104 Lord Rayleigh on the Optical Character 



explanation is to reject the dye theory and refer the colours 

 to interference. The facts recorded agree pretty closely 

 with what happens in the case of films of old decomposed 

 glass. 



Indeed, Walter, in a later passage, very candidly admits a 

 difficulty. He says (p. 98): — 



" Finally, it must not be passed over in silence that there 

 is a circumstance which makes a difficulty for the view here 

 propounded of the lustre colours of butterflies. This is the 

 fact that the lustre practically disappears in benzol and 

 bisulphide of carbon, whereas in treating the theory of sur- 

 face-colours we have several times insisted that a ray 

 strongly absorbed must under all circumstances be vigorously 

 reflected." 



Before leaving the question of the colours it may be well 

 to consider an objection strongly urged by Walter against 

 the interference theory, viz., that the colours of thin plates 

 change too much with obliquity. As regards a single thin 

 plate, which alone Walter seems to have contemplated, it is 

 true, I think, that the more pronounced colours of the 2nd 

 and 3rd order in Newton's scale change more rapidly with the 

 retardation* than could well be harmonized with what is ob- 

 servedof the animal colours. Butthe difficulty disappears when 

 we admit a structure several times repeated with approximate 

 periodicity. The changes in chlorate of potash crystals with 

 obliquity seem to agree well enough with what is required, 

 and this form of the interference theory has the advantage 

 of greater elasticity, e. g. meeting Walter's objection that 

 the colours of a single thin plate constitute a simple series 

 with but one independent variable. Indeed, the purity of 

 the reds often to be observed from beetles' wing-cases seems 

 to exclude an interference theory limited to a single thin 

 plate, inasmuch as the reds from such a plate are distinctly 

 inferior, especially when diluted with white light reflected 

 from an outer surface not forming part of the boundary of 

 the thin plate f. 



Michelson, who with his great authority supports the 

 surface-colour theory, mentions several tests under four 

 headings (p. 561). To my mind these tests are as well, if 

 not better, borne by an interference theory. But reliance 

 seems to be chiefly placed upon " the more rigorous optical 

 test of the measurement of the phase-difference and 



* See a diagram of the Colours of Thin Plates, Ed. Trans, vol. xxxiii. 

 p. 157 (1886); Scientific Papers, vol .ii. p. 498. 

 t Ed. Trans, he. cit. 



