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II. On the Cliange of Absorption produced by Fluorescence. 



Hi/ JOHN BURKE, M.A., Berkeley Fellow of the Owens College, Manchester. 



Communicated by Professor ARTHUR SCHUSTER, F.R.S. 



Received June 10, Read June 17, Revised December 14, 1897. 



Introduction. 



ABOUT two years ago, in the spring of 1895, in the course of a conversation with 

 Professor J. H. POYNTING on the nature of the phenomenon of fluorescence, in the 

 study of which I was at the time beginning to engage, the suggestion was thrown 

 out by him that possibly fluorescent bodies absorb differently, according us they are 

 fluorescing or not, the rays which they give out whilst fluorescing, thus that a 

 body A would absorb differently, according as it is fluorescing or not, the rays from 

 a similar body B in a state of fluorescence. Some fluorescent bodies undoubtedly 

 do, others do not, absorb, except to a very small extent, the rays which they emit. 

 A strong solution of fluorescin or eosin, for instance, hardly permits its fluorescent 

 light to penetrate even a very small thickness. Glass coloured with oxide of 

 uranium is much more transparent, but sulphate of quinine hardly absorbs these 

 rays a.t all. 



The question was whether during the act of fluorescing any change is produced 

 in the nature of the absorption itself, that is, whether during fluorescence there is 

 an increase or diminution of absorption in that part of the spectrum where the 

 emitted rays lie. For instance, with uranium glass the radiation takes place 

 chiefly between the D and E lines, so that the absorption power for -rays may bs 

 different according as the body is examined in the dark or in daylight in this part of 

 the spectrum. Of the five bright bands of which the radiation consists, three lie 

 between the D and E lines, the other two being of less refrangibility and of less 

 intensity in the red and orange (STOKES, 'Phil. Trans.,' 1852).* With the 

 spectroscope I have used I have not been able to see the band in the red, but the 



* The series of experiments have been made with uranium glass, as the compounds of uranium 

 exhibit many peculiarities which other bodies do not appear to possess when fluorescing. For instance, 

 the bright bands in the fluorescing spectrum of uranium glass seem to be noticeable in uranium com- 

 pounds alone. It is also remarkable that crystals of nitrate of uranium have the quality as well as the 

 quantity of their fluorescent light altered by depriving them of part of their water of crystallization. 



21.4.98 



