462 



Mr. S. S. Richardson on Magnetic Rotary 



The valne of a is known fairly accurately for the few 

 substances whose dispersions have been measured over a 

 large range of wave-lengths. Thus for quartz, fluorite, 

 sylvine, and rock-salt we have a Q = "37, *35, '25, *19 

 respectively, and the values of p calculated from (10) as 

 given below. 



Table IV. 



Substance. 



Quartz . 

 Fluorite 

 Rock-sait 

 Sylvine . 



(minutes). 



M. 



d. 



w D . 



4-9 



•01664 



60-4 



2-65 



1-5443 



•00897 



78-0 



3-18 



1-4339 



5-10 



•03280 



58-5 



2-15 



1-5443 



412 



•02670 



74-6 



1-95 



1-4904 



4-87 



In fluorite, rock-salt, and sylvine, Meyer * finds that the 

 fundamental formula 



S= * X^ (12) 



Amy) 1 d\ 



agrees well with the experimental results when n does not 

 involve infra-red influences. Hence for these substances 

 rthe dispersional period in the ultraviolet deduced from 

 imagnetic rotation should agree with that deduced from 

 dispersion, and if more than one period is involved the 

 effective ejm is therefore the same for both. Introducing 

 ,7] as above {vi = V2—v)'> Meyer's results give for 



Fluorite y- = 1*21 X 10 7 (e.m.u.) 



m 



Rock-salt „ = 1'51 



Sylvine „ = 1*42. 



The magnetic dispersion in quartz in the visible spectrum 

 'lias been subjected to very accurate investigation by Lowry f. 

 Taking the relative values obtained for *6708 fx and '4800 /a 

 inear the ends of the visible spectrum, namely, *646 and 

 1-318, with n = 1*5415 and 1*5501 respectively, and using 

 -equation (9) (Part I.), we obtain 



^ = •106 fi. 



The period thus deduced from the magnetic effect agrees 



* Ann. d. Phus. (30) p. 607 (1909). 



t Phil. Trans. A. vol. ccxii. p. 295 (1913). 



