S?* M. Melloni on the Radiations of Incandescent Bodies^ 



direction; nevertheless the difference is manifested in just the 

 opposite manner. Therefore the elongation of the yellow 

 rectangle in the central spectrum arises beyond all doubt, 

 altogether or in part, from a partial overlapping of the yellow 

 rays belonging to the spectra of the entire series of elementary 

 prisms, the rays from which traverse the dilated pupil of the 

 observer. This overlapping takes place perpendicularly to 

 the axis of the spectrum, and consequently in a vertical direc- 

 tion in the arrangement adopted in my experiment. 



The method proposed by Sir D. Brewster to determine the 

 composition of the solar spectrum does not therefore appear 

 to me adapted to the end in view : and until it shall have been 

 proved that the colours of a spectrum, perfectly pure, change 

 by the interposition of a medium placed near the prism, the 

 change persisting at every other distance, the existence of dif- 

 ferent colours in the same transverse element of the spectrum 

 ought to be regarded as entirely hypothetical, 



I can therefore no longer admit the existence of different 

 colours in the same part of the spectrum : I hold it as proved, 

 that every particular tint arises from a single ray possessing a 

 particular frequency of vibration, and a particular wave-length. 

 I think altogether with Newton, that colour is a characteristic 

 sign, distinctive of the different elements contained in the 

 beams of the sun and of luminous bodies; elements which are 

 separated only and solely under the action of the prism in 

 virtue of their different degrees of refrangibility. 



In thus rejecting a means of analysis, which has enjoyed 

 until now much favour among philosophers, we ought to re- 

 collect that Sir David Brewster is the author of very many 

 beautiful and important discoveries confirmed by experiment; 

 works, from the merit of which the conclusion here arrived at 

 can never detract, any more than the errors committed by 

 Newton on diffraction and the dispersion of luminous rays by 

 diaphanous bodies of different kinds, can ever lessen the glory 

 which must belong to his optical researches, and his discovery 

 of the system of the universe- 



To return to the researches of Professor Draper. I say 

 that they conduct, as do others heretofore known on light and 

 radiant heat, to a perfect analogy between the general laws 

 which govern these two great agents of nature. I will add, 

 that 1 regard the theory of their identity as the only one ad- 

 missible by the rules of philosophy ; and that I consider myself 

 obliged to adopt it, until it shall have been proved to me that 

 there is a necessity of having recourse to two different prin- 

 ciples for the explanation of a series of phaenomena which at 

 present appear to me to belong to a solitary agent. 



