Rotation of Light by Magnetic and Electric Forces. 309 



cross, for instance, and by bringing the eye as near as may 

 be to the glass, this difficulty is more or less overcome. 



2215. For the sake of supplying a general indication of the 

 amount of this induced rotating force in two or three bodies, 

 and without any pretence of offering correct numbers, I will 

 give, generally, the result of a few attempts to measure the 

 force, and compare it with the natural power of a specimen of 

 oil of turpentine. A very powerful electro-magnet was em- 

 ployed, with a constant distance between its poles of 2\ inches. 

 In this space was placed different substances: the amount of 

 rotation of the eye-piece observed several times and the ave- 

 rage taken, as expressing the rotation for the ray length of 

 substance used. But as the substances were of different di- 

 mensions, the ray lengths were, by calculation, corrected to 

 one standard length, upon the assumption that the power was 

 proportionate to this length (2163.). The oil of turpentine 

 was of course observed in its natural state, i. e. without mag- 

 netic action. Making water 1, the numbers were as follows : — 



Oil of turpentine . . . 11*8 

 Heavy glass (2151.) . . 6*0 



Flint-glass 2*8 



Rock-salt 2-2 



Water 1*0 



Alcohol less than water. 



iEther less than alcohol. 



2216. In relation to the action of magnetic and electric 

 forces on light, I consider, that to know the conditions under 

 which there is no apparent action, is to add to our knowledge 

 of their mutual relations ; and will, therefore, very briefly state 

 how I have lately combined these forces, obtaining no appa- 

 rent result (955.). 



2217. Heavy glass, flint-glass, rock-crystal, Iceland spar, 

 oil of turpentine, and air, had a polarized ray passed through 

 them; and, at the same time, lines of electro- static tension 

 (2149.) were, by means of coatings, the Leyden jar, and the 

 electric machine, directed across the bodies, parallel to the 

 polarized ray, and perpendicular to it, both in and across the 

 plane of polarization ; but without any visible effect. The 

 tension of a rapidly recurring, induced secondary current, was 

 also directed upon the same bodies and upon water (as an 

 electrolyte), but with the same negative result. 



2218. A polarized ray, powerful magnetic lines of force, 

 and the electric lines of force (2149.) just described, were com- 

 bined in various directions in their action on heavy glass 



