On Calorific Radiation. 261 
cold upon the thermometer by means of the conical reflector ; be- 
cause it was readily concluded, that were cold merely the priva- 
tion of heat, the result ought to be just the reverse; for in that 
case it was supposed, that when the thermometer was placed at 
the wider end, mostly all the heat which flowed from the one-half 
of its ball, behoved to be collected to the narrow end by reflec- 
tion; whereas, in the other position, a much less portion of heat 
would be conducted to the cold body. To a superficial inquirer, 
this afforded a very excellent demonstration of the existence of 
frigorific rays. 
However, upon a more careful examination of the subject, 
founded upon principles generally received, it may easily be 
shown, that a conical reflector can only collect rays which are 
but little inclined to its axis ; and consequently, that most of the 
rays of heat flowing from the thermometer when placed at the 
wide end, are either thrown out at that end or fall again upon 
the thermometer. 
For let the cold body E, five inches in diameter, be now placed 
at the narrow end of the cone, whereof the diameter is one inch 
(see fig. 4), and let F, the ball of the thermometer of two inches 
diameter, be stationed at the wide end, of which the diameter is 
five inches ; the inclination of A B to C D being 16°: we shall 
also suppose at first, that the heat issues from the ball of the 
thermometer in lines perpendicular to its surface: 
Then a ray FG falling upon the reflector so as to make the 
angle FGA=30", will be reflected to H, making 1GH also = 
30°; but angle GHC will evidently be 30° together with 16° 
the inclination of AB to C D, in all 46°. For alike reason, an- 
gle HIG =62°, IK H =78°, and KL 1 = 94°: always in- 
creasing by 16°. Further than this it cannot go; for at the next 
reflection the ray will fall back between K and H, and finally 
return to the wide end. This will obviously be the fate of the 
greater part of the radiation ; much of it, no doubt, falling again 
upon the thermometer. 
Now it is evident, by thus tracing out the true paths of the 
rays, that all the radiation that is more inclined to the axis of the 
cone than about 12°, will return again to the wide end, if not to 
the thermometer ; and therefore, the radiation of the surface of 
a spheric segment 24° in breadth, which is only 1-46th of the 
surface of the entire hemisphere, is all that can reach the cold 
body. But when the thermometer is at the narrow end, it is evi- 
dent that that end embraces a space of the ball’s surface 60° in 
breadth, which is about 2-15ths of the hemisphere ; and that all 
the radiation from this portion of the ball ean readily find its way 
to the cold body. 
Thus it appears, that the quantity of caloric which arrives at 
R3 the 
