334 0« the Powers of the prifniatic Culours 



of heat, which lies at the other extreme. By feveral experi-, 

 merits, which time will not allow me now to report, it appears 

 that the maximum of illumination has little more than half 

 the heat of the full reel rays; and, from other experiments, 

 I likewiTe conclude that the full red falls ftill fliort of the 

 maximum of heat; which perhaps hcs even a little beyond 

 vifibie refraftion. In this cafe, radiant heat will at leaft 

 partly, if not chiefly, confift, if I may be permitted the ex- 

 preffion, of iuvifib'.e light ; that is to fay, of rays coming from 

 the fun. th:it have ftich a momentum as to be unfit for vifion. 

 And admitting, as is highly probable, that the organs of 

 light arc only adapted to receive imprefllons from particles 

 of a certain momentum, it explains why the maximum of 

 illumination fhould be in the middle of the refrangible rays; 

 as thofe which have greater or lefs momenta are likely to 

 become equally unfit for impreffions of fight. Whereas in 

 radiant heat, there may be no fuch limitation to the mo- 

 mentum of its particles. From the powerful eflcdts of a 

 burning lens, however, we gather the information, that the 

 momentum of terreftrial radiant heat is not likely to exceed 

 that of the fun ; and that, confequently, the refrangibility of 

 calorific rays cannot e.^tcnd much beyond that of colour'ijic 

 light. Hence we may alfo infer that the invifiblc heat of 

 red-hot iron, gradually cooled till it cgafes to fliine, has thp 

 momentum of the invifible rays \yhich, in the folar fpeftrum 

 viewed by day-light, go to the confines of red ; and this will 

 afford an eafy fqlutioi; of the refleiStion of invifible heat by 

 ponqave mirrors. 



Application of the Txcfult of the foregoing Ohfcrvat'ions to the 

 Method of vieiv'ing the Sun adijjntageoujlj , ivith Tele? 

 fcopes of large Apertures and high JMagnif'ing Fczuers. ■ 



Some time before the late tranfit of Mercury over the 

 diflcof the fun, I prepared niy 7 -feet telefcope, in order to fee 

 it to the heft advantage. As I wiflicd to keep the whole aper- 

 ture of the niiror qpen, I foun cracked every one of thp 

 darkened flips of wedged glaffcs, which are generally ufed 

 with achromatic tclefcopcs : none of them could withfiaud 

 \\^^ ^ccumul;Ued Jicat in the focus of pencils, where thef^ 



clal]f,i 



