ERBIUM SALTS. 59 



the field. A crystal less than 1 mm. thick in a field of 2720 c.g.s. units 

 causes a difference of X in the middle of some bands. 



To explain the Zeeman effects observed we must consider whether the 

 external field is simply added to the intramolecular fields, or whether there 

 are variations produced there by the presence of the atoms. If one assumes 

 the same vibrators in every case, electrons with a value of e/ra of 1.8 (10) 7 , 

 then the internal magnetic fields must vary between 200,000 c.g.s. units 

 in one direction and about the same number of units in the opposite direc- 

 tion. Assuming, on the other hand, that the intra-atomic field simply has 

 the external magnetic field added then Becquerel gives the following values 



of e/m = 27r V -& 77, + referring to a positive electron: 



X 5206.5+ + , e/m= + 4.5 (10) 7 ; X 5211.3++, -0.3 (10) 7 ; 



A5215.5++, +2.1 (10) 7 ; A5221.5 + + , +1.6 (10) 8 ; 



X 5225.6, -1.4 (10) 8 ; X 5236.6++, + 2.9 (10) 7 ; 



X 5242.0+ +, +1.9 (10) 7 ; X 5245.8+ + , +4.5 (10) 7 ; 



X 5251.1, -7.1 (10) 7 ; X 6422.7, - 1.6 (10) 8 ; 



X 6434.5, - 1.4 (10) 8 ; X 6474.0, +3.8 (10) 7 ; 



X 6505.6 + , +5.1 (10) 7 ; X 6523.4+ + , -5.9 (10) 7 ; 



X 6537.0 + , -2.8 (10) 7 ; X 6542.5, +3.4 (10) 7 ; 



X 6564.4+, -8.6 (10) 7 ; X 6581.0+, +3.6 (10) 7 . 



For convenience we will consider that some of the above bands are 

 due to + electrons, and some due to electrons, according to the value 

 of e/m as calculated from the above formula. 



The magnetic rotatory polarization is very closely related to the Zee- 

 man effect, the + and electrons behaving differently in this case also; the 

 sense of the rotation being different for the + and for the electrons. 



J. Becquerel, in Le Radium, March, 1907, gives a theory for the mag- 

 netic optical effects which are observed in crystals. The general basis of his 

 theory is somewhat similar to that of Voigt. 1 The electrons are considered 

 to move along each coordinate axis independently. The three directions are 

 considered to be the same for each electron and independent of the period 

 of the exciting light. The form of the equations is then similar to those of 

 a pendulum experiencing great frictional resistance. The theory as here 

 developed explains some but not all of the different types of resolution found 

 experimentally. As different results have been obtained, especially by 

 Page, 2 the development given by Becquerel will not be considered in detail. 



Later papers by Becquerel 3 deal with the effect of a magnetic field 

 on the absorption spectra of certain crystals of xenotine, tysonite, parisite, 

 monazite, apatite, and zircon at low temperatures. The bands in general 

 are displaced towards the violet, especially for tysonite, as the tempera- 

 ture is lowered. Many changes of relative intensity occur and at low 

 temperatures the bands are invariably narrower and more intense. 



1 Ann. Phys., 6, 346 (1899); 6, 784 (1901^; 8, 872 (1902). 

 * Trans. Camb. Phil. Soc, vol. 20, No. 13, 291, 322. 



3 Compt. rend., Feb. 25, Mar. 25, May 13, June 17, Aug. 19 (1907); Le Radium, 

 Sept. (1907). 



