of some Brilliant Animal Colours. 105 



amplitude-ratios " when polarized light is reflected. I 

 agree that this is a cogent argument, and unless it can be 

 met the balance of evidence derived from simple observation 

 would perhaps incline to the surface-colour theory. It is, 

 I think, the tact that many beetles exhibit a less well-marked 

 polarizing-angle than could be reconciled with the usual 

 theory of thin plates constituted of non-absorbent material. 

 An escape from the difficulty might perhaps be found in 

 imagining a stratification composed of more than two mate- 

 rials, so that, for instance, the polarizing-angle for the first 

 and second might differ considerably from that correspond- 

 ing to the second and third. But such a structure seems 

 rather improbable, and any combination of thin plates com- 

 posed of two transparent materials only should give a 

 definite polarizing-angle, abstraction being made from the 

 minor deviations observed by Airy and Jamin. 



At this point it may be recalled that a well-marked 

 polarizing-angle and a sudden change of relative phase 

 through two right angles are more closely connected than is 

 sometimes realized. The latter without the former would 

 involve a physical discontinuity. Michelson considers that 

 in practice the phase-change affords the more delicate cri- 

 terion *, and that in most cases it is decisive in favour of 

 surface-colour. 



A circumstance which may perhaps be regarded as telling 

 upon the other side is afforded by the variety of colouring at 

 different parts, but at the same angle (e. g. at perpendicular 

 incidence) seen in certain beetles — Dr. Hodgkinson men- 

 tions Chrysocliroa fulminans. The " colours vary in an 

 indescribable manner when attentively examined at different 

 angles of incident light with the eye alone ; with the mirror 

 (viz., at perpendicular incidence) the wing-cases are seen to 

 be coloured successively from base to tip iridescent green, 

 yellow, orange, and red, and these tints remain unaltered 

 by change of position of the object." I have confirmed 

 generally this observation, and other beetles show something 

 similar. The explanation makes large demands upon the 

 surface-colour theory; but a moderate change of structure is 

 all that would be required by interference. 



A caution is perhaps required against regarding the two 

 theories as mutually exclusive. Both Walter and Michelson 



* Some of Michelsoivs diagrams are rather confusing in that they 

 su«i; est a phase-difference of 180° between the two polarized components 

 reflected perpendicularly, when evidently the distinction between the 

 two components disappears. 



