THE NATURE OF ELECTRICITY. 29 



the same result follows ; but when both are turned in 

 the same direction, there is no change in the brightness, 

 and no extinction of the light. Substitute one for the 

 other, and the same results are obtained. 



It is evident, then, that the light in passing through the 

 first plate has undergone a change which affects its trans- 

 mission through the second, in any position except when 

 the optical axes of both are parallel; extinguishing it 

 entirely when they are at right angles to each other. 



It is also found that this effect, termed polarization, 

 occurs to light transmitted through or reflected from any 

 transparent medium, as glass, selenite, Iceland spar, 

 and various liquids. Polished metals also produce the 

 same effect on reflected light. And this reflection or 

 transmission takes place at a certain angle, known as 

 the polarizing angle, which varies in each substance 

 by a certain definite amount. 



Now let the ray be transmitted through, or reflected 

 from, a small piece of glass of suitable size or shape, 

 placed at the proper polarizing angle, and let the plates 

 be turned till their optical axes are at right angles, so 

 as to produce extinction ; then let the glass be sub- 

 jected to a powerful electric strain and the extinguished 

 light will re-appear, continue during the electric action, 

 and disappear when it ceases ; which shows that this 

 electric action has counteracted the effects of polar- 

 ization. 



Similar experiments with various substances, too 

 numerous to detail here, show similar results. 



Without anticipating another branch of our subject, 

 it may be stated here, that electricity and magnetism 

 are so closely allied that whatever affects one must 

 have some important relation to the other. 



