[ 98 ] 



VII. On the Optical Character of some Brilliant Animal 

 Colours. By Lord Eayleigh, O.M., FM.S* 



IT is singular that the explanation of some o£ the most 

 striking and beautiful of optical phenomena should be 

 still matters of controversy. I allude to the brilliant colours 

 displayed by many birds (e. g. humming-birds), butterflies, 

 and beetles, colours which vary greatly with the incidence of 

 the light, and so cannot well be referred to the ordinary 

 operation of dyes. In an early paper f, being occupied, at 

 the time with the remarkable coloured reflexions from certain 

 crystals of chlorate of potash described by Stokes, and which 

 1 attributed to a periodic twinning t, I accepted, perhaps 

 too hastily, the view generally current among naturalists that 

 these colours were " structure-colours,^ more or less like 

 those of thin plates, as in the soap-bubble. Among the sup- 

 porters of this view§ in more recent times may be especially 

 mentioned Poulton and Hodgkinson. In Poulton's paper || 

 the main purpose was to examine the history of the very 

 remarkable connexion between the metallic colours of certain 

 pupse (especially Vanessa urticce) and the character of the 

 light to which the larvse are exposed before pupation. In a 

 passage describing the metallic colour itself he remarks : — 



11 The Nature of the Effects Produced. — The gilded appear- 

 ance is one of the most metal-like appearances in any 

 non-metallic substance. The optical explanation has never 

 been understood. It has, however, been long known that it 

 depends upon the cuticle, and needs the presence of moisture, 

 and that it can be renewed when the dry cuticle is moistened. 

 Hence it can be preserved for any time in spirit. If a piece 

 of dry cuticle be moistened on its upper surface the colour 

 is not renewed, but almost instantly follows the application 

 of spirit to the lower surface. Sections of the cuticle re- 

 semble those of Papilio machaon described in a previous 

 paper (Roy. Soc. Proc. vol. xxxviii. p. 279, 1885), and show 

 an upper thin layer and a lower, much thicker, finely 

 laminated layer which is also striated vertically to the 

 surface. With Prof. Clifton's kind assistance I have been 



* Communicated by the Author. 



t Phil. Mag. vol. xxiv. p. 145 (1887) ; Scientific Papers, vol. iii. p. 13, 

 see footnote. 



X Phil. Mag. vol. xxvi. p. 256 (1888); Scientific Papers, vol iii. 

 p. 204. 



§ Distinctly suggested by Hooke in his ' Micrographia ' (1665). 



|| Eoy. Soc. Proc. vol. xlii. p. 94 (1887). 



