From John Herschel   Nov. 13 1864

Collingwood Nov. 13 1864.

My dear Sir

Many thanks for your highly interesting paper on Luminous and obscure radiation.¹ I am very glad to see that you propose dividing some share of your attention to the extra spectral calorific rays which lie out very far beyond the extreme visible red as indicated in the note to my paper of 1840² to which you refer. And there are some further facts in that of 1842 §.205.³ in connexion with this which seem to indicate a continuous extension of the spectrum in this direction not due to merely calorific rays. It might be worth while to try & trace the visibility of these rays. That of the chemical rays to an almost indefinite extent beyond the violet is clearly proved by the exp[erimen]^t. recorded on Art. 56 of my 1^st. paper.⁴

The dynamical difficulties is in the way of a change of period of vibrations of a ray (such as Prof. Stokes has adopted for his explanation of fluorescence and such as D^r Akin has used to explain the incandescence of platinum wire in hydrogen flame) seem to me very great in the face of a theorem which as I fancy I have pretty rigorously demonstrated in my treatise on sound⁵ § 393 et seq. on the law of forced vibrations which seems quite to preclude such a change at least except through the intermedium of some indirect mode of action to me not easily imaginable.—Stokes, to whom I once mentioned this objection tried to see over it but I confess, as it appeared to me very unsatisfactorily. I once thought I could see my way to such a change by regarding the new vibration as some sort of secondary or tertiary harmonies (harmonies of harmonies) of the original—but nothing came of it.

As regards the incandescence of platinum⁶ any metal or other body very violently heated discharges white light—or at least rays ranging over all the spectrum more or less interrupted—for so far as common observation goes does it seem to matter at all how the heat is communicated to it—If a platinum wire be held in the flame of any burning body not containing hydrogen (suppose carbonated of sulphur) urged by an oxygen fan blowpipe—I suppose it would become hot, 1^st observing then red —then white hot, just as in the flame of hydrogen.

Your transcalescence of Iodine is very extraordinary—and as you have now the means of insulating the calorific rays would it not be possible to impress on moistened black paper an extension to make visible their rays of interference, so as actually to measure the lengths of their vibrations?

NB. In the propositions communicated in §. 26, 27 of your paper⁷ ought not the word “calorific” or “thermal” to be introduced into the enunciation before the word “radiation?”—The pile used to measure the deflection is sensible only to such radiations.

I have written a very rambling note but I hope you will attribute it to the great interest excited by your beautiful experiments & believe me

Yours v[er]y t[rul]y | J F W Herschel

RS HS 394b

RI MS JT/1/TYP/2/529–30