500 M. Verdet on the Optical Properties developed in 



Experiment No. VIII. 



Distance between the armatures, 62 miUims. 6 Bunsen's 



elements. 



Magnetic action 94-12 



Azimuth of the tint of passage 185° 0' 45" 



Azimuth after inverting the current . . . 188° 55' 45" 



Magnetic action 94*25 



Double rotation, 3° 55'; mean magnetic action, 94*19; 

 ratio, 2*49. 



Eocperiment No. IX. 



Distance between the armatures, 90 millims. 6 Bunsen's 

 elements. 



Magnetic action 68*5 



Azimuth of the tint of passage 185° 29' 30" 



Azimuth after inverting the current . . . 188° 23' 30" 



Magnetic action 69*5 



Double rotation, 2° 54'; mean magnetic action, 69*0 ; ratio, 2*52. 



Each of these observations ought to be exact, as I have before 

 said, to two or three minutes, and consequently the measure of 

 the rotations does not allow a greater error than five or six 

 minutes ; the probable error of the measurement of magnetic 

 action is half a division of a scale. Although these errors are 

 so small, we may rest satisfied that they are sufficient to account 

 for the differences which exist between the several values of the 

 ratio of the rotation to the magnetic action determined in the 

 same series of observations. 



It is important to notice, that the proportionality of the rota- 

 tion and of the magnetic action is confirmed with the same ex- 

 actitude whether the distance from the magnetic centres to the 

 transparent substance is changed, or whether the quantity of 

 free magnetism accumulated in these various centres suffers a 

 variation. This law of proportionality is demonstrated by our 

 experiments for transparent bodies of finite dimensions, all of 

 whose parts are equally affected by the electro-magnet; it is 

 consequently true for all the infinitely small plates of which we 

 can conceive the transparent body to he composed. It follows, 

 hence, that the formula can be stated in the following manner : 

 — The rotatory power developed by the action of a magnetic centre 

 in an infinitely thin plate of a monorefr acting substance varies 

 proportionally to the magnetic action, i. e. directly as the quantity 

 of magnetism accumulated in this centre, and inversely as the square 

 of the distance. 



