REPORT ON RADIANT HEAT. 9 
medium. The method of observation adopted is indirect, turn- 
ing upon the determination of the critical angle of total inter- 
nal reflexion. This was ascertained in a rock-salt prism, having 
two angles of 40°, and one of 100°. The sentient surface of 
the pile is so placed with regard to the prism, that it continually 
receives rays coming from the source of heat, after undergoing 
two refractions and one reflexion, whatever be the angle of 
incidence, which is effected by a very simple but ingenious 
mechanical construction. Every kind of precaution to avoid 
error was adopted. And in this way the author obtained a 
series of indices “ for the mean quality of the heat most abun- 
dantly contained in the rays obtained from various sources.”’ 
These values are given in a table, and are professedly but ap- 
proximate. Prof. Forbes has, however, subsequently favoured 
me with an unpublished communication, in which he states, 
that while the numbers may be regarded as relatively correct, 
in order to become absolutely so, they must all be reduced by 
about ‘05. This will give the corrected series of results as 
follows :— 
Index of refraction for 
Rock Salt. 
POC AAU ee ee ia te ee LOT 
Do. transmitted through alum. . . . 1°548 
—_—- BIASS. aise vero LO 
———-—--—— opake glass . .  1°543 
eee on Sere 
Incandescent platina Dee RES ee a IH 
Do. transmitted by glass . . . . . 1°538 
aS a a Opake mica... 1:534 
(2 BIRSE STRIVE da ap le Soden aman aaiaarenimara F259 I 
Do. transmitted by clear mica . . . 1°527 
PCTCUEY Be Abe Tee ae a a OD 
Source of Heat. 
Mean luminous rays . . . . . « « 1°552 
From the experiments described in this section, the follow- 
ing general conclusions are deduced :— 
1. The mean quality, or that of the more abundant propor- 
tion of the heat from different sources, varies within narrow 
limits of refrangibility. 
2. These limits are very narrow indeed, where the direct 
heat of any source is employed. 
3. All interposed media (including those impermeable to 
light), so far as tried, raise the index of refraction. 
4. All the refrangibilities are inferior to that of the mean 
luminous rays. 
