distinct Calorific Rai/s in the Sunbeam. \ 1 



spectrum, this thermometer seemed to be more aftected the 

 nearer it was placed to the red margin, and less as it ap- 

 proached the opposite or violet-colouied edge. But the most 

 remarkable effect of all was, that the thermometer indicated 

 the greatest heat when placed just without the red margin, 

 where none of the visible rays reached it at all. They there- 

 fore inferred that this effect was produced* by a set of dark 

 calorific rays which are less refrangible than any of the other 

 rays. On repeating these experiments, M. Berard obtained 

 similar results, except that he found the maximum of heat in 

 the red ray. 



But notwithstanding the high reputation of these philoso- 

 phers, their conclusion has been questioned by Professor 

 Leslie of Edinburgh, who by experimenting somewhat diffe- 

 rently was unable to detach any of these dark rays from the 

 light. Having rendered a circular spot opaque in the middle 

 of a large convex lens, he received the light transmitted by 

 the remaining transparent ring upon a surface of black wax, 

 held at such a distance that the light formed upon the wax 

 an iris, or ring, composed of a set of distinct concentric rings, 

 which severally possessed all the various colours of the com»: 

 mon spectrum. Mr. Leslie then carefully observed the effect 

 of these rings on the wax, and found that none of it was melted 

 beyond where it was covered by the iris ; whereas if a set of 

 dark calorific rays had existed, these ought to have more 

 thoroughly melted a larger ring than that whereon the light 

 fell ; for the dark rays, if less refrangible than light, would 

 have fallen without the margin of the red ring, which includes 

 all the others. 



Now, since this experiment of Professor Leslie is of a more 

 simple and decisive cast than any performed by the gentle- 

 men already mentioned, I am much inclined to give it the 

 preference, and to conclude with him that the rays of light 

 only produce or become heat when they themselves dis- 

 appear. It is also well known that the more perfectly any 

 surface reflects light, the less will that surface itself be heated 

 by the light. 



As I do not recollect to have seen any reason given why 

 Sir William Herschel's mode of experimenting ought to have 

 apparently produced the effect he observed, without calling 

 in the aid of dark calorific rays, I shall take the liberty of 

 briefly suggesting what I suspect to liave been the principal 

 source of deception. If a prism such as Sir William employed 

 be heated, a very delicate thermometer will, ctvteris pa>ib?/s, 

 be more affected when it is held opposite to one of the flat 

 sides of the prism than when opposite to one of its edges ; 



B 2 because 



