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Show 1896.] METALLIC C O L O U R S O F BIRDS. 285 almost invariably absent from the wings, but where, as in the above species, the male as compared with the female is characterized by the development of a special pigmental colour, this pigment is entirely absent from the wing-quills, though present in the wing-coverts. Having thus described some of the special peculiarities of distribution of the metallic tints of the two families, it may be well to consider what is known as to this kind of colouring. The most important paper is that of Gadow (" The Coloration of Peathers as affected by Structure," Proc. Zool. Soc. 1882, pp. 409-421, 2 pis.; see also Bronn's ' Thierreich,' Bd. vi. Abt. iv. S. 575-584); but more recently there has been published a research from the physicist's standpoint ('Die Oberflachen- oder Schillerfarben,' von B. Walter : Braunschweig, vi+122 pp., 8 figs., 1 pi., 1895). Gadow distinguishes metallic colours as subjective, and thus contrasted with objective unchanging structural colours such as the green of many Parrots' feathers. H e examined numerous feathers showing metallic colour, and found that all looked black when the eye was placed in the plane of the feather between the light and the feather, and also when the feather was placed under a similar condition between the eye and the light. In intermediate positions certain of the colours of the spectrum could be observed in the order in which they appear in the spectrum. Thus a feather which when looked at from above is green, when successively moved through the positions named above, shows the colours black, green, blue, violet, black; while a red feather would usually show a greater, and a blue a less range of colour. Further, on examining certain metallic feathers microscopically, Gadow found that " in any metallic feather the metallic colour is confined to the radii which are entirely devoid of cilia, and consist of a series of variously shaped compartments which overlap one another like the tiles of a roof." 1 The direct physical cause of the colour Gadow considers to be the transparent sheath of keratin which covers the compartments, and which according to him acts like a series of prisms. Such metallic radii always contain blackish-brown pigment (melanin). Gadow's theory that the metallic colour of birds' feathers is due to the dispersion of white light by prisms is strongly opposed by Walter (op. cit.) on physical grounds. Walter holds that all the structural colours of animals are " Schillerfarben." H e does not appear to distinguish between Gadow's subjective and objective colours, but compares the pigments of the coloured tissues to such colouring-matters as fuchsin and " diamond-green." This analogy hardly seems to be compatible with our present knowledge of the melanin pigments in birds, but the question is not one which directly affects the present discussion. Eeturning to Gadow's description of metallic feathers, it is obvious that if the type described by him is of universal occurrence, 1 A similar statement in the article " Colour " in Newton's ' Dictionary of Birds' is qualified by the words " as a rule," but no details are given. |
