1882.] DR. H. GADOW ON THE COLOUR OF FEATHERS. 415 
the light falling upon and passing through a blue feather is likely to 
undergo. First, part of the rays will be simply reflected from the outer 
surface of S S (fig. 1, 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 00006 mm. 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. 
We 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 drachnothera 
(Plate XXVIII. fig. 6) had a diameter of 0-007 mm. ; as there were 
about twelve such ridges, like a, (3, y, their distance could not be 
greater than on .7 =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 
must be smaller than .7=0°0015 mm. Another method of 
12:0 
12+12 
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 Zthopyga and in Sturnus ; 
but the ridges do not appear to be quite straight, moreover they are 
much finer; in Stwrnus only 0:00085 mm. 
Green feathers.—-Only in the Musophagide green pigment has 
been found. All other green feathers contain only either zoofulvin 
or a black-brown pigment. Krukenberg suggested therefore that 
