102 Prof. Forbes's Researches on Heat. 



a reference to the mode of reduction there employed, required 

 to be recomputed in order to give the index of polarization. 



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

 the three kinds of heat then employed (Argand lamp, incan- 

 descent platinum, and dark hot brass) would give throughout 

 the same result for the same kind of heat. This was far from 

 being the case ; the interposition of the depolarizing plate of 

 mica between the polarising and analysing 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, w^hich I had heedlessly adopted, and suf- 

 fered to remain unquestioned, that, in order to affect the in- 

 dex of polarization, the heat must have been modified by 

 transmission prmo2/5 to its falling upon the first or polarizing 

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

 took place between polarization and analysation *. Of course, 

 when I perceived this oversight, the confirmation of my views 

 was greater, because it was unforeseen. 



But the most material result of the examination of those 

 experiments was this. By a reference to the section on de- 

 polarization, it will be seen that five different thicknesses of 

 mica (varying from three to sixteen thousandths of an inch) 

 vf&ce 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 pre- 

 vail, — that whilst aJilTn of mica "003 inch thick scarcely altered 

 the characteristic properties of heat from different sources, as 

 shown by their variable indices of polarization, an increased 

 thickness of mica had almost no sensible effect upon the heat 

 from the Argand lamp, but it increased the index of polariza- 

 tion 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. 



When I had fully seized this conclusion, the explanation 



• 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 immaterial, provided it occur 

 between the source and the indicator of heat jfor whether the rays in ques- 

 tion are absorbed before or after polarization, those which ultimately 

 escape and reach the pile are the only ones of which the index of polari- 

 zation is measured. 



