( 360 ) 



will consist of a 



Cf. \(2p-l)(p-l) p }, 



whose missing pairs of elements can be united to sets of {p — 1), so 

 that a 



Cf. {2p(p-i) p , p(2p- 1V_,| 

 is formed which breaks up into p principal-(2p — l)-sides. 



As in the series /j = 4, just as for ju = 3, the two extensions of 

 the theorems of Netto can be applied, we can form, in the tirst set 

 of hundred, systems £(4, 2) for: 



rc = 13, 25, 49; 4, 16, 40, 52, 64, 76, 100. 

 The still missing values are : 



ra = 37, 61, 73, 85, 97; 28, 88, 

 of which 85 might be acquired by 28. 



Physics. — "Remarks on the Leyden observations of the Zeeman- 

 Effect at low temperatures" By W. Voigt. (Communicated by- 

 Prof. H. A. Lorentz). 



(Communicated in the Meeting of October 31, 1908). 



The observations of Kamerlfngh Onnes and Jean Bequerel l ) on the 

 ZEEMAN-Effecl at exceedingly low temperatures have led to some 

 surprising results, among which are two, very interesting from the 

 theoretical standpoint. These I will try to throw light upon in the 

 following. 



I. It has been found, when the observation was made along the 

 optic axis of an uniaxial crystal (where such a body acts as if 

 isotropic) a longitudinal magnetic field being used, that the compo- 

 nents of the ZEEMAN-doublet have different intensities. The component 

 on the side of the shorter wave-length had generally (though not 

 always) the greater intensity. 



This result seems to show, that in the crystal one sense of rotation 

 is preferred to the other. Therefore it might be interesting, to consider 

 firstly the effect of a magnetic field on a naturally active crystal. 

 I may at this point remark that, contrary to what should be expected, 

 the effects of the natural activity and of the magnetic field do not 

 superpose each other, but rather singular combined effects appear in 

 the neighbourhood of an absorption band. 



l ) J. Becquerel and Kamerlingh Onnes. These Proceedings February 29 th 1908. 



