From G. G. Stokes   27 February 1868

My dear Sir,

I told your son I fancied from a very hasty examination made a long time ago that the play of colours in the Peacock’s tail was a phenomenon of diffraction.¹ I fancied I recollected having seen very fine fibres or ciliæ which might account for it. But a more careful examination under the microscope leaves little doubt in my mind that it is the quasi-metallic reflexion exhibited by certain colouring matters having very intense power of absorption. Murexide and platinocyanide of Magnesium² are good examples, or to take substances more easily procured carthamine,³ which is sold in the form of the so-called pink saucers, and several of the analine colours, rosanaline for instance.⁴ If you pour 2 or 3 drops of a solution, tolerably pure, of these colouring matters on glass and allow it to evaporate, a film is left which is one colour by transmitted light and another by reflected light, and the reflected colour has that metallic aspect seen in the peacock’s tail. The leaflets, if I may so call them, of the feather (I describe the feather as a bipinnate leaf and speak of the 2^ndary leaflets i.e. the leaflets of the primary divisions) of the peacocks tail feathers, in those places where the feather shows that beautiful metallic green, are red by transmitted light.

I mean to make some further experiments on the subject.

You will find a paper of mine on the optical phenomenon in the Phil. Mag. for Dec. 1853 (Vol 6 p 393 of Series 4).⁵ I do not mention any feathers in the paper.

I am dear Sir | Yours very truly | G. G. Stokes

Cha^s. Darwin Esq