368 Prof. Forbes on the Refraction and Polarization of Heat. 



interpolated from the successive observations marked (I), (2), 

 (S), &c. which are made under similar circumstances. 



1835. Jan. \Q,— Source of Heat, Incandescent Platinum. Po- 

 larizing Mica Plates E and F. Film of Mica interposed. 

 No. I. 



Position of 

 Mica plates. 



[Eat 0°] 

 F at 90° 



Fat 0« 

 Fat 90° 

 Fat 0<> 

 F at 90° 

 Fat Oo 

 Fat 90° 

 Fat 0° 

 F at 90° 

 Fat 0° 

 Fat90o 

 Fat 0° 



Principal Sec- 

 tion of inter, 

 posed Mica, at 



(45° (1) 



\45 



/ 45 (2) 



\45 



/ 45 (3) 



^2 



\45 



/ 45 (4) 



(^ 



\45 

 /45 (5) 



145 



1 45 (6) 



(^ 

 145 



Multiplier. 



140-8 



2} 



4} 

 4} 



^i 

 3f 



8} 

 ^} 



?} 

 8} 

 «} 



5} 



Total Po- 

 larization. 



Depolari- 

 zation. 



Total Po.|Depolari- 

 larization zation 

 corrected , corrected 



-j_ 20-8^ 



+ 2 '2' 



6-0 



4 '2 



4-9 



4-5 



5 



5'1 



2°75 



2 1 



2-65 



2-2 



2-2 



Mean, 



Ratio. 



100 : 46 



100 : 50 



100:54 



100 : 49 



100 : 44 



100 : 43 



100 : 48 



When the analysing plate F is said to be at 0°, it is pa- 

 rallel to the plate E. When the principal section of the in- 

 terposed film is at 0°, it is parallel to the plane of incidence 

 at the plate E ; at 45° it is inclined 45° to that plane. The 

 signs + and — in the column of " depolarization," indicate 

 whether the effect of the interposed film was to increase or di- 

 minish the heat transmitted. 



72. The physical meaning of the expression for the inten- 

 sity of the depolarized light, E^ = F^ sin^ tt (— "^) will be 



found to be this. When the thickness of the interposed film 

 is such as to give a retardation of 0, X, or any whole multiple 

 of A, E is equal to nothing, or no light is depolarized, and 

 between those values the amount of E^, or the intensity of the 

 depolarized light, will gradually increase from the values 0, 



