I 



of Heat in the Prismatic Spectrum. 44 7 



beyond the red : nor are we told what relation this prism 

 bears to the others which Herschel examined. He merely 

 says (Phil, Trans. 1800, p. 442) that he also made experi- 

 ments with prisms of white glass, crown-glass, flint-glass, and 

 water; and that " he found with all of them invisible heating 

 rays beyond the visible red rays." He then merely mentions 

 a few instances of heat half an inch and an inch beyond the 

 red, but not of that in the red itself; thus leaving it uncertain 

 where the maximum of heat was found with these prisms. 

 Since Herschel was not sparing in the communication of his 

 experiments, he would not have withheld any comparative 

 experiments he had made on those prisms respecting the 

 heat in the two places mentioned. I therefore suppose that 

 he contented himself with simply investigating the heat be- 

 yond the spectrum, concluding that the effect would be the 

 same in all the rest of the spectrum as it was with his first 

 prism. It would have been very desirable that he should have 

 explained himself on this head, as well as on the quality of 

 his first prism, which I am inclined 'to -suppose to have been 

 of flint-glass. 



Nor did Herschel in his other investigations place the ther- 

 mometer in all the prismatic colours ; but only into the violet, 

 green, and red, and beyond the red, and once also beyond 

 the limit of the violet ; wherefore he missed the discovery that 

 with some prisms the maximum of heat falls into the yellow 

 and orange. 



Sir H. C. Englefield, who confirmed Herschel's experi- 

 ments, made use of only one prism ; but he does not state 

 whether it consisted of flint-glass or of any other kind of glass. 



Rochon and Leslie alone ex^wessly mention that they em- 

 ployed prisms of flint-glass in their experiments. But Rochon 

 never carried his thermometer beyond the limit of the spec- 

 trum ; and Leslie mentions but one experiment, and that made 

 in 1797 : from which, at least, it does not appear that he then 

 made any investigation respecting the heat beyond the spec- 

 trum ; as, in fact, no one seems to have done before Herschel. 

 Mr. Leslie, indeed, tells us that he repeated Herschel's experi- 

 ments, as soon as he was informed of them, with his highly 

 sensitive photometer ; but that he did not experience the least 

 effect either above or below, or on the sides of the spectrum. 

 How Mr. L. with that really very sensitive instrument could 

 find no evolution of heat beyond the red half of the spectrum, 

 I cannot conceive. For suppose even that he should have 

 placed the limit of the red further than Herschel, (who, how- 

 ever, in this respect followed Newton,) it cannot be imagined 

 that he should have carried it several inches below the bright 



red, 



