208 POLAEIZATION BY REFLECTION. [SECT. xxi. 



may be polarized at any angle of incidence by a sufficient 

 number of reflections. For, if a ray falls upon the upper 

 surface of a pile of plates of glass at an angle greater or less 

 than a polarizing angle, a part only of the reflected ray will 

 be polarized, but a part of what is transmitted will be polar- 

 ized by reflection at the surface of the second plate, part at 

 the third, and so on till the whole is polarized. This is the 

 best apparatus ; but one plate of glass having its inferior 

 surface blackened, or even a polished table, will answer the 

 purpose. 



