Transparent Plates upon Light. 



Fig. 3. 



31 



In order to demonstrate these views by an analysis of the changes 

 which the intromitted light experiences from the two refractions and 

 the intermediate reflexion of a transparent plate, I took a plate of 

 glass of the shape MN (Fig. 3.) having an oblique face M d cut upon 

 one of its ends. A ray of light R A, polarized -]- 45° and - 45°, 

 was made to fall upon it at A, at an angle of incidence of nearly 83°, 

 so that the inclination of the planes of polarization of the reflected 

 ray A P was about 36^°. Now the ray A C after reflexion in the 

 direction C S, without any refraction at B, where it emerges perpen- 

 dicularly to. M d, would also have had the inclination of its planes of 

 polarization equal to 36^° if there had, been no intermediate refrac- 

 tion at A 5 but this refraction alone being capable of producing an 

 inclination of 53° or a rotation of 53° — 45° = 8°, and this rotation 

 being in an opposite direction from that produced by the second re- 

 flexion at C, the inclination of the planes of polarization for the ray 

 C S is nearly 44^°, the reflexion at C having brought back the ray 

 A C almost exactly into the state of natural light. 



Without changing either the lighter the angle, I cemented a prism. 

 Mcc? on the face Mc7, so that cd was parallel to d'N, and I found 

 that the second refraction at b, equal to that at A, changed the incli- 

 nation of the planes of polarization to 53° ; that is, the two refrac- 

 tive actions at A and h had overcome the action of reflexion at C, 

 and the pencil h s actually contained light polarized perpendicular to 

 the plane of reflexion. 



In order to to put this result to another test, I took a plate M c N Q 

 (Fig. 3.) of the same glass, which separated the pencil b s reflected 

 at the second surface, from the parallel pencil A P reflected from the 

 first surface, and I found that at an angle of 83° the value of the 

 inclination I or <p for the ray was about 37 J°, while the value of I for 

 the ray b s was nearly 55°, an effect almost equal to the refractive 

 action of a plate at 83*^ of incidence. 



