gg INTERFERENCE OF RADIATIONS. [MEMOIR V. 



edition of which dates as early as 1776. It is entitled 

 "A Series of Experiments on Phosphori." He shows 

 that it is the more refrangible rays which excite the 

 phosphorescence of sulphide of lime, but the less refran- 

 gible ones extinguish it when shining. 



In 1801 Ritter found that chloride of silver which 

 had been blackened in the violet rays had its color par- 

 tially restored when placed in the red. He states also 

 that phosphorus, which is oxidized with the production 

 of fumes in the invisible red, is instantly extinguished 

 in the violet. 



The well-known experiments of Wollaston with guaia- 

 cum- served to show the opposite relations of the red 

 and violet rays. It is remarkable that he subsequently 

 abandoned this interpretation of the phenomenon, on 

 discovering that green guaiacum changed its color by 

 the application of a hot silver spoon. 



In 1839 Sir J. Herschel encountered the same action 

 in the case of some of the preparations of silver. His 

 first idea was that of a positive and negative polarity 

 of the spectrum ; but this was subsequently modified 

 for the reasons set forth in his memoir (Phil. Trans., 

 1840, 60, etc.). 



In 1842 I had obtained some very fine daguerreotype 

 impressions of the solar spectrum in Virginia, and since 

 that time have never doubted the actual existence of 

 these negative or protecting rays; and on this occasion, 

 when that existence is reasserted by Lerebours, Fizeau, 

 and Foucault, I will make known certain new facts, 

 premising that I do not think the views taken by M. 

 Becquerel are correct. They are founded on what seems 

 to me to be a misapprehension of the phenomenon of 

 the daguerreotype. 



A daguerreotype plate can exhibit three different 

 varieties of surface: 1st, a black aspect on those re- 



