1824.] Mr. Powell on Solar Light and Heat. 403 



bulb, and thus altering the state of equilibrium in favour of the 

 absorption of heat on its surface from the light. 



These experiments tend to confirm the conclusion maintained 

 in my former paper, and to extend the limits within which it 

 holds good. I do not attribute any other importance to it than 

 as contributing to lay a foundation of distinctness of ideas upon 

 which to proceed in the further examination of the subject, and 

 in comparing the radiant heating effect emitted from the sun 

 with that from incandescent and burning bodies. 



(21.) In reference to the validity of this opinion, however, we 

 may make this further observation. Those rays of the sun 

 which come within the reach of our examination have been here 

 shown to be entirely composed of one species characterised by 

 the definition before laid down. It must, however, be admitted, 

 as by no means improbable, that the sun may originally give out 

 a separate radiation of heat, distinguished by other properties, 

 and of the same kind as the radiant heat from hot bodies. None 

 of this kind reaches ns, but we must consider the very different 

 degree in which any medium, as air, absorbs or intercepts the 

 passage of those two sorts of radiant matter. The heat from a 

 hot body will not be perceptible at a short distance, while its 

 light will traverse an amazing extent of length ; and thus at 

 different distances the ratio between the two will be very differ- 

 ent. Some degree of simple heat, therefore, may actually be 

 initially radiated by the sun, and be lost before it reaches us. 

 We have no reason to believe that there is any medium between 

 the different parts of the solar system capable of absorbing heat. 

 The highest regions of our atmosphere into which observation 

 has penetrated are uniformly the coldest; but they are known to 

 have a greater capacity for heat. Thus though it is possible 

 that some heat may reach to that distance and be absorbed 

 without becoming sensible to us, its quantity must be very 

 small : if, therefore, we suppose any simple heat to be initially 

 radiated from the sun, it must be all or nearly all absorbed by 

 some parts or appendages of that luminary exterior to the part 

 where it is generated. 



(22.) From considering the heating power which so insepara- 

 bly accompanies the rays of light, which is always developed 

 wherever they impinge on a surface which, from its colour, 

 absorbs the rays, and which continues to act with very little 

 diminution of intensity when the rays pass through transparent 

 media of considerable thickness, we are led to observe some 

 remarkable instances in which such effects are produced. Such 

 aninstance is afforded in the case of the eye, and the phenomena 

 of vision. 



What may be the immediate cause of vision, and what effects 

 light may be capable of producing on the retina and optic nerve, 

 we are altogether ignorant; but we may with tolerable certainty 

 infer from well known facts and universal laws, that (among 



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