REPORT ON RADIANT PIEAT. 
261 
served that there was a very considerable degree of aberration 
in the focus from an exact position ; considerably nearer to the 
reflector than the true focus, the effect continued undiminished. 
(Inquiry , p. 64.) 
5. ) Alleged reflexion of cold. 
An account of the earliest experiments will be found in the 
Memoirs of the Florentine Academy, (Waller’s Transl. p. 103; 
also Gaertner, 1781.) 
Pictet with conjugate reflectors found the thermometer sink 
when ice was in the opposite focus. (. Essais de Phys. p. 82.) 
Count Rumford employed a tube, a frustrum of a cone, open 
at both ends; placing ice at the small end, the thermometer at 
the large end sunk very little. The ice being at the small end, 
the thermometer at the large end fell considerably. Rays re¬ 
flected by the inside of the tube from the body at the large end, 
would be concentrated on that at the other. 
6. ) M. Frevost ( Essai sur la Calorique rayonnant , Geneva 
1809, and Recherches sur la Chaleur , p. 15,) proposes a theory 
of radiation, that heat is a discrete fluid every particle of which 
moves in a straight line, and such motions are constantly taking 
place in all directions, whether there be more or less heat 
present. Hence all bodies, whether of a higher or lower 
temperature, are supposed to be continually radiating heat; 
and this going on mutually tends to bring them all to an equili¬ 
brium of temperature. 
On this theory explanations are given of the apparent radia¬ 
tion of cold. 
The thermometer in the conjugate focus, when nothing is in 
the other, remains stationary, because the rays reflected from 
all the surrounding space so as to cross at the focus of the op¬ 
posite mirror, and be reflected in a parallel state to the other, 
and thence on to the thermometer in the focus, are exactly 
equivalent to those which the thermometer radiates. But when 
a mass of ice is placed in the opposite focus, it intercepts and 
absorbs a portion of the rays which would otherwise have 
fallen on the first mirror, and so have reached the thermometer, 
which in consequence radiates more than it receives, and there¬ 
fore sinks. 
A similar explanation applies to Count Rumford’s experi¬ 
ment. (See Thomson On Heat , Sfc. p. 163.) 
In the Quarterly Journal of Science (June 1830, p. 378,) 
some observations are given on this subject, and an explanation 
offered, which, though very ingenious, appears somewhat com¬ 
plicated. 
