The Athenaeum | Royal Institution | 4th. Jan.
Dear Sir John,
At the time that you rendered me your debtor by sending me a mass of rock-salt your gift was accompanied by a note2 containing a short history of the piece of salt, and directing my attention to the heat spots of the spectrum which you discovered in 1841.3
I have not yet had time to touch the spectrum, and therefore have nothing to communicate regarding these extremely remarkable spots. I shall attack the subject in due time. Meanwhile, as I am writing a book on Heat,4 and have had occasion to refer to your piece of salt; it occurs to me that the printing of the few lines in your note would be a proper way of enabling you to take date should anything important arise out of the examination of the extra-red end of the spectrum.
Have you any objection to my printing the necessary extract?
Some time ago Dr. Bence Jones, our honorary Secretary, made me acquainted with a note from your son5 which encouraged the hope that we might hear him on some Friday evening at the R. I.6:—Would you kindly say to him that any assistance I could render him in the way of illustration should be rendered with the utmost pleasure.
Assuming that the action of aqueous vapour upon radiant heat is established beyond doubt—(which it is)7 we have in the vapour a body which absorbs the same rays as the water which produces it—in other words it shares the colour of water. Through the operation of this cause our atmosphere is certainly a blue medium. The quantity of vapour might not be sufficient to produce a sensible blue, if the rays went straight through it; but the reflections within the body of the atmosphere must be innumerable, and thus its virtual depth increased. I wonder is this likely to throw any light on the blue of the sky!8 Could you get your angle of polarization of 45° by reflection from air or vapour?9
Yours very sincerely | John Tyndall.
RS HS/392a
[1863]: the year is given by reference to Tyndall working on a book about heat (published in 1863), and that this letter is from the January following an 1862 letter from Herschel.
a note: see letters 1878 and 1880 (Tyndall’s reply to Herschel), Tyndall Correspondence, vol. 7.
you discovered in 1841: this refers to Herschel’s work on spectroscopy, published as J. Herschel, ‘On the Action of the Rays of the Solar Spectrum on Vegetable Colours, and on Some New Photographic Processes’, Phil. Trans., 132 (1842), pp. 181–214.
a book on Heat: LT noted on the letter, ‘published in 1863’. This refers to Heat as a Mode of Motion.
your son: Alexander Stewart Herschel.
we might hear him… at the R. I.: Alexander Herschel did indeed give a Friday Evening Discourse at the RI, ‘On Luminous Meteors’, on 24 April 1863 (Roy. Inst. Proc., 4 (1862–6), pp. 87–97; see letters 1960, 1980, 1981, 1982, and 1983.
(which it is): this point was, of course, very much disputed.
it shares the colour of water... of the sky!: Tyndall significantly changed his views on this point; see Ascent of John Tyndall, pp. 245–9.
Could you get your angle… from air or vapour?: see Herschel’s reply (letter 1960).
Please cite as “Tyndall1958,” in Ɛpsilon: The John Tyndall Collection accessed on