166 Professor Forbes on the Refraction 



power for dark heat in the thin mica film mentioned in (56) is 

 now easily explained. Its thickness was such as to polarize 

 (nearly) circularly, the mean luminous rays. Its retardation, or 



o — e was then rz g for these rays. But we know from Mel- 



loni's experiments, that the heating rays are less refrangible 

 than the luminous rays (I mean in heat from terrestrial sources, 

 as well as that of the solar rays), and that generally in propor- 

 tion to this obscurity. Therefore, on the undulatory hypothesis, 



their waves are longer. Hence a retardation of - for light, would 

 be a retardation of less than -g if X be the length of a wave of 



heat from an Argand lamp ; it would be still less for heat from 

 incandescent platinum, and least of all for dark heat ; hence, as 

 the retardation is a smaller fraction of x or approaches zero, the 

 depolarization or the value of E 2 approaches zero. This per- 

 fectly coincides with the experiment of (56). 



74. Without attaching much weight to the numerical accu- 

 racy of the following results, it is worth quoting them as confirm- 

 ing the general fact, that obscure heat has longer undulations 

 than luminous heat. The numbers derived from Plate No. 2, 

 (see 65), are most to be depended upon, and the agreement of 

 the different series made with dark heat is highly satisfactory. 

 The numbers correspond to those of the last column in the ex- 



ample of (71). 







Mica Plate, No. I. Retardation for Light, 

 or o — e = .00004 inch. 



Argand Lamp, 

 Incandescent Platinum, 



Ratio of Total Polarization and Depolarization, 



or F» : E 2 . 



Number of Comparisons. 



4 100:80 

 4 100:78 



Brass about 700°, 



4 



100 : 69 



Mica Plate, No. 2. Retardation for Light, 

 or o e = .00002 inch. 







Argand Lamp, 

 Incandescent Platinum, 



3 

 6 



100 : 66 



100 : 47 



Brass about 700°, 



7 



100:52 



Ditto, 



4 



100:51 



Mercury about 500°, 



5 



100:52 



