Ɛpsilon

From G. G. Stokes   20 December 1875

Lensfield Cottage Cambridge

20th Decr 1875

My dear Sir,

You may remember that some years ago you asked my opinion as to the cause of the colours in peacocks’ feathers.1 I made some experiments, or rather observations hardly deserving the name of experiments, about it. I felt however that it was a matter hardly to be attacked without a thoroughly good microscope which I did not then possess. I expressed a leaning to the opinion that the colours were reflection-colours connected with intense absorption, and similar accordingly to the reflection-colours seen when some of the aniline colouring matters are poured in solution on glass, and the solvent allowed to evaporate. I felt at the time I think misgivings as to whether so much play of colour as is observed could thus be accounted for—2

As I have helped to lead you wrong if you followed my guidance, it is but just that I should direct you to a right solution. Some little while ago I was with Mr. Sorby, who wished to show me some other experiments, and I found that he has been studying birds’ feathers, as to their colours, and had arrived at the solution of the cause of the play of colours in the feathers of peacocks, humming birds &c. I am not sure whether he has yet published his results   If he has, I have no doubt he will be happy to send you copies of his papers if he has not done so already.3 If not, I dare say he would explain to you in a few words their general nature. Though I feel confident of his permission, I refrain as a matter of principle from communicating to a third person what has been told me by a friend of his unpublished researches. Besides it is due to him not to spoil the pleasure he would feel in communicating to you for the first time his own results.

I am dear Sir | Yours sincerely | G. G. Stokes

Chs. Darwin Esq F.R.S.

1
See Correspondence vol. 16, letter to G. G. Stokes, 18 February [1868].
2
See Correspondence vol. 16, letter from G. G. Stokes, 27 February 1868. Reflection colours were thought to occur when most of the incident light was absorbed and the structure of the surface allowed a scattering of light in all directions; the molecular mechanism of absorption was, however, considered one of the most ‘subtle and difficult in physics’ (Tyndall 1875, pp. 35–7 and 39). The iridescence of peacock feathers was later shown to be caused by structures in the feathers that interfered with incident light in proportion to the angles of observation and illumination. This structural coloration can be combined with pigment colours.
3
CD had consulted Henry Clifton Sorby about the red pigment in the hairs of the common sundew (see letter to F. J. Cohn, 21 August 1875, n. 6). In two papers detailing the use of a spectrum microscope, Sorby mentioned that he had analysed the colours of feathers (see Sorby 1875a, pp. 202–3, and Sorby 1875b, p. 270), but he does not appear to have published a paper on the cause of the play of colours in feathers.

Please cite as “DCP-LETT-10310,” in Ɛpsilon: The Charles Darwin Collection accessed on 5 June 2025, https://epsilon.ac.uk/view/dcp-data/letters/DCP-LETT-10310