^6i6 RefraBlon ojinvifibk radiant Heat. 



method of comparative examination was adopted as in the laft e:£perlment. The refult alfo 

 was fjmilar, for which reafon I (hall confult brevity fo far as to avoid repeating the par- 

 ticulars in detail. 



Experiment 17. RefraBion of the inviftble rays of folar heat. One half of the before 

 mentioned burning lens was covered, and the prifmatic fpe£trum thrown upon the cover> 

 fo that the laft vifible red colour falling one-tenth of an inch from the margin of the pafte- 

 board, the invifible rays beyond the fpeftrum were fuffered to fall upon the lens. In the 

 focus of the red rays, or a very little beyond it, was placed the ball of one of the 

 thermometers, and the other thermometer as near to it as convenient. The thermometer 

 in the focp? was raifed 45 ° in one minute, whilft the other thermometer underwent no 

 ^hainge. 



As a little of the red colour was difcernible on the ball of the thermometer, the DodVor 

 was induced to try if the invifible rays could not be rendered perceptible to the fight by 

 condenfation ; and with this intention the next experiment was made. 



Experiment 1 8, Trial to render the inviftble rays of the fun vifthle by condenfation. The 

 laft defcribed experiment was repeated by withdrawing the lens till the extreme vifible 

 colour was -^s from the margin of the femicircular pafteboard cover. In thefe circum- 

 ftances there was no longer any tinge of colour or veftige of light to be feen on the ball of 

 the thermometer ; but, neverthelefs, the efFed of the invifible rays was fuch, that the 

 thermometer in the focus received 21° of heat, whilft the other near the focus remained 

 ftationary. The colour feen in the 17th experiment muft therefore, as the Do6tor ob- 

 feryes, have arifen from the imperfeft refradtion of a lens of fo great a diameter ; the 

 difB,culty of afcertaining the termination of a prifmatic fpeflrum in a room not perfe£lly 

 dark, and the gradual difFufion of rays of the fame kind over a confiderable fpace in con- 

 fequence of the breadth of the prifm. 



Experiment 19. RefraElion of invifible culinary heat. This experiment is attended with 

 (bme difficulties, on account of the feeble ftock of heat in a red hot lump of iron, or other 

 fjmilar thing, and the fpeed with which it is carried offl The Do£lor was prevented 

 fooip adopting fome contrivance to keep up this heat from the confideration that the 

 alternate rifing and falling of a thermometer in the focus of a lens, fucceflively covered and 

 uncovered, muft be afcribed to the refraftion of heat. The lens of 1,4 focus was placed 

 before the cylinder of iron of experiment 15, made very red hot, and the thermometer 

 placed in its focus was for each fucceffive two minutes firft expofcd to the heat, and then 

 fcreened from it by a fmall piece of pafteboard. The iron was at the beginning very red 

 hot. The changes, after the iron had ceafed to emit any light perceptible in a darkened 

 room, were eight in number, and regular, amounting to one degree at the beginning, and 

 at the end the effedl of the condenfed rays exceeded but half a degree the lofs of thofe 

 which were ftopped by the lens. 



Experiment ^Q. Confirmation of the lafl experiment. The 19th experiment was repeated 



with an afliftant thermometer. The principal thermometer being placed in the focus, as 



4 before 



