272 M. MeWowi on the Radiations of Inca7idescent Bodies^ 



ficiently pure to produce distinctly the black lines of Fraun- 

 liofer. It was attached to its support by one end, placed 

 fifteen feet from the window, and fixed horizontally in the 

 position of minimum deviation. Its anterior face was covered 

 for one-third of its extent with India ink. From the middle 

 of this there was removed a longitudinal space, which ran from 

 end to end of the blackened band, leaving free and uncovered 

 a horizontal line little more than a millimetre in width. The 

 slip of blue glass covered only two-thirds of the prism count- 

 ing from the painted extremity. 



Things being thus arranged, I observed the image of the 

 aperture successively through the uncovered part of the prism, 

 and then through the two portions on which the blue glass 

 was placed. The first operation gave me the normal spec- 

 trum ; the second, made after the method of Brewster, fur- 

 nished a complex spectrum ; the third, a spectrum arising 

 from a little portion which may be regarded as the mean 

 longitudinal element of the prism. Now, on comparing the 

 first image with the second, I observed the pha?nomena of 

 luminous and obscure zones so well described by Herschel. 

 On comparing then the third image with the second, I per- 

 ceived that the luminous zones belonging to the elementary 

 spectrum were much more sharp, although less intense, miich 

 nan-ffwer, and traversed by obscure zones, much deeper, 

 larger, and with contours more striking than those of the 

 spectrum which came from the unpainted part of the prism. 

 It was easy thus to convince myself, by the comparative in- 

 spection of the three images, that the differences of tint 

 between the second and third spectrum corresponded to the 

 colours which Sir D. Brewster imagines to have the same 

 refrangibility as the tints absorbed. In his spectrum, for in- 

 stance, the normal orange colour is replaced by an obscure 

 zone invaded on one side by the red, and on the other by the 

 yellow, from which he infers the presence of these two colours 

 in the orange. Now these invasions of the yellow and red do 

 not exist in my elementary spectrum, iii w/iich all the space 

 corresponditig to the orange is occupied by a dark zone ; the red 

 and the yelloiio which limit this zotie^ in the spectrum produced 

 by all the viiddle part of the prism covered by the blue glass, 

 are therefore independent of this spectru?n, and belong to spectra 

 of elementary layers superior atid inferior to the intermediate 

 line. 



This last conclusion is nevertheless not exempt from objec- 

 tions. For in a dark room the observer must necessarily have 

 his pupil much dilated, and his sight be more or less conl'used ; 

 consequently, on looking at one time through the uncovered 



