( 579 ) 



"solution d'erbinm fortemeiil paramagnétiqiie ; d'aillenrs M. Zeeman 

 "lni-mème Tavait déja clieirhé en vain pour le spectre d'émission 

 "de I'erbine chauifée. Des experiences sont en preparation pour déter- 

 "miner la rotation dans les raies d'absorption mémes et aux alentoui-s 

 "imniédiats". ^) 



After results had been published by Schmauss, Bates and Wood, 

 the agreement of which left much to be desired, the experiments in 

 question were taken up again in this laboratory in 1906, when one 

 of us actually obtained a very pecnliar dispersion curve of the 

 magnetic rotation within and near a narrow region of absorption.-). 

 At the same time such determinations proved to be subject to many 

 difficulties, which could only be surmounted by means of speciall}^ 

 adapted apparatus ; moreover, simultaneous measurements of other 

 optical properties of the absorbing substances are desirable for com- 

 pleteness' sake ; this more extensive investigation is now being con- 

 tinued with such improved apparatus. 



In the first negative experiments referred to a direct influence of 

 magnetisation in the form of a displacement of the dimly defined 

 absorption bands of an aqueous erbium-solution, in other woi'ds a 

 ZEEMAN-effect in the usual sense, could not be observed, the grating 

 used, however, being the same as that used in the present experiments. 

 Naturally the observation of the last-mentioned effect is much simpler 

 and more easily ff?asible than an adequate and trustworthy measu- 

 rement of the rotation. However, the relation between those two 

 modes of looking at one and die same phenomenon is so close that 

 either remains undetermined without a rather complete knowledge 

 of the other. 



Mr. Jean Becquerel Jr. ") resumed such an investigation on the 

 narrower and more sharply defined absorption bands of some exceed- 

 ingly i-are and small crystal fragments, which we had not at our 

 disposal: xenotime, tysonite, parisite and others, the spectra of which 

 had been formerly determined by Henri Becquerel Sr.^j. Besides, the 

 influence, already more or less known, of the tempei'ature on such 

 spectra was more fully investigated. The imffortant results obtained 

 may be assumed to be known. 



1) H. Du Bois, Rapp. Gongr. de l^hys, 2 p. 499, Paris 1900; Ann. d. Phys. (4) 

 7 p. 944, 1902. 



-) G. J. Elias, Physik. Zeitschr. 7 p. 981, 1906 (chloride of erbium). 



») J. Becquerel, Gompt. Rend. 142 pp. 775, 874, 1144, 190G. 143 pp. 7G9, 

 890, 9G2, 1133, 1906. 144 pp. 132, 420, 592, 682, 1032, 1336, 1907. 145 pp. 

 413, 795, 916, 1150, 1412. Also Physik. Zeitschr. 8 pp. 632, 929, 1907. 



^j H. Becquerel, Ann. Ghim. & phys. (6) 14 p. 170, 1888. 



