188 



PROFESSOR FORBES'S RESEARCHES ON HEAT. 



36. The following experiment was made with heat wholly unaccompanied 

 by light, and with the same mica plate. 



Dark Hot Brass : 14 inches from Pile, depolarizing Mica No. 3. 



Position of Pola- 

 rizing Plate K (I 

 being always atC''). 



AtO° 

 At 90° 



AtO" 



At 90° 



AtO° 



At 90° 



AtO° 



At 90° 



Position of 

 Neutral Sec- 

 tion of Mica. 



AtO° 



At 45° 



Galvanometer. 



Dynamical 



Effect. 



AtO° 



At 45° 



AtO° 



At 45° 



AtO° 



Mean 



Total Po- 

 larization 



[3.25]* 



4.0 



3.95 



4.0 



3.98 



Depolariza- 

 tion 



+ 3.75 



— 3.8 



+ 3.8 



3.3 



+ 3.7 



— 3.6 



3.64 



37. It now remains to explain how these observations have been discussed. 



The ratio ^ is at once obtained by dividing the second mean result by the first, 



and I have purposely quoted these observations, to shew how very nearly the 

 plane of polarization was thrown at right angles by the action of this particular 

 thickness of mica, especially in the case of dark heat, which appears to be owing 

 to its greater homogeneity, as we shall presently have reason to infer. 



38. We have seen above (ai't. 32) that 



r- = sin^ 180 

 F2 



, 1^1 



And therefore. 



£2 



• -1 /E' 



N'F2 



180'= 



Since the radical has an ambiguous sign, the equation will be satisfied by a 

 value of —^ equal to a fractional munber a, or by 1 — a, or 1 -|- a, or 2 — a, or 



* Omitted in the mean as manifestly too small, arising from the lamp being just lighted, and the 

 brass not fully heated. 



