To J. F. W. Herschel 11 May 
Down Farnborough Kent
Dear Sir John Herschel
Knowing that you are interested about Colours, I venture to mention the following fact, which if, as is most probable, you do not care about, please to take no notice of this note, which otherwise I should not think of troubling you with.
The genus Ibla is a Cirripede or Barnacle, of which the valves of operculum consist of a horny substance, instead of being as in all other genera calcified. This horny substance is hard, & translucent, & consists I believe of Chitine or Entomoline. The borders of the valves when a slice is scaled off are of a very fine blue colour, but which colour, (& this is the point) on slight pressure instantly changes into a translucent red like the clouds of sunset.1 It is curious under the microscope to press it with a needle & see spot after spot assume this fine permanent glow of red. Acetic & Muriatic acids produce the same effect without pressure. Caustic potash has no action on it. I could, if by chance you feel any interest in this point, send you a valve. The Ibla is a quite small animal from the Phillippines. Cirripedes are nothing but Crustaceans fixed to a support.
Pray believe me, | Yours sincerely | C. Darwin To | Sir J. W. Herschel Bart.
Describes colour changes from blue to red in valves of operculum of the cirripede genus Ibla.
Please cite as
Darwin Correspondence Project, “Letter no. 1175,” accessed on 27 March 2017, http://www.darwinproject.ac.uk/DCP-LETT-1175