274 Dr. L. Silberstein on Fluorescent Vapours 



4. Magnetic Rotation of the Fundamental Line. 

 For the fundamental line or the resonance spectrum we 

 have n = N and therefore, by (15), 



V k — Z , n . 



and, by (16), b/a = and 



tanf=-r (16 ) 



Thus, the fundamental line will continue to be rectilinearly 

 polarized, but its plane of polarization will be rotated round 

 the magnetic field through an angle f = (r, E)= arc tan(Z/&). 



It will be remembered that Z=-~yH, where H is the 



my 



absolute value or the intensity of the magnetic field. Thus 



Z is positive or negative according as q>0 or q<0. This 



settles the question of the sense of rotation. Whether the 



charge q is positive or negative, can only be decided by 



experiment. 



Let us, henceforth, write shortly 



qjmV=y, 

 so that Z = H?7, and 



tanf = H| (16 ) 



If f exists, and is great enough (for H of the order 10 4 ) 

 to be measured, the ratio rj/k, and therefore the product 



r)T, 



will be found experimentally (where r = 2jk is the relaxation- 

 or the extinction-time of the resonator). In order to find rj 

 and r separately, some further connexion is required, which 

 might be obtained from the first or the higher lines of the 

 resonance-spectrum. 



Before passing on to these it will be convenient to distin- 

 guish here a certain, particular, intensity H, viz. that 

 intensity which makes f = ±45°, i.e. that intensity which 

 makes 



Z = k for g>0, | 



Z=-k for q<0. \ ' 



Let us denote this particular intensity of the field by H c? . 

 so that 



H.=A:| 9 |. (17) 



We shall see later that, for iodine vapour, H c is of the 

 order of 10 4 gauss. 



