J882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 411 



tlie 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 laminae of the radii, or 

 by a thin coating of the transparent ceratiuine. 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 tomhacea, for instance, the thickness of such a barbule-plane, 

 where it contained only little or no pigment, was under the micro- 

 scope certainly less 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 O'OOOC mm. 

 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 lohich 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 Pitta (in 

 the green part of the feather figured), 1 found the distance between 

 the top of the two neighbouring barbules equal to 0'05 mm., or at 

 another part = 0*04 mm. 



Expla7iation 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 w hich 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 microsco})e, appear invariably brown. The 

 blue feathers of Parrots lose this colour if held against tiie 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 

 disai)pear if we destroy the supposed colour-producing parts. This 



