SELECTIVE REFLECTION 181 



polishing an elytron with carborundum powder, are very abrupt 

 and, in other respects, quite different from the colours of thin 

 films. Onslow points out that single films have been shown to 

 be very bright, but if the metallic colours of the insects in question 

 are due to such films, it is essential to suppose that they are of an 

 extraordinarily uniform thickness throughout, or the colour 

 would not be the same all over. Moreover, the thickness in all 

 individuals of a species must be the same, for the difference of a 

 fraction of a wavelength would cause a considerable alteration 

 of colour, and it appears very difficult to conceive of an organic 

 film complying with these requirements. He concludes that a 

 layer of organic material, which acts like a film of a dye or of a 

 metal, and gives rise to the metallic reflection, is a more con- 

 ceivable means of colour production. He is, however, faced 

 with the difficulty of not knowing how the surface layer is formed, 

 but concludes that it is difficult to imagine how a periodic structure 

 can arise in a homogeneous medium, about 0-5 /a in thickness, 

 during its extrusion to the exterior. 



Mason (1927b) further discusses the subject of smooth iridescent 

 integuments of the kind just referred to, and recognises the 

 " metallic type," common in many insects, and the " enameUed 

 type," seen in certain Cetoniid beetles. He comments upon the 

 fact that no selectively reflecting material is known which does 

 not absorb light very strongly and show any intense colour in very 

 thin layers. The brilliant colour and lustre of specimens bleached 

 to a pale straw colour, when viewed by transmitted light, is utterly 

 at variance with the properties of substances which exhibit 

 selective reflection. The idea that a small number of thin films 

 is responsible for the observed effects is not inconsistent with the 

 thickness of the material concerned, since he finds that in most 

 specimens the colour layer exceeds^ 1 /x in thickness. As with 

 iridescent wing membranes, he concludes that lack of sufficient 

 differences between the refractive indices of the presumed lamellae 

 renders it hopeless to expect to obtain their resolution under the 

 microscope. 



Notwithstanding their marked differences in lustre. Mason con- 

 cludes that " enamel-like " integuments similarly are explainable 



