498 M. Verdet on the Optical Properties developed in 



We see from these tables that the magnetic action and the 

 rotation may vary in the relation of 1 to 3, and still remain pro- 

 portional. It is further to be remarked, that in each series of 

 experiments the magnetic action is made to vary in two ways ; 

 by varying sometimes the intensity of the current, sometimes 

 the distance of the branches of the electro-magnet. From 4 to 

 20 Bunsen's elements were employed, and the distance between 

 the armatures was varied from 50 to 90 millims., so- that the 

 distance from each of the armatures to the middle of the 

 transparent substance might vary in the relation of 1 to 2. 

 In order to show that in both cases the law was the same, and 

 also to give an idea of the agreement in the individual observa- 

 tions, of which the preceding tables only contain the mean, I 

 shall give here the experiments I. and VI. in detail. 



Experiment No. I. 

 Heavy glass, No. 1 ; thickness, 40 millims. White light was 

 employed. Distance between the armatures, 60 millims 20 

 Bunsen's elements. 



The magnetic action was measured ; the mean 



of two consecutive observations was . . 143*25 



Azimuth of the tint of passage 182° 28' 45" 



Azimuth of the tint of passage after reversing 



the current 191° 42' 30" 



Second determination of the magnetic action 143*5 

 Double rotation, 9° 13' 45"; magnetic action, 143*37 ; ratio, 3-86. 



Experiment No. II. 



Distance between the armatures, 80 millims. 20 Bunsen's 



elements. 



Magnetic action 115 



Azimuth of the tint of passage 183° 21 



Azimuth after inverting the current . . . 190° 49' 30" 



Magnetic action 115 



Doublerotation,7°28'30"; mean magnetic action, 115; ratio,3*90. 



Experiment No. III. 



Distance between the armatures, GO millims. 10 Bunsen's 



elements. 



Magnetic action 112*75 



Azimuth of the tint of passage 183° 24' 45" 



Azimuth after inverting the current . . . 190° 42' 30" 



Magnetic action 112 



Double rotation, 7° 17' 45"; mean magnetic action, 112*37; 

 ratio, 3*89. 



