1896. ] METALLIC 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 pls.; see also Bronn’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+122 pp., 
8 figs. 1 pl., 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 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 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). 
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. ct.) 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 affects the present discussion. 
Returning 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, 
