27th March 1833
I thank you for the communication of your paper extracted from the Annales de Chimie. <1> I am not much of a Chemist, but sometimes amuse myself with experiments. You mention in your paper that Potash throws down the oxide of Uranium in the form of a yellow hydrate. This precipitate I examined with a microscope last year, when studying the subject of microscopic crystallisation) and found it to consist of transparent crystalline particles – I should like to know whether it does not contain Potash in combination with it? Theoretically speaking, ought water alone to give transparency to the oxide? Very curious phænomena attend crystallisns observed with the microscope, some crystals are always binate – that is, in their nascent state they appear two together in the form of a cross X, I cannot help thinking this may have something to do with the theory of uniaxial & biaxal crystals.
Brewster’s <2> yet unpublished discovery, by which he can increase the number of bands or striæ on the Solar Spectrum, or give them to the Spectrum of Ignited Lime (which has none of its own) is certainly a capital one. But he is a long time maturing his ideas on the subject for publication. I have kept his secret for him now for a twelvemonth, and one of my own (consequent upon his) which I should like to publish. As to his reducing all light to red blue & yellow I am not convinced of it. I want decisive proof that a pure prismatic ray can be altered in tint by any coloured infusion or other coloured medium. Let A be an aperture thro’ which any prismatic ray passes, [illustration] B a similar one covered with blue cobalt glass, which alters or at any rate weakens the light. Then by some contrivance let the intensity of the light A be diminished by slow degrees till it disappears.
If during this gradual disappearance the aperture A never acquires the tint of B, it is a proof that the light B has been not only weakened but altered, & when I see such proof I shall be more ready to give my belief to the theory of the 3 colours –
I always explained to myself the circumstance of some persons not being able to see red light, by supposing that in their eyes the crystalline lens is coloured green; will not that serve to explain it?
Believe me, Yours very truly
H. F. Talbot
1. John Frederick William Herschel, ‘Sur la Séparation de l’oxide de Fer, et sur un nouveau Procédé pour effectuer la Purification Complète de l’oxide d’urane’, Annales de Chimie et de Physique, v. 49, 1832, pp. 306–311.
2. Sir David Brewster (1781–1868), Scottish scientist & journalist.
3. This response to WHFT’s question is written in Herschel’s hand.