Second Series—Circular Polarization. 467 
time, proves the extension of one of the most important laws of 
light to the case of heat. 
37. It is well known that the following law holds for pola- 
nized light. When light, polarized in any plane, is reflected from 
a refracting surface ar the polarizing angle for that surface, it is 
wholly polarized in the plane of incidence. If it be incident at a 
SMALLER angle than the polarizing angle, the reflected light is (po- 
larized in a plane lying on the farther side of the plane of incidence 
from the plane of primitive polarization. If it be incident at a 
GREATER angle than the polarizing angle, the plane of polari- 
zation will be on the same side of’ the plane of incidence as at first. 
Now, this I have fully verified in the case of heat. Having po- 
larized heat by transmission through a mica bundle, mounted as 
in Fig. 5, in a plane inclined + 45° to the plane of reflection, 
which it subsequently underwent at the first surface of a thick 
mica plate, I examined its state of polarization by another simi- 
lar mica bundle interposed between the reflecting mica and the 
thermal pile. I found that at great incidences the plane of po- 
larization was on the same side of the plane of reflection as at 
first, whilst at smaller incidences it was thrown to the opposite 
side. I varied the incidence until the plane of polarization co- 
incided with the plane of reflection, when I concluded that I had 
reached the polarizing angle. This was found by the quantity of 
effect when the plane of analyzation was inclined + 45° and 
— 45° to the plane of reflection. With dark heat, from brass at 
700°, I estimated the polarizing angle to be 57° nearly (16th 
March 1836). By experiment I found that the polarizing angle 
for the same mica surface and for homogeneous red light was 59°: 
§5. On the Circular Polarization of Heat. 
38. In my last paper I shewed (art. 75) that heat may be 
circularly polarized, like light, by the doubly refracting action of 
a plate of proper thickness. This circumstance is indicated by 
VOL. XIII, PART II. 30 
