ON THE POLARIZATION OF LIGHT. 237 



glass at the angle of incidence of 56°, as was first discovered 

 by Malus in 1808, the two rays or beams into which it is 

 separated will have different properties on different sides, and 

 in the case of the glass, supposing we hold another plate of 

 glass over the first, it wiU be found that the reflected ray will 

 pass through it when held in some positions, and not In others : 

 if the glass be turned round without altering its angle to the 

 horizon, the light wiU be reflected in one quarter, transmitted 

 in a second quarter, reflected again in the third, and when the 

 circle is completed it will be again transmitted ; that is, a beam 

 of light has acquired the property of sides, on two of which it 

 can, and on the other two it cannot be reflected ; and as these 

 properties bear some analogy to the poles of a magnet, a ray 

 of light so modified is said to be ■polarized. In the case of the 

 Iceland spar the polarization is eflfected by double refraction, 

 but In that of glass by reflection and transmission. In order 

 to explain the polarization by reflection from glass, the 

 apparatus represented by fig. 154 has been contrived: it 



■W\- 



Fig. 134. 



consists of two tubes of wood, D C ; C having at one end a 

 plate of glass. A, capable of being turned round an axis, so 

 that it may form different angles with the axis of the tube ; 

 another tube, D, a little larger than C, carrying a similar 

 piece of glass, B, is made to fit over it, so that by turning 

 either of the tubes the two plates may be placed in any 

 position in relation to one another. 



If a beam of light, r s, be allowed to fall upon A at the 

 polarizing angle of 56° 45', it will be reflected through the 

 tubes and wiU fall upon the second plate, B ; If this plate be 

 also placed at the angle of 56° 45', and the tube to which it is 



