292 DR. BREWSTER ON THE PHENOMENA AND LAV^S 
On the authority of this experiment I was led to believe that metals acted 
upon light like crystallized plates ; and when I found that the colours were 
not only better developed, but more pure after successive reflexions, it was a 
natural though a rash generalization, to conclude as I did, and as M. Biot did 
after me, that each successive reflexion corresponded to an additional thick- 
ness of the crystallized film. 
In order to show the incorrectness of this deduction, let a ray polarized +45° 
be reflected twice from steel at angles of 75°. In this case the effect of the 
second reflexion should be to double the tint produced by the first, if the tints 
are those of crystallized plates. The result, however, is, that the whole of the 
light is polarized in one plane, in place of consisting of two pencils polarized 
in opposite planes. M. Biot got over this embarrassment by regarding the tint 
produced by two reflexions as the white of the first order, which, in conse- 
quence of its complementary tint being black, is the only one where the light 
is all polarized in one plane : but had he examined the light reflected four 
times, six times, or eight times at 75°, he would have still found it all polarized 
in one plane, a result entirely incompatible with the supposition of the tints 
rising with the number of reflexions. That the tint is not the white of the first 
order may be more easily proved by making it pass along the axis of calca- 
reous spar ; for we shall find that in place of producing an increment of tint, 
the effect of the second reflexion has been to destroy entirely the effect of the 
first, and to restore the ray to common polarized light. All this will appear 
by the perfection of the system of rings seen through the spar. If we examine 
in a similar manner the light which has undergone any number of reflexions 
between the plates, we shall easily ascertain that the effect never exceeds that 
of a quarter of a tint in Newton’s scale. 
Having thus ascertained that light polarized + 45°, and reflected at the 
maximum polarizing angle of metals, is neither common light nor polarized 
light, nor light constituted like that which passes through thin crystallized 
plates, I conceived the idea of its resembling circularly polarized light — that 
remarkable species of light which comports itself as if it revolved with a cir- 
cular motion during its transmission through particular media. 
According to Fresnel’s beautiful discovery, a ray of light polarized +45° is 
