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Published online by Cambridge University Press: 17 January 2013
1. The following paper is divided into three sections, containing three distinct yet intimately connected investigations. The two first on the Polarizability and Depolarization of Heat have arisen immediately out of the train of investigation contained in my two former papers, and the researches of others to which they gave rise. The third is on the Refrangibility of Heat, a point of the highest importance for theory.
2. The experiments on which these investigations are based have been performed almost exclusively during the past winter. Part of the experiments on Depolarization were, however, made in the winter 1836-7. The mode adopted for trying Refractive Indices I had long ago contemplated. It was not, however, put in practice until January last.
page 177 note * Comptes Rendus de l'Académie des Sciences, ii. 140.
page 177 note † Ibid. p. 194.
page 177 note ‡ To avoid circumlocution, I shall denote by I. II., &c. the First, Second, &c. Series of Researches, and by the succeeding Arabic numeral the Article referred to.
page 178 note * I might add, too, that, had he been aware of the extreme tenuity of the mica plates employed (of which more hereafter), he must have been led as a necessary consequence of his own reasonings to admit that the effect must be insignificant.—Ann. de Chimie, Mai 1837, p. 13, note.
page 181 note * Lest this confusion should, by possibility, occur to any one, as it did to myself, I will observe that the position of the sifting or modifying plate, absorbing the least refrangible rays, is quite immaterial, provided it occur between the source and the indicator of heat, for whether the rays in question are absorbed before or after polarization, those which ultimately escape and reach the pile are the only ones of which the index of polarization is measured.
page 182 note * Annales de Chimie, Mai 1837. At p. 17, &c., M. Melloni has given a minute account of that method of constructing the piles, which, “amongst several different ways, he considers the preferable one.” No one could doubt from his language that he is describing a new and improved form of the apparatus. I regret for a moment to descend to notice an apparent want of justice and courtesy towards myself; but it is impossible for me not to observe, that the procedure he so exactly details, is, to almost the minutest particular, identical with that which I myself used in June 1835, in constructing, in M. Melloni's presence, the first pair of piles used for polarizing heat which existed in France, at a time when M. Melloni expressed his unqualified scepticism as to the polarization of heat generally; which piles I left, at his desire, where I presume they now are,—in his own possession. This mode of construction I soon after abandoned, for the improved one alluded to in the text.
page 183 note * I do not state this as a new idea; it has been repeatedly remarked by M. Melloni, that, in proportion as substances are thinner, they possess a more equable diathermancy for heat of different qualities.
page 183 note † The part of the effect due to reflection, I had previously established to be nearly the same for different kinds of heat.
page 184 note * Kelland on Heat, Art. 166.
page 186 note * These numbers are obtained by doubling those due to the corresponding tints of thin plates of air in Newton's Table. In the case of the two last numbers, there might have been some doubt as to the order of colours to which they belonged, but this was removed by the measurements given farther on, which shewed that the pink of No 5. is a colour of the fourth order.
page 188 note * Omitted in the mean as manifestly too small, arising from the lamp being just lighted, and the brass not fully heated.
page 190 note * Most of the experiments on incandescent platinum were made early in 1837, the remainder during the winter 1837–8.
page 190 note † The interpolating line for Incandescent Platinum is in the engraving placed at rather too high an angle.
page 192 note * I do not mean to offer any opinion on the nature of light in a partially polarized ray generally; but, as in the present case, the angle of incidence is that of complete polarization nearly, I presume that the transmitted ray is undoubtedly composed partly of light polarized perpendicularly to the plane of incidence, and partly of common light.
page 193 note * Phil. Trans. 1802.
page 195 note * See Melloni on the Reflection of Heat, Annales de Chimie, Dec. 1835.
page 199 note * It may not be superfluous to state, that during the course of the experiments referred to in this series of papers, I have adopted a uniform and clear system of recording my experiments, which admits of subsequent reference, and, if necessary, of publication. The experiments have been fairly written out, averages and ratios taken, generally on the same day on which they were made. This methodical plan cannot be too strongly recommended. Much after anxiety is spared, the calculations are lightened, errors avoided in the reduction after minute circumstances have been forgotten, and suggestions are afforded by the result of past experiments for the conduct of new ones.
page 202 note * This observation was made with a very contracted diaphragm; the readings therefore were very small. It is omitted in the final reductions.
page 202 note † Omitted in the final reductions on account of the irregularity of the observations.
page 206 note * Such a one I have had executed.