69 ALBERT ST. REGENT'S PARK MY DEAR THOMSON I write to refer you to a paper of Foucault's in which he finds that the voltaic arc produces by absorption the fixed line D in light in wh. it did not before exist, and that the bright line D is of constant occurrence in the arc. L'Institut No 788. Jan. or Feb. 1849. MY DEAR THOMSON Yours very truly G. G. STOKES 69 ALBERT STREET REGENT'S PARK I was speaking to Foucault about the artificial dark line D. It is easily produced by arranging so that the coke pole shall be seen through the arc, the pole itself giving an uninterrupted spectrum in which the dark line D is seen by absorption while in the less bright spectrum formed by the arc itself is seen the bright line D as a continuation of the other. Yours very truly G. G. STOKES DEAR STOKES ADMIRALTY LONDON July 7/71 Many thanks for your letter and printed paper which are most valuable to me. It was certainly prior to the summer of 1852 that you taught me solar and stellar chemistry. I never was at Cambridge from the summer of 1852 until we came there in 1866, and the conversation was certainly in Cambridge. I had begun teaching it to my class in session 1852-3, (as an old student's note book which I have shows *) and probably earlier although of this I have not evidence. I used always to show a spirit lamp flame with salt on it, behind a slit prolonging the dark line D by bright continuation. I always gave your dynamical explanation, always asserted that certainly there was sodium vapour in the sun's atmosphere and in the atmospheres of stars which show presence of the D's, and always pointed out that the way to find other substances besides sodium in the sun and stars was to compare bright lines produced by them in artificial flames with dark lines of the spectra of the lights of the distant bodies. You very probably learned more from Foucault of what he had done when you met him in 1855, but you certainly told me of a Frenchman (I don't recollect the name you then gave me or even that you mentioned the name though it is most probable you did) having obtained the dark line by absorption. Yours truly W. THOMSON P.S. I think of using almost all you say of Herschel, in my address as from you. * I speak from memory. I believe it was of session 1852-3. I have it in Glasgow. MY DEAR THOMSON CAMBRIDGE 11th Jany 1876 You have pushed me far too much forward as to spectrum analysis. Whitmell sent me a syllabus of his lectures, in wh. I was mentioned far too prominently, and this led me to write to him my recollection of my ideas (and it is a distinct one) when we talked about the matter. It was you who proposed to extend the method so much. I rather thought you were allowing your horse to run away with you. Most of the absorbing (optically not mentally) matters that then came across me were things that would not stand the fire, and I don't know that I ever asked myself the question What would take place if a coloured glass were heated? Anyhow Stewart's extension of Prevost's Theory of Exchanges in the Proc. R. S. x 385 form (for I was not then acquainted with his Edin. Phil. Trans. papers though I might have been) came before me with all the freshness of a new discovery. I don't know whether you had known it before. As to the bright line D of a spirit lamp with salted wick. thought the flame gave such a shake to the Na Cl molecule as to make the Na bell ring, but did not think the more ethereal vibrations of D pitch could do it unless the Na were free, which I did not think of its being in the flame of a spirit lamp, though I thought the tremendous actions on the sun's surface might set it free. When later (1855) I heard from Foucault's lips of his remarkable discovery I still thought of the sodium being set free by the tremendous action of the voltaic arc of a battery of 40 or 50 elements of Grove or Bunsen. I thought from memory that it was when Foucault came to receive the Copley medal that he told me of it, though all I could feel certain of from memory that it was in the evening at Dr Neil Arnott's. On referring to your Glasgow address *, I was going to strike out the date, when I obtained proof positive that I was right. I feel certain that I should have mentioned Foucault's remarkable discovery in what I drew up for the President about his researches if I had then known of it (Proc. R. S. VII, 571). Yours sincerely G. G. STOKES [* Edinburgh address, 1871, loc. cit. p. 135 supra ?] INDEX TO VOLUME IV. [The references are to pages.] Absorption of light, mechanism of, 69; Achromatism of double object glass, Achromatism, practical investigations, Bessel functions, relations between con- Blood, oxidation and reduction of, 264, Cauchy, A. L., on double refraction, Chlorophyll, constitution of, 237, 247, Chromatic aberration, correction of Colours, nature of compound, 65; origin of natural colour, 153, 243; body Convergence, limited, of series, 80. Diamond, reflection from, 363. Dispersion, false, of light by small Divergent series, discontinuity in use of, Double refraction, report on theories of, Dynamical paradox explained, 334. Electric telegraph, theory of signalling, Enclosure, radiation belonging to tem- Equilibrium of gases in blood, 274. Eye, chromatic aberration of, 59. Foucault, L., on absorption of D lines, Fraunhofer, J., his achromatic com- binations, 63; on spectra, 368. Gravity, theory of Harton pit experi ment, 70. Green, G., on elastic propagation and Haemoglobin, 240; its reactions, 264. Hankel, H., on Bessel functions, 80. glasses, 339 seq. Hydrogen atmosphere stifles sound, 300. Kelvin, Lord (Thomson, W.), on begin- Limit, mathematical, approached dif- MacCullagh, J., theory of double re- Madder, optical analysis of, 123. rings by, 361; silver transparent in Miller, W. H., on coincidence of D and sodium lines, 370, 372. Optical properties as tests for organic bodies, 238, 249; of glasses, 339. Polarization produced by pile of plates, Polarized optical rings, peculiarity in Prisms, compensating, for investigating Quinine, metallic reflection of a salt of, Radiation, law of exchanges, 129; veri- Series, semi-convergent, discontinuity Solar eclipse, appearance at horns of Sounding-boards, necessity for, 300. Spectrum analysis, history of, 127; Spectrum, ultra-violet, 28; long, of Stewart, Balfour, on law of exchanges Tuning-fork, mutual interference of Uranium salts, fluorescent, 209, even Vibrations, communication to a gas, Vibrations, direction of, in polarized Wind, effect on sound, 110. CAMBRIDGE : PRINTED BY J. AND C. F. CLAY, AT THE UNIVERSITY PRESS. |