THIRD SERIES.— VARIABLE POLARIZABILITY OF HEAT. 181 



18. It occurred to me, that it would be satisfactory for the farther and in- 

 dependent confirmation of the conchisions just given (which were then only par- 

 tially obtained), to examine the index of polarization (by which I mean the per- 

 centage of the heat stopped in the crossed position of the polarizing and analyzing 

 plates) deducible for different sorts of heat, from a series of experiments made 

 wholly without reference to this question, I mean those on Depolarization, con- 

 sidered in another section of this paper, and which, it will be seen by a reference 

 to the mode of reduction there employed, required to be recomputed in order to 

 give the index of polarization. 



19. I at first imagined, that the experiments made with each of the three 

 kinds of heat then employed (Argand-lamp, incandescent platinum, and dark 

 hot brass) would give throughout the same result. This was far from being the 

 case ; the interposition of the depolarizing plate of mica between the polarizing 

 and analyzing plate, acting simply by transmitting only certain rays of heat, had 

 modified the index of polarization, and that more or less, as the thickness of the 

 interposed mica was more or less considerable. Such a result might have been 

 anticipated, as in exact conformity with the discovery I had formerly made ; but 

 I was misled by a false notion, which I had heedlessly adopted, and suffered to 

 remain unquestioned, that, in order to affect the index of polarization, the heat 

 must have been modified by transmission previous to its falling upon the first or 

 polarizing plate, whilst, in the experiments referred to, the modification took 

 place between polarization and analyzation.* Of course, when I perceived this 

 oversight, the confirmation of my views was greater, because it was unforeseen.. 



20. But the most material result of the examination of those experiments 

 was this. By a reference to the section on Depolarization, it will be seen that 

 five different thicknesses of mica (varying from three to sixteen thousandths of an 

 inch) were interposed successively, and the index of polarization determined for 

 each of the three kinds of heat. Now, upon examining the result of these fifteen 

 experiments, I clearly perceived (amongst occasional irregularities) this law to 

 prevail, — that, whilst a film of mica .003 inch thick scarcely altered the characteristie 

 properties of heat from different somxes, as shewn hy their variable indices of po- 

 larization, an increased thickness of mica had almost no sensible effect upon the heat 

 from the Argand-lamp, but it increased the index of polarization of dark heat so 

 fast, that, with a thickness of mica of .016 inch interposed, the apparent index of 

 polarization for heat from the Argand-lamp, incandescent platinum, and dark hot 

 brass, was almost the same. 



* Lest this confusion should, by possibility, occur to any one, as it did to myself, I will observe 

 that the position of the sifting or modifying plate, absorbing the least refrangible rays, is quite imma- 

 terial, provided it occur between the source and the indicator of heat, for whether the rays in question 

 are absorbed before or after polarization, those which ultimately escape and reach the pile are the only 

 ones of which the index of polarization is measured. 



