COLOUR 97 
and Megaloprepia, the surface of the rami and radii is smooth 
and quite transparent, while between it and the pigment exists a 
layer of small polygonal bodies, similar to those of blue feathers. 
Glue has not yet been discovered as a pigment. Blue feathers 
contain only orange or brownish pigment; the blue appears only 
on the shafts of the rami and larger radii. The structure of blue 
feathers seems to be always the same: (1) a transparent, colour- 
less layer of ceratine, from 0°004 to 0:007 mm. in thickness; (2) 
a layer of polygonal, more or less pyramidal, and often hexagonal 
columnar cells, each of which is colourless itself, and its walls are 
highly refractory and not unfrequently striated and ridged ;1 (3) 
the horny narrow cells of the inside of the radius, with brown, 
black, or orange pigment corpuscles. 
The blue naked parts of the skin of Cassowaries contain yellow 
or black pigment covered by peculiarly modified epidermal layers. 
Til. Subjective structural, prismatic, or metallic colowrs.—These 
colours change according to the position of the light and the eye 
of the observer, and they always change in the order of those in the 
rainbow. ‘They are restricted, as a rule, to the radii without cilia, 
and moreover to those parts of the feathers which are not covered by 
others. The metallic portions of the radii are composed of one row 
of compartments, which often partly overlap each other like curved 
tiles. In the inside black or blackish-brown pigment is collected ; 
and each compartment is covered with a transparent colourless layer 
of extreme thinness, eg. 0°0008 mm. in Sturnus. The surface of 
this coat is either smooth and polished as in Nectarinia, or exhibits 
very fine longitudinal wavy ridges when the feather is violet, or 
numerous small dot-like irregularities as in Galbula. The coating 
seems to act like a number of prisms, as indicated in the first 
figure. All metallic feathers appear black when their surface is 
parallel to the rays of the light in the same level with the eye and 
the light. To the eye of the observer at A, in the lower part of 
-the first figure, the metallic collar of Ptilorhis magnifica will 
appear absolutely black; the eye at B will see it bright coppery 
red, and at C rich green; the metallic feathers of the sides of the 
breast in the same bird will change from black to green at B, and 
to blue at C. The beautiful Pharomacrus mocinno changes from 
greenish bronze through golden green, green, and indigo to violet. 
Oreotrochilus chimborazo in position B exhibits the whole solar 
spectrum, namely, violet and red on the head, followed by orange 
and green on the back, blue, violet, and lastly purple on the 
1 In Pitta moluccensis I calculated the following measurements: width of 
one polygon 0°001 mm., height of same 0°015 mm., thickness of its transparent 
coating about 0°0012; distance between two of the longitudinal ridges on the 
surface of the polygon 0°0005, thickness of the transparent outer layer of the 
radius about 0°005 mm. 
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