REPORT ON RADIANT HEAT. 
m 
3. ) Refraction by lenses. 
Lambert collected the rays of a fire by a large lens, and 
found the heat scarcely sensible to the hand. 
Sir W. Herschel (Phil. Trans. 1800, pp. 272, 309, 327,) 
received the rays of a candle on a lens, with a pasteboard 
screen, having an aperture nearly equal to that of the lens ; the 
thermometer in the focus rose 2\° Fahr. in 3 min. ;—the same 
with the rays from a fire, and from a mass of red hot iron. 
Mr. Brande found the rays of a flame, concentrated by a lens, 
produced an effect on a blackened thermometer in its focus; 
the lens did not become heated. (Phil. Trans. 1820, Part I.) 
4. ) Dr. Ritchie found that if Leslie’s photometer be placed 
opposite a ball of iron heated almost to redness, no effect 
whatever will be produced ; but if the temperature of the ball be 
raised so as to shine in the dark with a dusky red colour, the fluid 
in the stem of the black ball will sink a considerable number of 
degrees. If the temperature of the ball be raised still higher, 
it will produce a greater effect upon the instrument than the 
flame of the finest oil-gas, though the one possesses a much 
greater illuminating power than the other. 
Dr. Turner and Dr. Christison have found that Leslie’s pho¬ 
tometer “ is powerfully affected by heat’’ when placed “ before 
a ball of iron heated so as not to be luminous, or even before a 
vessel of boiling water.” The opposite result of Dr. Ritchie 
may possibly be owing to some difference in the surface, sub¬ 
stance, or thickness of the black bulb employed. ( Edinb • 
Journ. of Science , iv. 321.) 
I have found differences, which I am at a loss to account for, 
between the effects on a differential thermometer with the bulbs 
of equal height, and one in which they are in a vertical line. 
5. ) That there exist essential differences between the con¬ 
stitution of the heating power of luminous hot bodies, and 
that of the same power proceeding from those which are non- 
luminous, was remarked by former experimenters. But it is a 
point which does not seem to have excited any close or syste¬ 
matic inquiry until the subject was taken up by M. De la 
Roche, whose researches are justly entitled to the high cele¬ 
brity they have acquired. The Report of the French Institute 
upon them will be found in the Annals of Phil. O. S. ii. 161 ; 
and a full account of the experiments in Biot’s Traite de Phys « 
iv. 640. 
The whole series of results is as follows 
