1824.] 



Mr. Powell on Solar Light and Heat. 



293 



Simple lens. Aperture 1 inch. Focal distance 7 "5 inch. 



15 



Small instrument. Bulb painted with Indian ink. 

 In case. 



Just without the focus. 

 Away under shadow. 



10 12 

 4 



No case. 



16 



Away. 



2 3 



No case. 

 Focus near black bulb. 



9 10 



Focus near plain bulb. 



4 



It was with this instrument that the experiments in the former 

 paper were made. (Annals, Aug. Art. 1, §44.)* 



In conclusion I will merely say, that still admitting a consi- 

 derable degree of uncertainty to hang over this subject, I shall 

 endeavour to prosecute the further examination of it whenever 

 it may be in my power to vary and extend the experiments. 

 Meanwhile 1 beg to repeat my request to those who have lenses 

 of large size to examine the phenomena which they present. 

 A delicate conmon thermometer would be amply sufficient to 

 display the effects ; and my hope that some experimenter may 

 be thus induced to take up the subject, and to extend it also to 

 ihe case of concentration of light by reflection, has been my 

 chief motive for publishing these experiments under their present 

 imperfections. 



* In the experiments referred to, an error has crept in which I wish to correct. For 

 J inch without the rays," read \ inch. 



