( 603 ) 



In the case of the iiilVa-rcd rays whose absorption lias been 

 measured by Angstrom we are probably concerned with the vibrations 

 of charged atoms of oxygen or carbon. The mass of an atom of 

 hydrogen being about 1,3 X lO-^* gramme, I shall take 

 m = 2X 10-23. 

 The result then becomes 



iV=6 X 10'^- 



^ 11. The above method is not available for a sodium flame. 

 Hallo has however observed that the value of JSF for this body may 

 be deduced from his measurements of the magnetic rotation of the 

 plane of polarization and Geest has shown that the magnetic double 

 refraction in the flame may serve for the same purpose. In what 

 follows I shall only use one of H.\llo's results. 



In the first place it must be noticed that in the case to be con- 

 sidered, £ is much larger and much smaller than unitv. The 



fe 5' + ïj' 



radical in (24) may therefore be replaced by 



^ S' + n' 

 and the formula becomes 



c 5 



- = 1 + 



2 (5^ + n') 



Now, if there is an external magnetic field, the velocities of pro- 

 pagation tUj and to, of right and left circularly polarized light can 

 be calculated by a similar formula. We have only to replace § by 

 S, — ? and by § + ?.') From the results 



— = 1 H ;: ~ and — = 1 



2[(|-?r + V] ^, 2 !(§ + ?)' + VI 



we liiid for the angle of rotation per unit length 



_1 /"l.lV^il !jii i±i ! .31. 



In order to determine iV^ by means of a measured value of y, 

 we begin by observing that, in virtue of the equation (28), for 

 which we may write 



5 ^^ 2 m' n^ («„ — w), 

 each value of i determines a certain point in the spectrum whose 

 distance from the middle of the band is proportional to t. At the 



1) See LoRENTZ, Sur la theorie des phenomènes magnéto-optiques, etc., § 16. 



