Third Series. — Depolarization of Heat, 111 



linear relation between the thickness of the plate and the 



o^~e 

 quantity , (both vanishing when the thickness = 0,) and 



A 



inspection of the figure will show that no doubt can attach to 

 the choice of the ambiguous numbers, and also that the straight 

 line represents in general remarkably closely the course of 

 those points. 



There is one' exception to this statement, and it is an im- 

 portant one. It will be observed that the interpolating line, 

 instead of passing through any of the dots set off for the mica 

 plate No. 3, bisects exactly tisoo dots. These dots are nearest 

 to one another in the case of dark heat, — wider apart with 

 incandescent platinum, and widest of all in the case of the 

 Argand lamp. The explanation is complete and satisfactory. 

 The interpolating line in all these cases gives a value of 



o—e . , E^ 



= 1, which gives a value of-™- = 1 ; in other words, 



infers a total polarization of the heat in the horizontal plane 

 (or in the case of light total darkness, when the polarizing 

 and analysing plates are parallel) which we know can only 

 occur when the heat is absolutely homogeneous. The want 

 of mathematical coincidence in this case infers the admitted 

 physical condition of want of homogeneity in the incident 

 rays. Hence, we infer that dark heat is most homogeneous ; 

 next, that from incandescent platinum ; and, least of all, that 

 from the Argand. 



The proportions give numerical results almost identical for 

 the three sources of heat ; a result so far contrary to what I 

 expected, that it shows that by this method we cannot hope 

 to discriminate the different lengths of waves of these kinds 

 of heat, as I had formerly supposed, and shows that the varia- 

 tion of A must be very small, or else (what is improbable) that 

 it is constantly proportional to the variation of the retarda- 

 tion o~e. 



In all the three cases we have as nearly as possible a value of 



l*^ for at a thickness of depolarizing mica, equal to 



A 



•020 inch, or '07 for a thickness of '001 inch. Let us com- 

 pare this with the case of light. The sum of the retardations 

 for the various mica plates, as given in art. 33, amounts to 

 •000199 inch ; the sum of the thickness in the next article is 

 •0361 inch, consequently the mean value of the retardation or 

 o—e is •OOOOOSS for a thickness of mica of one thousandth 

 of an inch. But the length of A for extieme red is '0000266, 



