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1919]                          Or SOME BRILLIANT ANIMAL   COLOURS                              593
from the absence of a well-marked polarizing-angle, and the details need not be insisted on. No surprise is felt at the deficiency of polarization in the light reflected from impressed and unglazed paper, of which the fibres are quite large enough to be the seat of interference effects. In illustration the transverse reflexion from glass rods and fibres may be mentioned. When we examine with a nicol the reflexion from a rod \ inch (6 mm.) in 'diameter, we can verify the extinction, at a suitable angle of the light reflected from the first surface, although abundance of other light still reaches the eye. When we replace the rod by a fine fibre, this discrimination is lost, and the rotation of the nicol may make no difference, or even a difference in the wrong direction.
The greater part of the preceding discussion was written about a year and a half ago. I am now able to supplement it with further observations of my own and of others who have been kind enough to help me. Most of my experiments have been made on wing-cases of beetles found in my garden (June and July 1917). Usually attention is first attracted by the display of a vivid green coloration, but on indoor examination the variation with angle is found to be about the same as is observed with brilliant specimens from abroad. At perpendicular incidence the colour is an. orange with approach to red, passing with increasing obliquity through yellow and green to a blue-green. Ordinary solvents such as water even at the boiling-point, ether, alcohol, benzol, bisulphide of carbon, acetic acid, etc., seem to be without effect, evpn when the precaution is taken to separate a wing-case into two parts so as to allow access to the interior of the cuticle. A treatment with hot caustic potash has more effect, in one experiment shifting the colour at perpendicular incidence from orange to a brilliant scarlet. By the action of hot somewhat diluted nitric acid the black underlying pigment may be removed without much affecting the dye, or the structure, which is the seat of the coloration.
Several experiments were made to test whether air-cavities existed. For this purpose the wing-case was exposed for some time to the action of vacuum, into which afterwards water or benzol was admitted. But no distinct evidence of the penetration of liquid could be recorded. I understand that Prof. Poulton has had a similar experience.
Again, it has been noticed by Mr H. Onslow and myself that considerable pressure fails to alter the colour of beetles and of the wings of some iridescent dragon-flies, though (Poulton, Mallock) effective in some other cases. It would seem that the hypothesis of air-cavities must be abandoned.
In the absence of air-cavities the alternating structure demanded by the
interference theory would require two kinds of matter capable of resisting
pressure and of sensibly different refractive indices.   Probably both would be
solids;  and since the range of relative index is then much restricted, the
K. iv.                                                                                           38or 7? = 27rJR/X = 2 occurs when //= +-5, where /u, is the cosine of the angle between the secondary (or scattered) ray and the backward direction of the incident ray. W. F. S.]spheres are easily demonstrated.