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Continued from Colour in Birds and
Chemical or Absorption Colors
The second class of colors, Objective structural colours, are
those which are produced by the combination of a certain pigment with a
special structure of, the superimposed colourless parts. Hereto always
belong violet and blue, green almost always, and occasionally yellow. Such
a feather, when examined under transmitted light, i.e. held against
the light, appears only in the color of its pigment. For instance, the
deep blue or green feathers of a Parrot will then appear only grey or yellowish.
The same happens when their polished surface is scratched or crushed, the
blue colour instantly disappears, showing only the blackish underlying
pigment, or yellow pigment in green feathers. When thoroughly wetted in
a bath, the feathers of the back of an Amazon Parrot appear brown without
a trace of green.
Microscopical examination of such colors reveals
the following structures :-
Yellow. The radii and rami of many
yellow feathers are in reality without pigment, but their surface shows
a number of longitudinal ridges and furrows, as for instance in Ara, Rhamphastus,
Coereba, Icterus, Xanthomelas, and Picus. Some of the radii of the yellow
fluffy pectoral tufts of Arachnothera have a diameter of 0.007 mm.; their
surface exhibits irregular ridges, separated by as many furrows;
the width of one ridge is less than 0.0007 mm., and the distance from ridge
to ridge about 0.002, so that the theory of colors of a system of narrow
gratings can well be applied to explain these colours.
Orange is occasionally produced by red pigment
with a yellow superstructure;
Green,
except in the case of the Musophagidae
mentioned above, is always due to yellow, orange, or greyish-brown pigment
with a special superstructure, which consists either of narrow longitudinal
ridges, as in Psittacula and in Pitta, or else, as in Chrysotis, Pitta,
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.
Blue feathers normally 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 usually the same:
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A transparent, colourless layer of ceratine, which
is from 0.004 to 0.007 mm. in thickness.
-
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
(In Pitta moluccensis the following measurements have been calculated:
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).
-
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.
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