102 Lord Rayleigh on the Optical Character 



appear to change too little. Neither in the case o£ fuchsin 

 nor of diamond green Gr — the second dye specially discussed 

 by Walter, — or with any other dye hitherto examined*, have 

 I seen an adequate change of colour without the use of the 

 nicol to eliminate vibrations in the plane perpendicular to- 

 that of incidence. In the absence of a nicol there is little 

 sign of the blue seen with it from fuchsin at 70° incidence. 

 Much greater changes with more saturated colour are 

 exhibited by the wing-cases of beetles when so examined. 



As to the adequacy of the surface -colours Michelson him- 

 self remarks: — "indeed, it may perhaps be objected that the 

 (animal) colours are far more vivid than any of the reflexion 

 hues of the aniline dyes, or of any other case of ' surface- 

 colour ' hitherto observed." But perhaps this objection should 

 not be very much insisted on in our ignorance of nature's 

 operations and with regard to the known existence of power- 

 ful dyes, e. g. in feathers. It is rather the rapid loss of 

 purity with obliquity in surface-colour which appears 

 significant. 



If a dye capable of surface-reflexion is present, there are 

 still alternatives open. The pure or nearly pure dye may be 

 on the outside so as to be in contact with air, or it may 

 be overlaid by a colourless skin of horny material (chitin) 

 in optical contact with it. The former case would be the 

 more favourable for vivid and variable colour, but then one 

 would expect to be able to remove the dye by solvents. So 

 far as I am aware this has not been done, and my own 

 trials with various solvents upon the wing-cases of beetles 

 have not succeeded. The most satisfactory demonstration of 

 the surface-colour theory would indeed be the extraction 

 of the dye and its exhibition as a thin layer spread upon 

 glass. 



If, on the other hand, the dye is imprisoned within a layer 

 of colourless chitin, the range of obliquities available in 

 ordinary observation would be restricted and the difficulty 

 of accounting for the variety of nearly saturated hues 

 actually seen would be increased, more especially when we 

 remember the dilution with white light reflected at the 

 external surface. 



* Through the kindness of Sir J. Dewar I have had the opportunity 

 of experimenting- with a good many dyes from the Badische Anilin- 

 Fabrik. Following Walter, I have used warm alcoholic solutions 

 spread upon previously warmed glass plates. Latterly I have examined 

 some more dyes, for which I am indebted to Prof. Green. In no case 

 have I seen any considerable change of well-developed colour unless the 

 light was polarized. 



