Refrangibllily and Heal oj the invifibU Rays of tht Sun, ga^ 



meters ufed in the former experiments, mounted upon their fmall inclined planes, and 

 placed fo that the centres- of the fliadows of their balls fall upon the interfedion of thefe 

 lines. C D is the prifra at the window, and E is the fpedtrum thrown upon the table, fo 

 as to bring the lad quarter of an Inch of the red colour upon the ftand. In this arrange- 

 ment all the fpeilrum, except the vaniftiing laft quarter of an inch, pafled by the ed^e 

 of the ftand, and falling upon the table, could not interfere the experiments. The room 

 was darkened. 



The trials of heat were made by caufing the fpe£l;rum to fall either upon or oppofite the 

 outer thermometer, while the other two were kept as ftandards out of the plane of the 

 • refradlion ; and when it was judged neceflary, in repetitions, the thermometer, which had 

 been fubje£ted to the heat, was changed for one of the- others. In three experiments with 

 the fame thermometer, the heat produced — by rays affording no illumination, but falling 

 half an inch beyond the extreme confine of the red colour, was 6| degrees in lo minu:es } 

 — by rays falling one inch beyond that confine, it was 5^ degrees in 13 minutes j— and by 

 rays falling i\ inch beyond that confine, it was 3^ degrees in 10 minutes. At the other 

 extremity of the fpe£lrum there was no augmentation of heat produced beyond the confine 

 of violet rays. The diftance of the prifm was 52 inches- 



Hence It followed clearly that there are rays coming from the fun which are lefs re- 

 frangible than any which afFeft the fight ; and that they are invefted with a high power 

 of heating, but with none of Illuminating bodies, which explains the reafon why they have 

 hitherto efcaped unnoticed. 



As the heat was before found not to correfpond with the meafure of illumination in the 

 fpe£lrum, and in thefe experiments It extended beyond that limit, it became an objed of 

 intereft and importance to afcertain the place where the calorific power Is greJiteft. This 

 maximum of heat -was found by experiment to be about half an inch dijiant from the boundary of 

 the red colour, and the heat at one Inch was fully equal to that of the middle of the red 

 colour itfelf. The boundaries of what may be called the calorific fpedtrum lie between 

 the extreme of violet to an undetermined fpot at lead li Inch, or i|° beyond the boundary 

 of the red colour. 



In his concluding fummary, befides fome remarks and recapitulation, which are In- 

 cluded in the preceding account, the author adds, that If we may Infer the quantity of 

 the efficient from the efFcdt produced, the invlfible rays of the fun probably far exceed the 

 vlfible ones in number 5 and that If we call light thofe rays which illuminate objefts, and 

 radiant heat thofe which heat bodies, It may be Inquired, whether light be eflentlally 

 different from radiant heat ? In anfwer to which he fuggefts, that we are not allowed, by 

 the rules of phllofophizing, to admit two different caufes to explain certain effefls, If 

 they may be accounted for by one. A beam of radiant heat, emanating from the fun» 

 confifts of rays that are differently refrangible. The range of their extent, when difperfed 

 by a prifm, begins at violet coloured light, where they are moft refradted, and have the 

 leaft efficacy. Thefe calorific rays have been traced throughout the whole extent of the 

 S prifmatic 



