106 Lord Rayleigh on the Optical Character 



admit exceptions, and certainly there is no improbability in 

 surface reflexion playing a part. It may be that both causes 

 are operative in a single specimen and even at the same part 

 of it. 



The next contribution to the discussion is an important 

 one by .\lallock *, who brings to bear the instinct and expe- 

 rience of a naturalist as well as of a physicist. His 

 observations were mainly on the feathers of birds and the 

 scales of insects, and they lead him to regard interference 

 rather than selective reflexion as the origin of the iridescent 

 colours. 4; The transparency or, at any rate, the vanishing 

 of the characteristic transmitted colour in the case of all 

 animal tissues when immersed and permeated by a fluid of 

 the same refractive index is strongly in favour of inter- 

 ference being the source of the colour, but even stronger 

 evidence is given by the behaviour of the structure under 

 mechanical pressure. 



" If the grain or peculiarities which favour the reflexion 

 or transmission of particular colours is of molecular size, 

 there is no reason to suppose that pressure insufficient to 

 cause molecular disruption would alter the action of the 

 material on light. On the other hand, if the colours are 

 due to interference, that is, to cavities or strata of different 

 optical properties, compression would alter the spacing of 

 these, and thus give rise either to different colours or, with 

 more than a very slight compression, to the transmission and 

 reflexion of white lipht." 



" In every experiment of this kind that I have made either 

 on feathers or insect scales the effect of pressure has been 

 to destroy the colour altogether. . . . With many feathers 

 the colour returns when the pressure is taken off. but with 

 insect scales the structure seems to be permanently injured 

 by compression, and though when allowed to expand again 

 the material is not colourless, the brilliancy which belonged 

 to the uninjured scale is gone, and the colour in general 

 changed. " 



" The facts above mentioned seem to offer stronger reasons 

 in favour of interference than the polarization phenomena 

 referred to by Michelson and Walter do against it." 



I have already commented on the importance of the 

 evidence afforded by observations with polarized light ; and 

 if we have to choose between selective reflexion and thin 

 plates of the type usually considered in theoretical writings, 

 we may find ourselves in a position of much difficulty. The 



* Proc. Roy. Soc. vol. lxxxv. a, p. 598 (1911). 



