312 RADIANT HEAT. 



Sir W. Herschel (Phil Trans., 1800, p. 297,) placed a candle at 

 twenty-nine inches from a concave "metallic reflector; the focal ther- 

 mometer in five minutes rose 3|°; another out of the focus was not 

 affected. 



The same took place with a fire, and with red hot steel. 



2.) Polarization by reflection. 



Berard (Memoir before cited) tried the polarization of heat from 

 luminous sources, and found a considerable diminution in the position 

 when the light ceases to be reflected. 



There was of course here no distinction drawn between the heat 

 accompanying the light and the simple heat. Of the latter nothing- 

 is proved; the former may be merely an effect of the absorption of light, 

 and if so, the term polarization is applied to the heat without any 

 proof. 



I repeated these experiments, and, after all precautions, thought 

 there was a small perceptible effect, (when the simple heat was cut 

 off by a glass screen,) which was diminished in the position of non- 

 reflection for the light; when the whole heat was admitted no propor- 

 tional diminution took place. — [Edinb. Journ. of Science, vi, 303.) 



c.) Effect of surfaces on emission of heat. 



Nothing ascertained under this head, unless we except some remarks 



in the Edinb. Journ. of Science, No. ii, p. 302. 



I 



d.) Effect of surfaces on absorption of heat. 



All experimenters have usually blackened their thermometer. — 

 (Cavallo, Phil. Trans., 1780.) 



Prof. Robison exposed a thermometer on charred oah under a glass 

 cover to the rays of a fire, when it rose to 212° Fahr. — (Black's Led., 

 i, 547; Thompson, i, 127.) 



e.) Effect of screens. 



1.) Mariotte interposed a glass screen between the fire and concave 

 mirror, and found the heat no longer sensible at the focus. — (Biot, iv., 

 606; Mem. Paris, i, 344.) 



Scheele interposed a glass screen in the experiment before men- 

 tioned, and found the heat of a fire so much intercepted as to be no 

 longer sensible to the hand — not even sensible in the focus of a re- 

 flector. 



Pictet with the conjugate reflectors interposed a glass screen. The 

 focal thermometer, which had risen 10°, fell 7° in nine minutes; on 

 removing the screen it rose again. — (Essais de Phys., p. 63.) 



2.) Sir W. Herschel tried experiments on this point. — (Phil. Trans., 

 1800.) Two moveable objects illuminated by a lamp were viewed by 

 the eye, one through an open hole, the other through a hole covered 

 successively by different transparent media. One object was moved 

 to greater or less distance, till they appeared equally bright; the 



