170 



Exp. 4 was 



made with equal 

 apertures and 

 fpcftra j but t*he 

 one being within 

 the locus was 

 1'ormrd by con- 

 vergent rays, 

 and the other 

 withomt by di- 

 vcrfeat rays. 



No fenfible dif- 

 ference oc- 

 curred. 



c. rumford's experiments on heat and light. 



times ojf heating_was not greater, as it appears to me, than 

 we might have expecled to find it, fuppofing it to be occafioned 

 folely by the difference between the quantities of light adling 

 on the refervoirs. 



The refult of the following experiment will eftablitn this 

 point beyond doubt. 



Experiment IV. 



Having replaced the lens belonging to the refervoir B, I 

 adjufled this refervoir to fuch a diftance between the lens and 

 its focus, that the folar fpe&rum was one inch in diameter; and 

 I placed the refervoir A at the fame diftance beyonds its 

 focus. 



As the quantities of light directed toward both were equal ; 

 and the, diameters of the fpectra, confequently the denfities 

 of the light that formed them, were alfo equal; there courd 

 be no difference between the refults of the experiments with 

 the two refervoirs, except what was occafioned by the differ- 

 ence in the direction of the rays that formed- the fpectra. On 

 one hand thefe rays were convergent, and on the other divergent; 

 and I had inferred, that if parallel rays were in reality lefs 

 efficacious in exciting heat than convergent rays, as fome phi- 

 lofophers have fuppofed, divergent rays rauft be fiill lefs effi- 

 cacious than parallel rays, and confequently much lefs than 

 convergent rays. 



Having made the experiment with all poffible care, I found 

 no fenfible difference between the quantities of heat excited 

 in a given lime ; by divergent and convergent rays. 



The following are the particulars of the progrefs and refults 

 of this experiment: 



General refults 

 of this laft ex- 

 periment. 





From 



