On the Solar and lerrejlrlal Ray ivhich occafton Heat. Jt 



being put together as they are placed when experiments of achromatic refra£Vions are to be 

 made, were found to give a fpe£trum nearly without colour. The thermometer being 

 placed in the middle of this fpeftrum, had its temperature raifed two degrees ; but 

 a thermometer placed near either end of the fpeftrum was not afFefted, This refult (hews 

 that the different refrangibility of heat, as well as that of light, admits of prifmatic cor- 

 reftion, and it alfo confirms the conftant ratio of the fines of refraftion to thofe of 

 incidence. 



Experiment 23. In burning glqffes the focus of the rays of heat is different from the focus of 

 the rays of light. A burning lens was placed in the rays of the fun, its aperture being 

 reduced to three inches, in order to leflen aberration. -The place of the luminous focus 

 was afcertained by throwing hair powder into the air with a pufF. A ftick of fealing wax 

 was then held 1", 6 or 4 beats of a chronometer in the contraded pencil half an inch 

 nearer to the lens than the focus. In this time no impreflion was made upon the wax. It 

 was then applied half an inch farther from the lens than that focus, and in eight- 

 tenths of a fecond or two beats of the fame chronometer it was confiderably fcorched. 

 When the fealing wax was alfo expofed to the focus of light, it was equally afl'efled in the 

 fame time. Whence the Dodtor apprehends we may fafely conclude, notwithftanding the 

 little accuracy to be expeded from fo coarfe an experiment, that the focus of heat was 

 certainly farther from the lens than the focus of light, and probably not lefs than one 

 quarter of an inch ; the heat at half an inch beyond the focus of light being ftill equal to 

 that in the focus itfelf. 



In the next place our author proceeds to the fubjeft of the tranfmiffion of heat through 

 diaphanous bodies. The experiments made by the prifm, the lens, and the mirror, are 

 fubjedi to certain natural imperfedlions, which muft impair the progrefs of fcientific in- 

 duftion. The prifm refraCls, refleds, tranfmits, and fcatters them at the fame time, and 

 the laws by which it afts in every one of thefe operations require to be inveftigated. A 

 lens not only partakes of the errors of the prifm, but alfo is fubje£l to the aberration from 

 its fpherlcal figure. And a mirror, befides its natural incapacity of feparating the rays of 

 light from thofe of heat, fcatters them very profufedly ; but our author adds that the fcanty 

 provifion of materials to a£l upon thefe rays has been partly our own fault, as every 

 diaphanous body may become a new tool in the hands of a diligent obferver. 



{To be continued.^ 



VU.—OutliKef 



