1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 417 
position A; if we turn the bird to position B it will, let us say, look 
green; and halfway between B and C this bird will assume a blue 
colour, which again passes into violet before appearing black again 
in position C. If the bird begins with bronzy red, it will change 
through golden green to pure green, then through bluish green to 
blue and violet. There is not a single feather which, if moved from 
B to A, changes from green to blue, in other words from the violet 
to the red end of the spectrum. Thus we are able to predict into 
which colour a bird can change if we know its colour in position B. 
Thus a blue feather can only become purple or violet ; a green one 
has more changes, and a golden-green one still more. It is, 
however, very rare that a feather changes through more than half 
of the spectrum; a coppery-red feather will generally cease with 
green; a violet feather cannot change at all, except into black, since 
beyond the violet there are no visible colours in the spectrum. 
Another important fact is that metallic feathers can appear in any 
colour which is represented in the spectrum, but not in any which, 
like brown or grey, are not spectral colours. All these circumstances 
induce me to explain the changeable metallic colours as prismatic ; 
and in order to prove this we have to examine the feathers for their 
prismatic structure. In any metallic feather the metallic colour is 
confined to the radii which are entirely devoid of cilize, and consist 
of a series of variously shaped compartments which overlap one 
another like tiles of a roof. 
Fig. 1, Plate XXVIII., represents a barbule of a violet feather of 
Aithopyga; it consists of about fifteen compartments, each 0-03 
mm. long and 0:03 mm. broad, and each forming a plane. Fig. 9 a, 
Plate XXVII., shows part of a barbule of Nectarinia famosa, brilliant 
green in position B, blue in C, black in A; each compartment was 
0:027 mm. long and 0:015 mm. broad. If turned on the edge and 
looked upon sideways, they look like fig. 94; each compartment is 
convex-concave, with the convex side lying uppermost turned towards 
the light. In Jacamar, coppery red in positions B and A, green in C, 
they measure 0:040 by 0°018 mm. Every one of these compart- 
ments is surrounded by a transparent horny coat, the thickness of 
which varies from 0°00085 mm. (Sturnus) and 0:0015 mm. (Jaca- 
mar), 0°0012 (Athopyga) to 0°0022 mm. ( Galbula tombacea). The 
surface of this coat is either perfectly smooth and polished, like in 
Nectarinia famosa, or, in violet feathers, contains very fine longitu- 
dinal ridges, or, as in Jacamar, it showed very fine and numerous 
little dots. Below this transparent and apparently colourless sheath 
lies brownish to black pigment evenly dispersed. 
Now, then, let us take a transverse section of such an arrangement 
(SS being the transparent sheath, P the pigment), and put it in a po- 
sition that the light falls upon it from L (fig. 2, p. 418). A very small 
part of the orbit of a circle or any curve may be treated as a straight 
line ; we thus may regard this sheath as consisting of a number of 
small prisms. We know that a prism in such a position, with the 
top or one edge directed upwards, breaks the light in such a way as 
to produce a spectrum on the side furthest from the light, with the 
