1824.] Mr. Powell on Solar Light and Heat. 287 



Article XIV. 



Remarks on Solar Light and Heat. By B. Powell, MA. FRS. 



(46.) I concluded a former portion of these inquiries with 

 some remarks on the small development of a heating effect 

 which is observable exterior to the cone of light formed by a 

 lens. The investigation of its nature appearing to me a topic 

 of considerable interest as bearing upon many other parts of the 

 science of light and heat, I have been led to try several experi- 

 ments upon the subject. The results of these trials in which 

 the circumstances and conditions of the case have been varied 

 in several different ways, I here propose to give in a tabular 

 form, and to introduce them by a few remarks on the nature 

 and object of the experiments, as well as on the sort of conclu- 

 sion, which can safely be deduced from them. 



(47.) The existence of the effect in question being admitted, 

 two suppositions obviously present themselves as to its nature. 

 It may be attributed to certain rays of light refracted to a posi- 

 tion beyond the principal body of rays which go to form the 

 focus. These may be rendered invisible or nearly so from the 

 proximity of other rays of infinitely superior intensity. And 

 thus the phenomenon may be nothing more than simply the 

 ordinary heating effect of these rays displayed on the black 

 bulb of the instrument. Again, it may be supposed (which 

 seems to have been the idea of the first observer of this and 

 kindred phenomena), that it is owing to some sort of radiant 

 heat. From his analogical view of the subject, it would have 

 followed that these were rays of a peculiar kind different from 

 those of terrestrial heat, and exhibited as existing in a separate 

 state from the rest of the solar rays by the refractive powers of 

 the lens ; rays in fact of a sort of intermediate character between 

 light and common heat. If, however, because we cannot see 

 the rays producing this effect we should think it necessary to 

 infer that they must be rays of simple heat, or if we had any 

 better experimental reasons for such a conclusion, still it would 

 not, I conceive, be at ail necessary to suppose any thing pecu- 

 liar in their nature, or that they were really an emanation entirely 

 sui generis ; for there would still be no proof whatever that they 

 had passed through the thick glass of the lens in the form of 

 rays of simple heat. It is obvious that we may, in all respects, 

 as well suppose them to have originated, or have been separated 

 in some way from the deflected rays of light after their passage 

 through the lens ; and it would seem that such a supposition is 

 the more incumbent on us, when we admit what has, I conceive, 



