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Show 1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 415 the light falling upon and passing through a blue feather is likely undergo. First, part of the rays will be simply reflected from the outer surface of S S (fig. l,p.413); secondly, the rest, before passing through this stratum, will be variously broken and reflected before reaching the coating ps, since the stratum SS is not homogeneous, but consists apparently of several irregular scales and secondary strata; thirdly, the coating, ps, breaks the rays again and partly reflects them, and, if it is only 0*0006 m m . thick, as in Pitta, it is thin enough to allow the application of the theory of thin-plate colours; fourthly, the system of ridges ; fifthly, some rays will reach the layer of brownish pigment. How much of them is absorbed, how much reflected as brownish light, and what the changes are of this brown light before it comes up again to the surface, we cannot tell. Again, the ray a will be under different conditions to the ray c. To follow and to calculate all these changes would be almost a superhuman task. W e know only the result, namely blue colour. By the application of the theory of colours of narrow ridges we are enabled to explain several other colours, fortunately under less difficult circumstances. We have seen before that many yellow feathers owe their colour to a yellow pigment. But several of them do not contain any pigment. The thin rami and radii of the downy part of a feather of Pitta, for instance, appear coloured (yellow) only under direct light, but they are colourless if examined under the microscope with transmitted light. Now in yellow feathers, no matter if they contain pigment or not, the surface shows very fine longitudinal ridges, which are more or less parallel to one another, and which appear as straight lines. This I found was the case in the yellow feathers of Pitta, Psittacula, Arachnothera, Picus, and Parus. The distance between the top of two neighbouring ridges varied from 0*001 to 0*0005 of a millimetre. That there are real ridges on the surface we can see on a transverse section of a yellow radius. The radius of a yellow pectoral tuft-feather of Arachnothera (Plate XXVIII. fig. 6) had a diameter of 0*007 mm.; as there were about twelve such ridges, like a, ft, y, their distance could not be greater than --r-. x =0*0018 mm. In Pitta the radius of a half-downy feather had a diameter of 0*012 mm. All round there were about twelve ridges, and the breadth of one ridge was rather smaller than the interstices; therefore the breadth of one ridge 120 must be smaller than fo^TTo '7 r = = 0'0 0 1 5 mm' Another method of calculating gave 0*0012 as the breadth of one ridge. Violet feathers.-Similar ridges exist on the surface-coating of violet metallic feathers, as, for instance, in Miliopyga and in Sturnus ; but the ridges do not appear to be quite straight, moreover they are much finer; in Sturnus only 0*00085 m m. Green feathers.-Only in the Musophagidse green pigment has been found. All other green feathers contain only either zoofulviu or a black-brown pigment. Krukenberg suggested therefore that |