ON POLARIZED LIGHT. 191 



will become brighter and brighter till the tube has been turned 

 round 90, when the plane of reflection from B is coincident with 

 and parallel to that from A. In this position the reflected ray, E 

 is brightest. If the tube be turned again, the light will grow 

 more and more faint, until another 90 are arrived at, when it 

 will again undergo reflection. Thus, changes will take place in 

 every quadrant of 90 until the starting-point is again reached, 

 the ray of light being alternately faint and visible. 



The same effect will be produced if we cause a ray of light, 

 R, Fig. 62, to pass through bundles of glass plates, A, B, in- 



clined at the proper angle. If the bundle of plates, B, be 

 placed as [in the figure, the ray s >, polarized by passing 

 through the bundle, A, will be incident on B at the polarizing 

 angle, and not a particle will be reflected, but it will be trans- 

 .mitted, as seen at v w. If B is now turned round its axis, 

 the transmitted light, v w, will gradually dimmish, and more 

 and more light will be reflected by the plates of B, till, after 

 a rotation of 90, the ray, v w, will disappear, and all the 

 light will be reflected. Alternate transmissions and reflections 

 will thus take place in every quadrant, as in the former case. 

 For the ray passing through the tube in Fig. 61, or the ray, 

 s r, in the last figure, we may substitute one of the polarized 

 rays formed by double refraction in a rhomb of Iceland spar, 

 as seen in Fig. 60, or we may employ with even greater ad- 

 vantage the single image prism of Mr. Nicol, who employed a 

 rhomb of calcareous spar divided into two equal portions, in a 

 plane passing through the acute lateral angles, and nearly 



