108 Lord Rayleigh on the Optical Character 



the effects have been calculated in this case, but it is evident 

 that similar phenomena would be produced by obstacles of 

 other and perhaps more probable forms. The obstacles 

 must have a different index from that of the medium in 

 which they are embedded, and there is no need for absorption. 



It may perhaps be objected that though a layer of 

 spheres may give a specular reflexion there would be an 

 accompaniment of light dispersed at other angles, forming 

 in the case of a regular pattern "diffraction spectra." 

 It is uncertain whether or not this occurs. If it does not, 

 the explanation may be that the pattern is too fine. 



The above remarks are intended merely to attenuate 

 the difficulty arising 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 

 J 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. WJien we replace the rod by a fine fibre, this dis- 

 crimination is lost, and the rotation of the nicol may make 

 no difference, or even a difference in the wrong direction. 



r> 



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 experi- 

 ments 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 g'"een 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 bine-green. Ordinary 

 solvents such as water even at the boiling-point, ether, alcohol, 

 benzol, bisulphide of carbon, acetic acid, &c, seem to be 

 without effect, even 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 



