Sir George Airy, which are seen in passing the polarizing angle of diamond, and which have been more recently extended by M. Jamin to the generality of transparent substances*; and if these pass by regular sequence to those I have described as seen with metals beyond the limit of total internal reflection, it follows that the latter would not be completely embraced in the application of Fresnel's formula, modified to suit an intensely absorbing substance and an angle of incidence given by a sine greater than unity +. [* Traced by Lord Rayleigh for the case of water, mainly, but not entirely, to the presence of a surface film of transition arising from contamination, Phil. Mag. 1892, Scientific Papers, III, p. 496; reference is there made to similar observations by Drude on recently formed cleavage planes in crystals.] + It was long ago observed, both by Professor MacCullagh and Dr Lloyd, that when Newton's rings are formed between a glass lens and a metallic plate, the first dark ring surrounding the central spot, which is comparatively bright, remains constantly of the same size at high incidences, although the other rings, like Newton's rings formed between two glass lenses, dilate greatly as the incidence becomes more oblique. See Proceedings of the Royal Irish Academy, Vol. 1, p. 6. PRELIMINARY NOTE ON THE COMPOUND NATURE OF THE LINESPECTRA OF ELEMENTARY BODIES. By J. N. LOCKYER, F.R.S. (Extract.) [From the Proceedings of the Royal Society, XXIV, 1876, p. 352.] March 3, 1876. MY DEAR LOCKYER,-You might perhaps like that I should put on paper the substance of the remarks I made last night as to the evidence of the dissociation of calcium. When a solid body such as a platinum wire, traversed by a voltaic current, is heated to incandescence, we know that as the temperature increases, not only does the radiation of each particular refrangibility absolutely increase, but the proportion of the radiations of the different refrangibilities is changed, the proportion of the higher to the lower increasing with the temperature. It would be in accordance with analogy to suppose that as a rule the same would take place in an incandescent surface, though in this case the spectrum would be discontinuous instead of continuous*. Thus if A, B, C, D, E denote conspicuous bright lines, of increasing refrangibility, in the spectrum of the vapour, it might very well be that at a comparatively low temperature A should be the brightest and the most persistent; at a higher temperature, while all were brighter than before, the relative brightness might be changed, and C might be the brightest and the most persistent, and at a still higher temperature E. If, now, the quantity of persistence were in each case reduced till all lines but one disappeared, the outstanding line might be A at the lowest temperature, C at the higher, E at the highest. If so, in case the vapour showed its presence by absorption but not emission, it follows, from the correspondence between absorption and [* Kayser regards this as still unsettled, Handbuch der Spectroscopie, II, 1902, p. 331.] emission, that at one temperature the dark line which would be the most sensitive indication of the presence of the substance would be A, at another C, at a third E. Hence, while I regard the facts you mention as evidence of the high temperature of the sun, I do not regard them as conclusive evidence of the dissociation of the molecule of calcium. Yours sincerely, G. G. STOKES. APPENDIX. CORRESPONDENCE OF PROF. G. G. STOKES AND PROF. W. THOMSON ON THE NATURE AND POSSIBILITIES OF SPECTRUM ANALYSIS. [THE following letters from Lord Kelvin came to light in arranging the scientific correspondence of Sir George Stokes. On supplying their dates to Lord Kelvin, he was able to extend the record. The parts relating to spectrum analysis are here printed, with Lord Kelvin's permission, as supplementary to the extracts contained in pp. 127-136 of this volume: cf. p. 136. It may be recalled that Prof. Stokes became Secretary of the Royal Society in 1854.] 2 COLLEGE, GLASGOW MY DEAR STOKES It is a long long time since I have either seen you or heard from you, and I want you to write to me about yourself and what you have been doing since ever so long. Have you made any more revolutions in Science? or done any of the exp1 research on the friction of air? I saw a notice of your lecture at the R. I. Tell me any new discoveries you have made, &c. However I do not mean to impose upon you by demanding all this, but if there is anything short and good you can tell me I shall be glad to hear it. I want to ask you about artif1 lights and the solar dark lines. Is there any other substance than soda that is related to D? Are bright lines corresponding to it to be seen where soda is not present? Have any terrestrial relations to any other of the solar dark lines been discovered (or to the dark lines of any of the stellar spectra)? Are all artificial lights subject to dark lines? I should be greatly obliged by your telling me in a word or two what is known on these questions, which I suppose you will easily do. Yours always truly WILLIAM THOMSON PEMBROKE COLL. CAMBRIDGE MY DEAR THOMSON Now for your questions. I am not aware that there is any pure substance known to produce the bright line D except soda. See end *. It would be extremely difficult to prove, except in the case of gases or substances volatile at a not very high temperature, that the bright line D, if observed in a flame, was not due to soda, such an infinitesimal quantity of soda would be competent to produce it. It is very common in ordinary artificial flames (such as a candle &c.) but I think in such cases it may be attributed with probability to soda. In a spirit lamp I feel satisfied it is derived from the wick, for I find that alcohol burnt in a clean saucer does not give it, except perhaps a flicker now and then. Miller told me (and I have verified the observation) that it is not found in an oil lamp, and I find that when the wick of a candle is cut short, so as to be surrounded by gas, and not to project into the luminous envelope where the combustion goes on, D disappears. Sir D. Brewster states (Brit. Ass. Report, 1842, p. 15 of the 2nd part) that the flame of deflagrating nitre contains bright lines corresponding to the dark lines A and B of Fraunhofer1, and implies that other of the bright lines of this flame correspond to the dark lines of the solar spectrum. I saw somewhere a statement, I think, by Sir D. B., that the flame, I think of burning potassium, certainly some flame in which potash was concerned, gave 7 bright lines corresponding to the dark lines forming Fraunhofer's group a. These are the only cases I know of in which identification has yet been established, but the subject has barely yet been attacked. A vast deal of measurement has yet to be gone through. I think it likely that very interesting results will come out. You will find in Moigno's Répertoire d'optique moderne (part III p. 1237) much information on the subject of your questions. You ask "Are all artificial lights subject to dark lines?" No, it is quite the exception. When there are lines of any kind it is usually bright lines. The flame of nitrate of strontia (i.e. a flame coloured red by nitrate of strontia) shews such dark lines in the red, but then these same dark lines are found in the spectrum of [1 See however p. 135 supra.] |