14 Experiments on the Refrangibilily 



It will now be eafy to draw the refult of thefe obfervations 

 into a very narrow compafs. 



The firft four experiments prove that there are rays coming 

 from the fun which arc lefs refrangible than any of thofe that 

 affect the fight. Thev are inverted with a high power of 

 heating bodies, but with none of illuminating objects ; and 

 this explains the reafon why they have hitherto efcaped un- 

 noticed. 



My prefent intention is, not to affign the angle of the lead 

 refrangibilily belonging to thefe rays, for which purpofe 

 more accurate, repeated, and extended experiments are re- 

 quired. But, at the diftance of 52 inches from the prifm, 

 there was ftill a confiderable heating power exerted by our 

 invifible rays, one inch and a half beyond the red ones, 

 meafured upon their projection on a horizontal plane. I 

 have no doubt but that their efficacy may be traced ftill 

 famewhat further. 



The fifth and fixth experiments fhow that the power of 

 heating is extended to the utmoft limits of the vifible violet 

 rays, but not beyond them ; and that it is gradually impaired 

 as the rays grow more refrangible. 



The four laft experiments prove that the maximum of the 

 heating power is veiled among the invifible rays, and is pro- 

 bably not lefs than half an inch beyond the laft vifible ones 

 when projected in the manner before mentioned. The fame 

 experiments alfo (how, that the fun's invifible rays, in their 

 lefs refrangible ftate, and confidcrably beyond the maximum, 

 ftill exert a heating power fully equal to that of red-coloured 

 light; and that, confequently, if we may infer the quantity 

 of the efficient from the effect produced, the invifible rays of 

 the fun probably far exceed the vifible ones in number. 



To conclude, if we call light, thole rays which illuminate 

 objects, and radiant heat, thofe which heat bodies, it may 

 be inquired, Whether light be effentially different from ra- 

 diant heat ? In anfwer to which I would fuggeft, that we 

 are not allowed, by the rules of philofophifing, to admit two 

 different caufes to explain certain effects, if they may be ac- 

 counted for by one. A beam of radiant heat, emanating 



from 



