5 
plane inclined 45° to that of reflexion will again be plane 
polarized in a plane inclined at a certain angle ¢ (which is 
17° for steel) to the plane of reflexion ; and we must have 
72 
tan p=". (13) 
Also, at the maximum polarizing angle we must have 
o’—d= 90°. (14) 
And these two conditions will enable us to determine the 
constants m and x for any metal, when we know its maximum 
polarizing angle and the value of ¢; both of which have been 
found for a great number of metals by Sir David Brewster. 
The following table is computed for steel, taking m = 33, 
026 | .026 | .526 
575 | .475 | .525 
638 | .407 | .522 
.729 | .808 | .5 8 
850 | .240 | .545 
947 | 491 | .719 
1. 1. | 
The most remarkable thing in this table is the last 
column, which gives the intensity of the light reflected when 
common light is incident. The intensity decreases very 
slowly up to a large angle of incidence, (less than 75°,) and 
then increases up to 90°, where there is total reflexion. This 
singular fact, that the intensity decreases with the obliquity 
of incidence, was discovered by Mr. Potter, whose experi- 
ments extend as far as an incidence of 70°. Whether the 
subsequent increase which appears from the table indicates a 
real phenomenon, or arises from an error in the empirical for-_ . 
mule, cannot be determined without more experiments. It 
should be observed, however, that in these very oblique inci- 
dences Fresnel’s formule for transparent media do not repre- 
sent the actual phenomena for such media, a great quantity 
