( 5'78 ) 



1 think it ai)pro|u'icite liowexei' to call attention to the fact that tho 

 change of wav-elengiii of the middle line of a triplet seems not to be 

 confined to the light emitted by a (leissler tidte. 



During the writing of this paper my attention was arrested by a 

 passage in a Thesis of W. Hartmann ^) : 



"Es mag schon an dieser Stelle erwahnt werden, dass der 

 Abstand der Ordnungen beim Einschalten des Magnetfeldes sich 

 mehrfach anderte, nnd zwar im allgemeinen mit waehsender Feld- 

 starke kleiner wurde. 



Dieser Anderung wiirde rein ausserlicli betrachtet cine Verkiirzung 

 der Wellenlange entsprechen, doch konnle eine wirkliche Gesetzmas- 

 sigkeit nicht constat irt werden." 



The observations of Hartmann were made by means of an echelon 

 spectroscope, the source of light being the self-induction spark 

 in vacuum after Michelson's arrangement. Hartmann's negatives 

 concerning copper, iron, gold and chrominum were made with fields 

 ranging from 8000 to J 2000 Gauss. Perhaps the author would have 

 expressed his opinion with less reserve, if he had operated with 

 stronger fields, in which case the phenomenon is more definite. In 

 the light however of our own observations there seems to be suffi- 

 cient evidence to conclude, that also the middle lines of the triplets 

 of other metals undergo the kind of change existing in the case of 

 mercury. 



Physics. — "The mjluence of temperature and magnetisation on 

 selective absorption spectra". By Prof. H. E. J. G. du Bois 

 and G. J. Elias. (Communication froui the Bosscha-Laborator3^) 



§ 1. As soon as the unequaTled paramagnetic properties of the 

 compounds of so-called rare earth-metals had been demonstrated "), 

 attention was drawn to the fact that most likely also the magueto- 

 optic phenomena would show important peculiarities ; this was 

 done in the following words: "La polarisation rotatoire magnétique 

 "a le signe positif on négatif pour les composes des différents 

 "métaux de cette série, comme d'ailleurs pour ceux de la série 

 "du fer. Je n'ai pas pu constater jusqu'ici un effet particulier de 

 "l'aimantation sur le spectre d'absorption tres caractéiistique d'une 



1) Walther Hartmann. Das ZEEMAN-Phaenomen ini sichtbaren Spectium von 

 Kupfer, Eisen, Gold und Ghrom. Dissertation, Halle a. d. S. 1907. p. 10. 



2j H. DU Bois & O. Liebknecht, Ann. d. Physik (4) 1 p. 196, 1900; St. Meijer, 

 Ann. d. Physik (4) 1 p. 064, 1900. 



