject being out of the track of most American ornithologists.
Color may be the result of any one or more of the following causes: a pigment, interference and diffraction of light in their various phases, fluorescence, and phosphorescence. Of these causes only three have been called upon to explain the colors of feathers, the last two apparently playing no part. The fluorescence noted by Dr. Krukenberg in solutions of certain feather-pigments probably plays no part, or at most an insignificant one, in the colors of feathers. Pigments act by absorbing all rays of light but those which enter into their color, that is turn them into heat.
Interference acts in several different ways, all of which are based on the same principle, and so films may be taken as an example. If a beam of light, xy (figure 1), is allowed to fall on any thin plate, or film, part of the rays will be reflected in the direction yz, the angles byx and ayz being equal. The rest of the rays will pass through the film to the other surface, being slightly refracted in their course. Here part will be reflected, and being again refracted at the first surface, will emerge in a line wz′ nearly coincident with yz, the balance passing out into the air. Now the waves composing the white light of two beams yz and wz′ will run together and partially obliterate each other, after the manner of ripples on water. Accordingly certain waves will be obliterated, and since white light is due to the blending of waves of the different colors, the light reflected from the film will be that of the colors not interfered with, the waves thus obliterated depending upon their length and the thickness of the film traversed. So as we look at the film from different points the conditions vary, and with them the resultant color.
Interference may also produce colored light by means of fine particles diffused through another substance, as milk in water, the particles in the air, and the like. Colored light produced in this way is known as opalescent, the transmitted light tending to the red end of the spectrum, and the reflected to the other portions. This result can be obtained by mixing black and white grains, an experiment which all have tried as school boys, by soaking chalk in ink, the result being a bluish color.
Diffraction acts apparently by bending the light rays different amounts, and thus spreading out the spectrum. Explanations of the various phenomena of this sort are difficult, and need not be entered into here.
Feathers are classed, according to their appearance, into ordinary, metallic and iridescent, the peculiarities of which are well known and so need not delay us.
The ordinary feathers are colored by simple pigments, by contrast of light and darkness and mechanically, as in the case of the Bluebird (Sialia sialis). Pigments of various colors are known to occur in feathers, and have received special names, as turacin, zoönerythrin, zoöfulvin, zoöxanthin, zoöchlorin, zoömelanin. These evenly distributed, as turacin, zoönerythrin, and zoöfulvin, or in patches, as zoömelanin, impart their respective colors to the feather parts in which they exist.[[67]] The color of the mass of the feather may, however, owing to various colors in the small feather parts, be different from that of any part.
Of these pigments none seem to be peculiar except turacin. This pigment is altered by wetting the feathers, and comes from the feathers into the water in which the birds bathe, a fact of considerable interest, since the birds maintain their normal color, thus necessitating a new supply of pigment.
White feathers are the result of the light being reflected as a whole from the finely divided feather parts. Some grays are the result of small black nodes in the barbules, which nodes are of considerable size, and do not disperse the light, being distributed along the barbules. Other grays are the result of a small quantity of black pigment.
Yellow feathers colored with zoöfulvin receive their hue from this pigment, which is pretty evenly distributed through the texture like a dye.