144 PROCEEDINGS OF THE AMERICAN ACADEMY 



In 1800 Sir Wm, Herschel published in the Philosophical Trans- 

 actions a series of experiments on this subject, which were, without 

 doubt, as accurate as the means at his command would allow, and 

 which later experiments have amply confirmed. 



He allowed the spectrum formed by the passage of the sun's rays 

 through a glass prism to fall upon a screen, in which there was a slit, 

 which could be moved to different parts of the spectrum. Behind 

 this slit he placed a very sensitive thermometer, and, moving the slit 

 through the spectrum, found that, after successive exposures of ten 

 minutes each, it rose in the violet l°.l ; in the green, 1°.8 ; in the 

 red, 3°. 8. Moving the slit still farther and beyond the limits of the 

 luminous spectrum, he found that on the very border of luminosity 

 the thermometer stood at 3°.6, and even at a distance of five centi- 

 meters from this point the reading was 1°.7. In the ultra-violet 

 there was no effect. 



The field having thus been opened, various experimenters took up 

 the subject, in order to find the form of the curve of distribution, and 

 the position of maximum heating effect. 



Leslie,* in repeating Herschel's experiments with what he claimed 

 to be a delicate differential thermometer, found, as the experimenters 

 previous to Herschel had, that the maximum was within the luminous 

 spectrum, and, in a somewhat severe criticism of Herschel's work, at- 

 tributed his results to the heating of the inclined plane on which the 

 thermometers were placed. 



Mickle,f in experimenting on the subject, found that the prism 

 itself became somewhat heated, and concluded that Herschel's appar- 

 ently anomalous results were due to radiations from the prism. 



Englefield^: found, as Herschel had done, that the maximum was in 

 the ultra-red. 



An accurate series of experiments carried out by Wiinsch,§ and a 

 perhaps still more praiseworthy series, which tended only to confirm 

 the first, made by Seebeck,|| showed that the form of the curve and 

 the position of the maximum was largely dependent upon the material 

 of the prism used, and that Herschel's results were, for the kind of 

 glass used, entirely correct. 



In 1833 Melloni H published in the Ann. de Chimie et de Physique 

 the results of a very accurate and extended series of experiments on 



* Nich. Journ., iv. t Phil. Mag., lxv. 



$ Journ. Roy. Inst, 1802. § Gehler's Journ., vi. 



|] Abhandl. d. Berl. Akad., 1819. 1" Ann. de Chimie, liii. 



