160 Professor Forbes on the Refraction 



verify, was the complementary nature of the transmitted heat, 

 when the plane of analyzation is parallel, and when it is perpen- 

 dicular, to the plane of polarization. 



64. It is well known in the case of light, that when no crys- 

 tal is interposed between the Polarizing and Analyzing Plates, 

 or when the crystal has its Principal Section parallel or perpen- 

 dicular to the plane of primitive polarization, the whole of the 

 light is stopped * when the plates are perpendicular or crossed ; 

 the whole is transmitted when they are parallel. If the Princi- 

 pal Section of the crystal be now inclined 45° to the plane of po- 

 larization, the depolarizing effect is a maximum, a portion of 

 light now being transmitted to the eye, the plates remaining 

 crossed, which was not transmitted before, and, in like manner, 

 a portion of the light which was formerly transmitted when the 

 plates were parallelbemg now stopped. Now these two quantities 

 are equal to one another, and therefore the sum of the intensi- 

 ties of illumination in the two cases (plates parallel and plates 

 crossed) is a constant quantity. Now these two pencils corres- 

 pond to the ordinary and extraordinary image in an analyzing- 

 prism of calcareous spar. Let us call these intensities O 2 and E 2 . 

 Let the whole quantity of polarized light, or the value of O 2 , 

 when the principal plane of the crystal coincides with that of po- 

 larization, be F 2 , and, under the same circumstances, E 2 — zero. 

 Then since the two effects are complementary, whatever be the 

 position of the principal plane, O 2 + E 2 — const. =: F 2 , 



and E 2 = F 2 -0 2 ; 



or the whole intensity gained by the extraordinary pencil (which 

 at first was zero), by the depolarizing influence of the crystal, is 

 equal to that lost by the ordinary pencil. 



65. That the same law holds in the case of heat, the expe- 



* That is, not reflected when the light is analyzed by reflection, or not transmitted 

 when it is analyzed by refraction. In these experiments the latter method was al- 

 ways used. 2 



