Transparent Plates upon light. - 33 
“A pencil of light reflected from the second surfaces of trans- — 
parent plates, and reaching the eye after two refractions and an in- 
termediate reflexion, contains at all angles of. incidence from 0° to 
the maximum polarizing angle, a portion of light polarized in the 
plane of reflexion. Above the polarizing angle the part of the: pen- 
cil polarized in the plane of reflexion diminishes till cos (¢-+7)= 
(cos (t—2’) )?, when it disappears, and the whole pencil has the char- 
acter of common light. Above this‘last angle the pencil contains a 
quantity of light polarized perpendicularly to-the plane of reflexion, 
which increases to a maximum and then diminishes to zero at 90°. 
Let us now examine the state of the pencil C S’ that has suffered 
only one refraction and one reflexion. Resuming’ the formula 
tan = = aot it is evident that when (cos (1-7 ))?=cos 
(t+), @ 1A, and consequently the light is restored to common 
light. This takes place in glass at an angle of 82° 44’. At all an- 
gles beneath this the Saint contains light polarized in the plane of 
reflexion; but at all angles above it, the pone contains light 
zed perpendicular to the plane of reflexion, the quantity incr 
from 82° 44’ to its maximum, and rénareiing to its minimnm at 90°. 
_By comparing these deductions with the formula and table for 
reflected light given in my paper On the Laws of: the Polarization 
of Light by Refraction, the following approximate law will be ob- 
served. When ‘ 
Cos (i—7’) + =cos (t+7’) All the incident light i is reflected. 
(Cos (t—7’))? =cos (¢+7/) Half the incident light is reflected. 
(Cos (¢ —1’))*=cos (t+7/) A third of the incident light is reflected. 
(Cos (i- <> =cos (t-+2’) An nth part of the incident light nearly 
is reflected. 
This law deviates from the truth by a este progression as ” 
increases and always ities the value of the reflected light in de- 
fect. Thus 
Angles of Incidence. Values of n. Differences. 
ewe ee ee 
oe ee 8 a, ee ee + 
75 38 4 21 
68 56 . 8 38 
4 11 43 
Vos. XXIL*—Ne. 1: = 
