INTERFERENCE COLOURS 179 



is in agreement with Suffert, consequently does not accept Onslow's 

 explanation, and points out that the ribs, seen flatwise, produce 

 no colour, although in this position they should give maximum 

 brilliancy according to Onslow's theory. 



3. The Entimus type (Fig. 65), where the lamellae fill the 

 interior of scale, and are inclined in different directions in sharply 

 defined areas, with the result that corresponding colour patches 

 are exhibited. This type is prevalent in the Diamond beetles 

 Entimus and Cyphus, and previous observers, including 

 Biedermann and Mallock, regarded the colour as being due to 

 the presence of thin films. Michelson (1911) concluded, on the 

 other hand, that since the colours exhibited by these scales are 

 so varied and bright, and change so rapidly with varying incidence, 

 the effect must be due to diffraction caused by a grating-like 

 structure on the interior surface of the scale. He calculated that 

 there were from 5,000 to 10,000 striae per centimetre, a figure which 

 agreed with actual counts made under a magnification of 

 1,000 diameters and oblique reflected illumination. Under these 

 conditions the striae appear clearly defined in small patches. It 

 was necessary, in order to satisfy his theory, to postulate that the 

 grating structure is of an asymmetric saw-tooth type, since all 

 the light is concentrated in one spectrum. Onslow comments 

 upon the fact that Michelson's explanation of the vivid colouring 

 of Entimus is supported by the similarity between these scales 

 and certain artificial saw-toothed gratings ; nevertheless, he does 

 not wholly accept the diffraction theory. He finds evidences of 

 stratification clearly displayed in some of his microscopical 

 sections in addition to the vertical striae already alluded to. He 

 then proceeds to discuss certain objections to Michelson's views, 

 and concludes that, although diffraction may participate in 

 producing the colour phenomenon observed, it is not the sole cause, 

 and interference by thin films also plays a part. Mason holds 

 that the multiple film theory alone affords the true explanation, 

 and for the theoretical and experimental evidence upon which his 

 conclusion is based the reader is referred to his original paper 

 (1927a). 



Scattering of Light by Minute Particles. This familiar 



