166 Professor Forses 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 = ; for these rays. But we know from Met- 
Loni’s experiments, that the heating rays are /ess 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 > if 2 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 4 or approaches zero, the 
depolarization or the value of E* 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 Prate, No. 1. Retardation for Light, Ratio of Total Polarization and Depolarization, 
or 0 —e = .00004 inch. or F2; E?, 
Number of Comparisons. 
Argand Lamp, . ‘ . : 4 100: 80 
Incandescent Platinum, d : 4 100: 78 
Brass about 700°, J , r 4 100 : 69 
Mica Prate, No. 2. Retardation for Light, 
or 0 —e = .00002 inch. 
Argand Lamp, .- 3 : 4 3 100: 66 
Incandescent Platinum, 6 100 : 47 
Brass about 700°, 7 100: 52 
Ditto, 5 ‘ 4 100: 51 
5 100: 52 
Mercury about 500°, 
