1896.] ME*AI.tItl COLOURS OF BIRDS. 285 



almost invariably absent from the wings, but where, as in the 

 above species, the male as compared with the female is charac- 

 terized 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 

 Feathers as affected by Structure," Proc. Zool. Soc. 1882, 

 pp. 409-421, 2 pis. ; see also IBronn's ' Thierreich,' Bd. vi. Abt. iv. 

 S. 575-584) ; but more recently there has been published a re- 

 search from the physicist's standpoint ('Die Oberfliichen- oder 

 Schillerfarben,' von B. Walter : Braunschweig, vi-1-122 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. He examined 

 numerous featliers 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 micro- 

 scopically, 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." ' 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). 



Gadovv's theory that the metalUc 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." He 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 ailects 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, 



' A similar statement in the article " Colour " in Newton's ' Dictionary of 

 Birds ' is qualiBed by the words " as a rule," but no details are given. 



