10 THE COLOUES OF ANIMALS 



of that particular rate is wanting from the reflected 

 light, which therefore appears coloured. 



Opinions differ as to the relative importance of 

 animal colours due to thin plates and to diffraction. 

 Many which were believed to result from the latter 

 are in all probability due to the former. The irides- 

 cent colours on the inner surface of many sheila 

 (mother-of-pearl) are at any rate partially caused by 

 diffraction, for an accurate cast of the surface exhibits 

 traces of the colours.^ The shell is, however, a 

 laminated structure, and the colours may therefore 

 in part be caused by thin plates. 



Colours due to refraction (prismatic colours) 



When hght passes through a wedge-shaped trans- 

 parent substance (or prism) with greater refractive 

 power than the surrounding medium, it is bent in the 

 same direction at both surfaces, but its different con- 

 stituents are bent unequally. The slowest vibrations 

 (red) are bent least, the most rapid (violet) most ; and 

 when the substance possesses a sufficiently high re- 

 fractive power, all the colours of white light are seen 

 arranged like the rays of a fan in the order of their 

 rates of vibration. Prismatic colours like those of the 

 diamond are due to refraction. 



* Professor C. Stewart informs me that he has repeated Brewster's 

 original experiment, upon which the above statement depends. He 

 found that the colour was due to a thin layer of shell which had been 

 stripped off and adhered to the surface of the wax. 



