Dr. Seebeck on the Heat of the Prismatic Spectrum. 331 



nor in the red rays, and indeed not in the colours at all ; and 

 it is still well remembered what a sensation was created by 

 this discovery, as well as by his doctrine of invisible rays de- 

 duced from it, especially of heating rays emanating from the sun; 

 what doubts and contradictions it experienced on one side, 

 and what approbation it met with on the other. -It is also 

 known that Leslie*, one of the first antagonists of Herschelf, 

 says that he found not the least change on his photometer, 

 which surpasses in sensibility the best of thermometers, either 

 above or below the spectrum, but that the greatest heat is 

 found within it, and only in the red rays; that Englefi«ld :{:, 

 on the other hand, soon after confirmed Herschel's discovery, 

 and affirmed that the greatest heat always fell beyond the 

 red. 



Seeing results so contradictory, from the experiments of 

 such celebrated natural philosophers, it was obvious that only 

 the difference of apparatus, or of the processes employed in 

 the investigation, could have occasioned them ; and that (as 

 is but too probable in a new field of inquiry) things have been 

 overlooked, or not sufficiently noticed, which must have in- 

 fluenced the results. This consideration alone called for in- 

 quiry : but I was still more imperatively urged to it from the 

 desire of gaining ocular information on the effect of coloured 

 illumination in all the functions of light, and to inquire whether 

 t\ie polar contrast of colours discovei'ed by Goethe would also 

 become proved and confirmed in the effect of coloured light 

 on bodies. I first engaged in an investigation of its chemical 

 effect and its effect on muriate of silver, some of the most 

 important of which I communicated in Goethe's Farbenlehre 

 (System of Colours). In the year 1806 I undertook a series of 

 expei'iments on the excitation of heat in the prismatic spec- 

 trum, and continued them in the summers of 1807 and 1808. 

 I shall give a detailed account of their results. 



For the first experiments I made use of a mercurial ther- 

 mometer with a blackened bulb. There were distinct dif- 

 ferences of temperature between the blue and violet half, as 

 compared with the red and yellow half of the spectrum ; they 

 were however but small, so long as the coloured light fell im- 

 n)ediately from the prism on the bulb of the thermometer; 

 yet the difference of temperature of the individual colours on 



* If any one lie desirous to know more of the [liotometer employed by 

 Leslie, we would recoimnend the perusal of a tract lately published, en- 

 titled A short Account of Experiments and Instruments depending on the 

 Relation of Heat and Air to Moisture ; by J. Leslie. 



J Nicholson's Phil. Journ. vol. iv. p. 344. 

 Journ. of the Roy. Inst. J 802, p. 202. 



T t 2 the 



