and Polarization of Heat. 165 
E; at 45° it is inclined 45° to that plane. The signs -+ and — in 
the column of “ depolarization,” indicate whether the effect of 
the interposed film was to increase or diminish the heat trans- 
mitted. 
72. The physical meaning of the expression for the intensity 
of the depolarized light, E? = F* sin’ z — will be found to 
a 
be this. When the thickness of the interposed film is such as to 
give a retardation of 0, 2, or any whole multiple of a, E? is equal 
to nothing, or no light is depolarized, and between those values 
the amount of E%, or the intensity of the depolarized light, will 
gradually increase from the values 0, a, 2a, &c. to the values 
aA BA 5A 
2’ 2’ 2? 
limit. When the retardation is + = = &e. half the light exactly 
is depolarized ; it is then circularly polarized ; in other cases, it is 
plane or elliptically polarized. 
73. Similar effects might be expected to occur in the case of 
heat. But we must recollect that it is even more difficult to ob- 
tain homogeneous heat, than homogeneous light, and that we shall 
have portions of heat differently depolarized by the same plate, 
(in consequence of the different character of refrangibility, indi- 
cating a different length of undulation), exactly as when we ope- 
rate upon white light. We know that heat of various degrees 
of refrangibility constitutes the solar heat, and probably all 
other kinds. Hence, no one plate can completely depolarize all 
these varieties. As far as my experiments go, made similarly to 
that of (71), heat unaccompanied by light is generally less de- 
polarized by a plate of given thickness than heat vividly lumi- 
nous. In the case of contrasting heat from an Argand lamp with 
that from incandescent platinum, and heat quite dark, this is 
strikingly marked, though not so decisively in comparing the 
two last kinds. If the inaccuracy be not in the experiments, it 
may very probably arise from the want of homogeneity in the 
heat just alluded to. The want of any apparent depolarizing 
1 
&e. and again diminish in the same manner to the next 
