1824.] Mr. Powell on Solar Light and Heat. 289 



balance the effect. This was done in Exp. 8, 9, and 10, upon 

 the bulb before plain, by attaching to it a small piece of light- 

 brown silk. In these a decided action was produced on this 

 bulb ; whilst an equal one in the contrary direction was displayed 

 by the smooth black surface ; but this might be occasioned by 

 the greater power of absorbing light now acquired by the bulb 

 before transparent. In the next experiment, therefore (No. 11), 

 I attached a similar piece of silk to the black bulb; the effect, if 

 merely due to light, ought by this means to have been consider- 

 ably diminished ; but it is evident on inspection that it was quite 

 equal to that on the plain black surface. Some additional 

 effect of simple heat must, therefore, have made up for the dimi- 

 nution which must have been occasioned by the decreased 

 absorption of light. 



Exp. 12 was tried immediately after No. 11, and shows more 

 decidedly that the alteration of the aperture makes no difference 

 in the effect. It would seem then (since with a diminished 

 aperture the total number of rays is less), to increase with the 

 more accurate convergence, instead of being diminished, as it 

 would be if owing to the straggling rays of light, or to less 

 refrangible rays of heat. 



(51.) According to the view of the phenomena before adverted 

 to, an exterior and invisible heat accompanying the concentra- 

 tion of light by a lens was considered as analogous to the sepa- 

 ration of peculiar invisible heating rays beyond the red end of 

 the prismatic spectrum. It would, however, admit of consider- 

 able question, whether the distance from the rays at which the 

 former effect is perceptible is not much greater compared with 

 space occupied by the coloured edges of the section of the cone, 

 than would be at all proportional to the distance of the exterior 

 heat in the other case beyond the visible boundary of the red 

 rays. If, again, according to the experimental authority on 

 which both facts were originally brought forward, the exterior 

 effect in the one case is the maximum, ought there not to be 

 some analogous result in the other ? It might, perhaps, from the 

 difference of circumstances, be impossible to ascertain this satis- 

 factorily ; but if only the smallest portion of the exterior red 

 fringe of the cone be made to glance upon the surface of the 

 bulb, the effect is enormous compared with the greatest indica- 

 tion while any sensible space intervenes between the bulb and 

 the light. These considerations led me to the idea of compar- 

 ing the exterior effect with a common simple lens, and a com- 

 pound achromatic one of the same aperture. This would show 

 whether it were owing to any thing connected with the disper- 

 sion or different refrangibility of the coloured rays ; if so, the 

 effect with an achromatic lens would be altogether or nearly 

 imperceptible. The lenses I employed were a small achromatic 

 one made by Dolland, of about one inch aperture, and 7-5 focal 



New Series, vol. vm. u 



