'824.] 



Mr. Powell on Solar Light and Heat. 



93 



(44.) Observations of the heat exterior to the cone of light 

 formed by a lens. Aperture, 3*25 inches ; focal length, 7*5. 

 Bulb of photometer coated with Indian ink, in glass case. 



The two last observations show how much of the effect is to 

 be attributed to reflected light. 



(45.) The experiment of Sir W. Herschel, from which a maxi- 

 mum heating effect further from the lens than the focus of 

 greatest light is inferred, will be found in the Phil. Trans. 1800, 

 No. 15, Ex.23. It there appears that sealing-wax was scorched in 

 the same time in the focus, and at half an inch further from the 

 lens ; whilst at half an inch nearer, no effect was produced in 

 double the time. It can, perhaps, scarcely be inferred, that 

 this effect is due to the same cause as that which operates 

 outside of the luminous cone ; since it is obvious, that beyond 

 the focus the light again diverges, and we cannot with certainty 

 distinguish the effects due to light under the 

 peculiar modifications to which it may there 

 be subjected, from those which may arise 

 from some peculiar development of heat in 

 the same position. The mere inspection of 

 the adjoining diagram will illustrate the di- 

 rections which the differently coloured rays, 

 separated by the dispersive power of the lens, 

 are made to assume ; and with their different 

 combinations it is highly probable that very 

 different heating effects are produced. This 

 is a topic of great interest, and one which, if 

 more thoroughly examined, seems likely to 

 lead to a more complete acquaintance than 

 We at present possess with the nature of the 

 heating effects developed both by the rays of light themselves, 

 and at short distances from them. 



