226 



On Polarization of Light by Refraction. 



able, — namely, that the impolarized part of the pencil, in place of 

 having suffered any physical change, retains the condition of common 

 light. 



T shall now proceed to. apply to this subject the same principles 

 which I have already applied to the polarization of light by reflexion, 

 and to establish on the basis of actual experiment the true laws of the 

 phaenomena. 



The first step in this inquiry is to ascertain the law according to 

 which the polarizing force of the refracting surface changes the posi- 

 tion of the planes of polarized light, — a subject which, in as far as I 

 know, has not occupied the attention of any other person. 



If we take a plate of glass deviating so slightly from parallelism as 

 to throw off from the principal image the images formed by reflexion 

 from its inner surfaces, we shall be able to see, even at great obli- 

 quities, the transmitted light free from all admixture of reflected light. 

 Let this plate be placed upon a divided circle, so that we can ob- 

 serve through it two luminous discs of polarized light A, B, (Fig. 1.) 



Fig. 1. 



formed by double refraction, and ha- 

 ving their planes of polarization incli- 

 ned -1-45° and —45° to the plane of 

 refraction. At an angle of incidence 

 of 0°, when the light passes perpen- 

 dicularly, the inclination of the planes 

 of polarization will suffer no change ; <^ 

 but at an incidence of 30° they will 

 be turned round 40' ; so that their in- 

 clination to MN or the angle aec will 

 be 45° 40'. At 45° their inclination 

 will be 46° 47'. At 60° it Will be 

 50° 7' ; and it will increase gradual- 

 ly to 90°, where it becomes 66° 19'. 

 Hence the maximum change produ- 

 ced by a single plate of glass upon the 

 planes of polarization is 66° 19' — 

 45°=21° 19', an effect exactly equal 

 to what is produced by reflexion at 

 angles of 39 or 70°. It is remarka- 

 ble, however, that this change is made in the opposite direction, the 

 planes of polarization now approaching to coincidence in a plane at 



