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Darwin Correspondence Project

From G. G. Stokes   27 February 1868

The Royal Society, | Burlington House, London. W.

27th Feb. 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 2ndary 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

Chas. Darwin Esq


See the letter to G. G. Stokes, 18 February [1868]. Stokes probably refers to George Howard Darwin.
Murexide, also known as Roman purple, was obtained from uric acid extracted from guano (Nieto-Galan 2001, p. 187). For Stokes’s experiments with magnesium platinocyanide, see Stokes 1853, p. 398–9.
Carthamine is a red pigment extracted from flowers of Carthamus tinctoria, also known as safflower red (Stokes 1853, pp. 393, 398; Blackshaw and Brightman 1961, p. 151).
Stokes misspelt ‘aniline’.
Stokes 1853. Stokes had conducted extensive research on optical phenomena such as fluorescence; see DSB.


Nieto-Galan, Agustí. 2001. Colouring textiles: a history of natural dyestuffs in industrial Europe. Boston Studies in the Philosophy of Science vol. 217. Dordrecht; London: Kluwer Academic Publishers.

Stokes, George Gabriel. 1853. On the metallic reflexion exhibited by certain non-metallic substances. Philosophical Magazine 4th ser. 6: 393–403.


On the play of colours in the peacock’s tail.

Letter details

Letter no.
George Gabriel Stokes, 1st baronet
Charles Robert Darwin
Sent from
Royal Society
Source of text
DAR 84.1: 47–50
Physical description

Please cite as

Darwin Correspondence Project, “Letter no. 5943,” accessed on 29 January 2020,

Also published in The Correspondence of Charles Darwin, vol. 16