and Polarization of Heat. 165 



E ; at 45° it is inclined 45° to that plane. The signs -f 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 2 = F 8 sin 2 % (^7^) will be found to 



be this. When the thickness of the interposed film is such as to 

 give a retardation of 0, A, or any whole multiple of A, E 2 is equal 

 to nothing, or no light is depolarized, and between those values 

 the amount of E 2 , or the intensity of the depolarized light, will 

 gradually increase from the values 0, x, 2 A, &c. to the values 



a 3a 5a 



2> -g-> y? &c. and again diminish in the same manner to the next 



limit. When the retardation is -' -r-> — , &c. 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. Tn 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 



