96 COLOUR 



it when dry. When bathing they colour the Avater red, and the 

 red feathers, when wet, are distinctly shot with blue. 



Tiiracoverdin is the only instance of a green pigment, and is only 

 found in the Musophagidae ; it contains comparatively much iron, 

 but no copper. 



Brown is the result of a mixture of red and black colouring 

 matter. 



JFhite 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 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 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 siu-face 

 is scratched or crushed, the blue colour instantly disappears, shew- 

 ing 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 colours reveals the following 

 structures : — 



Yellow. The radii and rami of many yelloAV feathers are in 

 reality without pigment, but their surface shews a number of 

 longitudinal ridges and furrows, as for instance in Ara, Ehaniphas- 

 tus, Ccereba, 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 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, 



