of Heat in the Prismatic Spectrum. 343 



These expei'iments have not been carried further, as would 

 have been desirable, since the last experiment was not made 

 on a favourable day. Their result, however, seems to be, that 

 the effect of the prismatic red was more weakened by the 

 orange-coloured glass, than the glimmering light beyond it, 

 which had a yellowish tinge proceeding from the glass. For 

 that the effect of the prismatic light is weakened generally and 

 more considerably by the coloured glass, appears by the last 

 experiments. 



On the same day, too, experiments were made with the flint- 

 glass prism. No. 2, with the intention of inquiring whether 

 the opposite colours (which, if they do not destroy, at least 

 weaken, the prismatic colours) would produce a considerable 

 effect. 



Exp. 54. — Distance of the thermoscope, six feet ; station of 

 the same in the place of experiment, 5" 1'". 



a. A violet pane of glass fixed to the outer side of the flint- 

 glass prism. 



In the red 5" 4'" 



Close under the red 6 2 



h. The violet glass on the inner face of the prism ; an hour 

 later. 



In the red 6" 0'" 



Close under the red 6 9 



The difference in the first experiment was 10'", and in the 

 second 9'"; and in both instances the heat was, as usual in this 

 prism, greater under the red than within it. 



Exp. 55. — The same prism, with a blue glass fixed to its 

 outer surface ; the distance of the thermoscope as before. 



In the red 7" 2'" 



Close under the red 7 2 



Thus the blue glass produced an equality of heat in those 

 two places where before it had always been different : indeed 

 the prismatic red had been considerably weakened and ren- 

 dered paler by it. In the preceding experiment with the 

 violet glass, this had not been so strongly the case ; for this 

 glass gave itself a reddish violet image, which may be the cause 

 why in that instance neither the effect nor the pi'ismatic co- 

 lour had suffered any change, although upon the whole the 

 light was also weakened, as we may see from the small diffe- 

 rence in the increase of heat in the spectrum above the tem- 

 perature of the place. 



[To be continued.] 



LV. On 



