Slough
March 25
1833.
1833.
My dear Sir/
I shall be very happy to be useful to you in the way you mention <1> at the Cape provided you will give me full instructions what to do being myself entering without judgement in all matters relating to the vegetable world. In return I will gladly avail myself of your obliging offer of your assistance in forwarding officially such occasional communications from Scientific friends as would be ruinous if transmitted by post – and are not of magnitude enough to secure them from risk of loss on the voyage if made into parcels.
Dr B Sir D Brewster’s Expts & speculations on the spectrum – appeared in the Ed Phil Journal. <2> So far as the facts go on which he rests the divisions of the solar rays into 3 primary tints red yellow & blue. I think my own experiments described in my paper on Absorption and in the Articles on that subject in my Light <3> (which are almost wholly my own) have entitled to a considerable share in establishing them – As to the theoretical part (ie. that the sensations of mixed colours are compounded of 3 primary sensations to the calling up of which one or other of which every solar ray is adequate per se) that is not Brewster’s but Mayer’s <4> and I take leave to doubt it altogether and I do so on the evidence of those eyes who see only two colours. Of these I have of late collected much testimony & I think I have wherewithal to prove that Sir D. B. has entirely mistaken this matter, in assigning as its cause an insensibility to the red rays. The fact is that all rays excite their retinas as light but that red rays (I mean perfectly homogenous) act on their sensoria as yellow. I know not whether I shall publish anything further on this matter however. Brewster has shewn so much asperity to every other person who has ventured to meddle with the subject of vision that I could hardly expect to do so without involving myself in a quarrel with him which I have no desire to do – Yet I must confess I think in one point in the article in question, he has done me much injustice and made me say what my soul abhors and maintain the spectrum to be composed of 4 primary colours red yellow blue – and violet!! on the strength of a figure in my work on light where I have made the red curve, after coinciding (as to sense) with the abscissa for a considerable span in the green & blue parts of the spectrum, rise again into a sensible swell at the violent End, thus, and so by the mixture of [illustration] red & blue producing that ruddy looking blue which we call violet – I set him right as to my meaning on this point by letter – but he has not so far rectified his misstatements in any subsequent paper.
I do not know whether you are chemically disposed – if so the process for the purification of uranium annexed <5> may interest you
I am dr Sir Yours very truly
JFW Herschel
H.F. Talbot Esqr MP.
31 Sackville Street
Piccadilly
London
Notes:
1. That is, collecting and sending seeds of interesting native plant life.
2. Sir David Brewster (1781–1868), Scottish scientist & journalist, ‘On the Absorption of Specific Rays, in reference to the Undulatory Theory of Light’, Philosophical Magazine, v. 2, 1833, pp. 360–363.
3. John Frederick William Herschel, ‘On the Absorption of Light by Coloured Media and the Colours of Certain Flames’, Edinburgh Journal of Science, v. 2, 1825, pp. 344–348, and in his article ‘Light’, Encyclopaedia Metropolitana, 2nd Division: Mixed Sciences, v. 2, 1830, pp. 341–586.
4. Probably Johann Tobias Mayer (1723–1762), German astronomer who developed lunar tables and published numerous books on astronomy.
5. In Doc. No: 02670, WHFT thanks Herschel for this paper, ‘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.