OCR Text |
Show 1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 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 laminse 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 microscope certainly less than 0*1 of one smallest division of the micrometer. The index of actual value for one division, with the power applied, was 0*0063, thus giving an actual value less than 00006 m m. The so called iridescence of feathers might be thus explained. An underlying pigment complicates the problem a little. A smooth, glossy surface may 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 represented 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. W e 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 Pitta (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 m m . Explanation of the Objective structural Colours, i.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 transmitted 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 |