164 



Experimental Investigah 



As a considerable part of the light that fell on the 

 lens before the reservoir A was lost in passing through 

 it, it is evident that the quantity received by this reser- 

 voir was less than that received by the reservoir B, 

 which was exposed to the direct rays of the sun ; and 

 we have seen that the latter was heated more rapidly 

 than the former. 



As we know not exactly how much light was lost in 

 passing through the lens, we cannot determine from the 

 results of this experiment whether convergent rays be 

 more or less efficacious in exciting heat than parallel 

 rays ; but the difference in the times of heating was not 

 greater, as it appears to me, than we might have ex- 

 pected to find it, supposing it to be occasioned solely 

 by the difference between the quantities of light acting 

 on the reservoirs. 



The result of the following experiment will establish 

 this point beyond doubt. 



Experiment No. 4. Having replaced the lens belong- 

 ing to the reservoir B, I adjusted this reservoir to such 

 a distance between the lens and its focus that the solar 

 spectrum was one inch in diameter; and I placed the 

 reservoir A at the same distance beyond its focus. 



