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.