on the terrestrial Rays that occasion Heat. 521 



duced to this single point. Is the heat which has the refrangibility 

 of the red rays occasioned by the light of these rays ? For, should 

 that be the case, as there will then be only one set of rays, one fate 

 only can attend them, in being either transmitted or stopped, 

 according to the power of the glass applied to them. We are 

 now to appeal to our prismatic experiment upon the subject, 

 which is to decide the question. First, with regard to light, I 

 must anticipate a series of highly interesting observations I have 

 made, but which, though they certainly claim, cannot find room 

 in this Paper. These have given me the means of acting sepa- 

 rately upon either of the extremes, or on the middle of the pris- 

 matic spectrum ; and by them I am assured that red glass does 

 not stop red rays. Indeed the appearance of objects seen through 

 such coloured glasses, till I can give those observations, will be 

 a sufficient proof to every one that they transmit red light in 

 abundance. Next, with regard to the rays of heat, the case is 

 just the reverse ; for, by our preceding table, the red glass stops 

 no less than 6g<2, out of a thousand, of such rays as are of the 

 refrangibility of red light. The incipient stoppage, moreover, or 

 that in two minutes, of which something will be said hereafter, 

 amounts even to 750 rays. 



Now, if it should be suspected, " that on account of the great 

 " breadth of prism, some invisible heat may be thrown upon the 

 " spot where the red colour falls," I do not only agree to it, but 

 am certain it cannot be otherwise : but this again, will give addi- 

 tional weight to our present argument ; for, by the 153d experi- 

 ment, as our last table shews, it has already been ascertained, that 

 all such heat will be transmitted through a red glass ; so that, 

 were it not for some of this admixture, the stoppage might be still 

 greater. 



