1882.] DR, H. GADOW ON THE COLOUR OF FEATHERS. 411 
the colour itself, is the result of their surface being smooth and 
polished : if the surface is rough, the colours given to the feather by 
pigment appear more or less dull; but if polished, they will appear 
with a more or less strong gloss, and they will look much more 
saturated, e.g. brilliant red. The polished surface is produced by 
the horny substance of the feathers. 
3. Interference of colours and colour of thin plates. The thin 
plates are represented by the extremely thin laminze of the radii, or 
by a thin coating of the transparent ceratinine. These parts appear 
with a certain colour simply because they are thin; but instances 
of this are very rare, although the planes of the barbules are certainly 
thin enough to allow the application of colours of thin plates. In 
Galbula tombacea, for instance, the thickness of such a barbule-plane, 
where it contained only little or no pigment, was under the micro- 
scope certainly Jess than 0°1 of one smallest division of the micro- 
meter. The index of actual value for one division, with the power 
applied, was 0-0063, thus giving an actual value less than 0-006 mm. 
The so called iridescence of feathers might be thus explained. An 
underlying pigment complicates the problem a little. A smooth, 
glossy surface inay likewise be produced by a fine film of oil on the 
surface of the feathers, e.g. in water-birds. 
Application of the Theory of Colours which are produced by a 
system of narrow ridges. 
Almost every fine feather exhibits a sort of iridescence if we look 
through it towards the light. The system of fine lines is then re- 
presented by the series of radii or barbules on either side of the 
rami or barbs. That these parts are minute enough for this is 
proved by observation. We know that “ Gitterfarben’’ begin to be 
visible to the naked eye if there are about twenty interstices to 
a millimetre. Now in a feather taken from the neck of Péta (in 
the green part of the feather figured), I found the distance between 
the top of the two neighbouring barbules equal to 0°05 mm., or at 
another part = 0°04 mm. 
Explanation of the Objective structural Colours, 
z.e. colours which are due to a particular structure of the feather- 
substance, which contain a pigment differently coloured from the 
colour actually observed, and which are not variable. 
Blue feathers.—All attempts made by chemists to find a blue or 
a violet pigment in feathers have been unsuccessful. Such feathers 
contain only a black-brown to yellow pigment. The simplest proof 
of this astonishing fact is that such feathers, if examined with trans- 
mitted light under the microscope, appear-invariably brown. The 
blue feathers of Parrots lose this colour if held against the light, 
i. e. if examined under indirect light. 
Moreover, we can make a crucial test. If certain colours result 
from a particular surface-structure of the feathers, these colours must 
disappear if we destroy the supposed colour-producing parts. This 
