92 Mr. Powell on Solar Light and Heat. [Aug. 



June, (19), (20), it is equally obvious that the same distinction 

 must be attended to ; but if the lower bulb were only coated on 

 the halfoi its surface exposed to the sun, the effect (if any were 

 produced) would be greater, since here the ratio of " greater 

 inequality " must operate. In this way I have repeated the 

 experiment with a half coating of chalk, but with results so pre- 

 cisely the same as before, that no diminution was perceptible. 

 In order to try the effect with a coating of still greater absorp- 

 tive power, I repeated the experiment with a bulb half coated 

 with white silk pasted on ; the other being entirely painted with 

 Indian ink. No diminution took place, as will be evident from 

 the following results. The instrument was of a larger construc- 

 tion, and not graduated by Prof. Leslie's scale. 



Large differential thermometer. Bulbs, Indian ink ; and 

 white silk on half. Graduation from white. 



Glass over white bulb, 3 inches. Both exposed. 



14° 14° 



15° 15° 



12° 11° 12° 13° 



16° 15-5° 15° 14° 



16° 16° 



■ (43.) The question above alluded to (31, &c.) as to the exist- 

 ence and magnitude of a heating effect exterior to the cone of 

 light formed by a lens, is one of the greatest curiosity and inte- 

 rest, especially as connected with what appears to be the analo- 

 gous effect in the case of the prismatic spectrum. In a supple- 

 ment to a paper on the latter subject, communicated some 

 months since, and now before the Royal Society, I recorded a 

 few imperfect experiments, in which it appeared to me that this 

 phenomenon was clearly perceptible with a lens of about three 

 inches aperture, and 7*5 focal length, by means of the differen- 

 tial thermometer ; and I have since repeatedly observed the 

 same thing, though from the smallness of the effects observed I 

 am inclined to suppose that they could hardly have interfered in 

 any sensible degree with the experiments described in the present 

 paper. From the very small intensity of the effect in question, 

 I have experienced great difficulty in applying both the test of 

 its transmissibility through glass, and that of its relation to sur- 

 faces, so as to come to any decisive conclusion. I hope shortly 

 to be able to bring forward some investigation of these points. 

 Meanwhile, as connected with the subject of the present paper, 

 I may be permitted to give the results of a few experiments, 

 which clearly establish the existence, and convey an idea of the 

 quantity of this effect ; and which were made with a different 

 instrument, and under different circumstances, from the few just 

 alluded to. 



