LIGHT. 65 



If a cardboard, with a long slit in it, be held over the string, 

 the string will vibrate in one plane only, viz., in that of the slit. 

 Instead of one cardboard, two or more may be used, and as long as 

 the slits are in one plane the vibrations will proceed in that plane. 

 If, however, the slit of one card be set at a right angle to that of 

 another card placed over the string, then vibrations will no longer 

 be propagated. 



To make the action of the tourmaline plates still more intelligible, 

 we may compare it to that of a grating, A, Fig. 26, formed of parallel 

 vertical rods which permit the passage of all vertical planes, as 

 aa, but intercept that of horizontal planes, cc. As long as the rods 



Explanatory diagram of the action of tourmaline plates. 



of the gratings A and B are in the same plane, a string may be 

 made to vibrate through these gratings parallel to the rods, but when 

 set at a right angle this becomes impossible. 



When a ray of light strikes a plate of tourmaline it permits the 

 passage of those undulations which are parallel with its axis, but it 

 absorbs those undulations which are in planes at right angles to its 

 axis. The rays which pass through the plate produce the polarized 

 light. 



Polariscope. As the unaided eye cannot differentiate between 

 common and polarized light, instruments are constructed by which 

 the phenomena of polarization can be studied, and these instruments 

 are known as polariscopes, polarimeters, or, in a special case, sac- 

 clmrimeters. They all contain some substance, known as a polarizer, 

 such as tourmaline, Iceland spar, etc., serving as a polarizer of light ; 

 and a second substance, called an analyzer, for the detection of that 

 light. In the above experiment with tourmaline plates the first 

 plate serves as a polarizer and the second as an analyzer. 



The material used generally for polarization is Iceland spar, as it 

 is more transparent than tourmaline. Instead of using plates of 

 this material, prisms, known as NicoVs prisms, are made by sawing 



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