on the Nature of the Light emitted by a Substance. 237 



" This was the last experimental research that Faraday 

 made." 



In 1875 we have a paper by Prof. Tait, who has kindly 

 sent me a copy, " On a Possible Influence of Magnetism on 

 the Absorption of Light, and some correlated subjects " (Proc. 

 Roy. Soc. of Edinburgh, Session 1875-76, p. 118). Prof. 

 Tait remarks that a paper by Professor Forbes, read at the 

 Society, and some remarks upon it by Maxwell, have recalled 

 to him an experiment tried by him several times, but which 

 hitherto has led to no result. Then the paper proceeds : — 



"The idea is briefly this. — The explanation of Faraday's 

 rotation of the plane of polarization of light by a transparent 

 diamagnetic requires, as shown by Thomson, molecular rota- 

 tion of the luminiferous medium. The plane-polarized ray is 

 broken up, while in the medium, into its circularly-polarized 

 components, one of which rotates with the aether so as to have 

 its period accelerated, the other against it in a retarded period. 

 Now, suppose the medium to absorb one definite wave-length 

 only, then — if the absorption is not interfered with by the 

 magnetic action — the portion absorbed in one ray will be of 

 a shorter, in the other of a longer, period than if there had 

 been no magnetic force ; and thus, what was originally a 

 single dark absorption line might become a double line, the 

 components being less dark than the single one." 



Hence here the idea is perfectly clearly expressed of the 

 experiment, tried in vain ; an idea closely akin to that of § 15 

 above, both being in fact founded on Kelvin's theory of the 

 molecular rotation of the luminiferous medium, though not 

 directly applicable to the experiment of § 9, in which case 

 the lines of magnetic force are perpendicular to the axis of 

 the tube. 



In the second place I have to mention two papers by the 

 late M. Fievez, to which attention has been drawn by M. van 

 Aubel, in a letter to Prof. Onnes and intended for communi- 

 cation to the Academy of Sciences, Amsterdam. Prof. Onnes 

 read the letter at the January meeting, and made at the same 

 time some explanatory remarks of which in the following I 

 make free and extensive use. The papers referred to are : — 

 M. Fievez, " De 1' Influence du Magnetisme sur les cpracteres 

 des Raies spectrales " {Bulletin de V 'Acad, des Sciences de Bel- 

 gique, 3 e serie, tome ix. p. 381, 1885) ; and Fievez, a Essai 

 sur l'Origine des Raies de Fraunhofer, en rapport avec la 

 Constitution du Soleil" (/. c. 3 e serie, tome xii. p. 30, 1886). 

 Here experiments are described as in §§ 4 and 13 of the 

 present paper. Nothing, however, is observed about the 

 widening of the absorption-lines, nor about the polarization 



