96 COLOUR 
it when dry. When bathing they colour the water red, and the 
red feathers, when wet, are distinctly shot with blue. 
Turacoverdin is the only instance of a green pigment, and is only 
found in the Musophagide ; it contains comparatively much iron, 
but no copper. 
Brown is the result of a mixture of red and black colouring 
matter. 
White is never due to pigment ; in every white object its colour 
is due to there being an innumerable number of interstices between 
its molecules, or the air-cells in its substance. The whole substance 
of a white feather, the “ceratine,” is colourless, but its texture forms 
a fine network which diffracts and reflects the light. 
The gloss of feathers, independent of the colour itself, is the 
result of their horny surface being smooth and polished, when 
rough they appear more or less dull. 
II. Objective structural colowrs 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, ie. held against 
the light, appears only in the colour 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, shew- 
ing only the blackish underlying pigment, or yellow pigment in 
ereen 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 colours reveals the following 
structures :— 
Yellow. The radii and rami of many yellow feathers are in 
reality without pigment, but their surface shews a number of 
longitudinal ridges and furrows, as for instance in Ara, Rhamphas- 
tus, 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 colours 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 Musophagidz 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, 
