523 Dr. Herschel's Experiments on the solar, and . 



Here then we have a direct and simple proof, in the case of 

 the red glass, that the rays of light are transmitted, while those 

 of heat are stopped, and that thus they have nothing in common 

 but a certain equal degree of re frangibility, which, by the power 

 of the glass, must occasion them to be thrown together into 

 the place which is pointed out to us by the visibility of the rays 

 of light. 



The manifest use of the union of these rays, arising from 

 their equal refrangibility, will be explained at a future opportu- 

 nity, when I may perhaps throw out several hints that have 

 already occurred to me, where the contents of this Paper may 

 be applied to the useful purposes of life. 



There still remains a general argument, that heat and light 

 are occasioned by different rays, which ought not to be omitted. 

 This, on account of the contracted state in which the experi- 

 ments have been given, cannot appear from my Paper ; but, by 

 an inspection of them at full length, it is proved, that the stop- 

 page of solar heat, setting aside little irregularities, to which all 

 observations are liable, has constantly been greater in the first, 

 second, or third minute, than in the fourth or fifth; or, more 

 accurately, nearer the beginning of the five minutes, than about 

 the end of them. Now this does not happen in the transmission 

 of light, which, as far as we know, is instantaneous ; at least a 

 failure in the brightness of an object, when first we look at it 

 through a glass, amounting to one, two, or even three minutes, 

 could not possibly have escaped our observation. This seems 

 to suggest to us, that the law by which heat is transmitted, is 

 different from that which directs the passage of light ; and, in 

 that case, it must become an irrefragable argument of the diffe- 

 rence of the rays which occasion them. 



