392 Dynamic Theory. 



If a full round beam of light is allowed to puss through a transparent 

 body which causes it to twist or rotate as it goes through, there is no 

 way of discovering that fact. If you roll over a saw-log it still pre- 

 sents the same appearance. If its grain is twisted from end to end it 

 will, as a whole, be still the same shape. But if a plank is made to go 

 through a crack which twists it, the amount of twist is seen upon the 

 emergence of the plank. Polarization is a process by which a single 

 plank of a beam of light is cut out of the middle of it and saved for use, 

 the rest of the beam being thrown away. So the vibrations of plane 

 polarized light are in only one azimuth, all the rest being cut off. We 

 may imagine a number of beams polarized side by side, in which case 

 we should have a number of planks piled together in a uniform manner. 

 There is more than one wa}^ of obtaining polarized light. As said above, 

 the light passing through crystals of Iceland spar is polarized in two 

 azimuths, one at right angles to the other, so that two planks of a beam 

 get through, all the rest being absorbed by the peculiar construction of 

 the molecules of the crystal. Tourmaline is a crystal which polarizes 

 light in only one azimuth cuts a single board out of a beam and re- 

 duces the rest to heat. If light be presented to a slice of tourmaline, 

 it allows only a flat board of it to get through, that is, the light is polar- 

 ized. We can prove this by allowing the light to pass through two 

 slices of tourmaline. The light which gets through the first will also 

 pass through the second, provided it is held so that its plane of polar- 

 ization corresponds with that of the first. But if the second be rotated 

 to the right or left 00, the crack which allowed the board of light from 

 the first to pass through, is now turned crosswise, and the board cannot 

 pass. But if the second crystal be turned another 90 it will be flat- 

 wise again and the light passes. Both of these crystals are polarizers, 

 but when used as above the first is called a polarizer and the second an 

 analyzer. Crystals of agate also have the property of polarizing light, 

 and so have artificial crystals of iodo-disulphate of quinine. Light can 

 also be polarized by re/lection from different substances having smooth 

 or polished surfaces ; the angle required to produce the effect being dif- 

 ferent with different substances. When reflected from water the angle 

 is 52 45 ' from the normal ; with glass for a reflector the angle is from 

 54 35 ' to 58. The angle of polarization depends on the refractive 

 power of the body, " the index of refraction being tangent to the angle 

 of polarization." And at the angle of polarization the reflected ray is 

 perpendicular to the refracted ray. When a ray of common light is 

 polarized by reflection, the plane of incidence and reflection is the plane 

 of polarization also. 



When polarized light is passed through thin lamina?, or plates of vari- 

 ous crystals, as mica, selenite, quartz, &c. . ( when placed between a 



