VOL. XC.] PHILOSOPHICAL TRANSACTIONS. 705 



that in the focus, so as to participate of the alternate screening, and also to receive 

 a small share of one side of the invisible N o 2 N o 3 



heat-image, which, though unseen, we Min. In the Focus. In the Edge of it. 



know must be formed in the focus of the ***£ ^ gj » 



lens. Here, if our reasoning be right, Open 11 64>\ 64 



the assistant thermometer should be af~ greened 12 J <& 6s| 



fected by alternate risings and fallings; Screened 16 6"2£ 63 



but they should not be so considerable as °P en 18 63 £ 63 l 



those of the lens. Here the changes of the thermometer N° 2 were — 4. -f 4. 

 — 14. + 4- — 1 + i ; and those of N° 3 were — 4. + .j. _ 4- -f i — 4. -f. 4. All 

 which so clearly confirm the effect of the refraction of the lens, that it must now 

 be evident that there are rays issuing from hot iron, which, though in a state of 

 total invisibility, have a power of occasioning heat, and obey certain laws of re- 

 fraction, very nearly the same with those that affect light. 



As we have now traced the rays which occasion heat, both solar and terrestrial, 

 through all the varieties that were mentioned in the beginning of this paper, and 

 have shown that in every state they are subject to the laws of reflection and of re- 

 fraction, it will be easy to perceive that I have made good a proof of the first 3 

 of my propositions. For, the same experiments which have convinced us that, 

 according to our 2d and 3d articles, heat is both reflexible and refrangible, establish 

 also its radiant nature, and thus equally prove the first of them. 



Explanation of the Figures. — Plate 12, fig. 1, shews the arrangement of the apparatus used in the 

 2d experiment, a is the small mirror with its adjusting screws m, n. N° 2, is the thermometer in the 

 focus of the mirror. N° 3, the assistant thermometer, b, a small screen for the thermometer N° 2. 

 c, the candle, d, the poker which, in the 4th and 10th experiments, is to be placed in the situation 

 of the candle 5 the rest of the apparatus being brought nearer to it. 



Fig. 2, shews the apparatus used in the 3d and 8th experiments, a, the mirror. N° 2, the ther- 

 mometer, bcd, a desk adjustable to different altitudes, e, the prism receiving the sun's rays through 

 an opening in the window shutter f. 



Fig. 3, a b is the front of the apparatus, which in the 5th experiment, is exposed to the fire of the 

 chimney, c, is the opening in the front plate ab, for the admission of heat, d, is the small mirror which 

 reflects the rays of heat, e, is the hole through which the heat passes to the thermometers. N° 1 and 

 N° 4, are the thermometers, f, is a prism, which, in the 6th experiment, is to be placed in the room 

 of the mirror d. 



Fig. 4, a, is the board that holds the apparatus used in the 7th experiment, b, the prism, c, the 

 spectrum, thrown partly on the paper with parallel lines, and partly on one of the small tables which 

 support the board, d, the mirror which reflects the rays of heat sideways. N° 1, the thermometer 

 which receives the reflected rays. N° 2, the standard thermometer. 



Fig. 5, ab, is the front which, in the 9th experiment, is put close to the flat side of a heated iron 

 stove, c, is the mirror, d, the feather-edged slip of deal, on two pins. N° 2, the thermometer 

 which receives the rays condensed in the focus of the mirror. N° 3, the standard thermometer, e, a 

 small screen tied to N° 3, to guard it from reflected heat. 



Fig. 6, A, the lens in the apparatus used for the 12th experiment. N* 2, the thermometer placed in 

 its focus, b, the screen with an aperture for admitting the rays of heat, c, the eccentric pivot for 

 turning away the lens, d, the candle. 



Fig. 7, a, the burning lens, covered ; with the prismatic spectrum thrown on an opening, left for it, in 



VOL. XVIII. 4 X 



