226 On Polarization of Light by Refraction. 
able,—namely, that the unpolarized part of the pencil, in placeiof 
having suffered oy physical change, retains the condition of —_— 
ee, now signed 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 bedis.of actual experiment the true laws. of the 
ena, me be 
The first step in this inquiry is to ascertain the law contiiaatii 
which the polarizing force of the refracting surface changes the posi- 
tion of the planes of polarized Jight,—a subject which, in as far as L 
know, has not occupied the attention of any other person. 
If we take a plate of glass _ so slightly from parallelism as 
to. throw off from the principal image the images formed by reflexion 
from its inner surfaces, we shall be be sae 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,. — 
formed by double refraction, and ha- Fig. 205 eee 
ving their planes of polarization caoies ae 
ned +-45° and — 45° to the plane of 
refraction, At an angle of incidence 
of 0°, when the light passes perpen- 
—" 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- 
elination 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° 197, 
Hence the maximum change produ- 
ont oh! a single plate of glass upon the 
* se —— is 66° = 
