To George Gabriel Stokes   18 February [1868]¹

My dear Sir

I have ventured to send my son² to you to obtain a little information for me, on one point, if in your power to give it, and by this means you will be saved the trouble of answering this note.

Have you ever attended to feathers, & can you tell me whether the splendid colours of the eye of a Peacock’s tail depends on colouring matter, or on reflection.³ If on the latter, as appears the case, I much want to know, whether any change of structure,—as the distance of a film, or the distance of fine lines or points from each other—gradually, but perhaps not equally, increasing or diminishing, would account for the series of colours, which surrounds the eye, & passes into the general tint of the barbs at the circumference of the feather. Will you be so kind as to look at the feather, & tell my son anything you can.—

Pray forgive me for troubling you & believe me | My dear Sir | Yours sincerely | Ch. Darwin