Polytechnic Association. 



921 



^ 



chrysogen, and No. 6 the same as displaced by solution in benzole } 

 and it will be seen that the upward displacement corresponds with 

 that of the fluorescent bands. 



So again, in Nos. 7 and 8, which represent the absorption spectra 

 of solid and dissolved thallene, a like connection is to be noticed. 



But yet more than this, when a pure spectrum is thrown on a 

 screen of paper painted with these substances, decided maxima of 

 fluorescence are observed, corresponding in position with these absorp- 

 tion bands, and when tanks containing solu- 

 tions of the same bodies are substituted for 

 these screens, the maxima are again strongly 

 developed and found to be in exact corre- 

 spondence with the displaced absorption bands 

 of the same solution. 



The method by which this observation is 

 made, is exhibited by part of Fig. 3, where 

 A is again 

 the port 

 lumiere, 

 with a 

 narrow 

 slit at 



B, admitting light to a prism, C, from which it passes to a corrected lens 

 at D, by which a pure spectrum is thrown on the screen E. 



The other parts of the figure are for another purpose, and will be 

 alluded to further on. 



It would thus appear that the absorption bands correspond, in all 

 respects, with the maxima of excitability ; but there is this curious 

 exception, namely, that with thallene solutions a brief exposure, 

 thirty minutes, to strong sunlight removes all the absorption bands, 

 but only destroys the first maximum of fluorescence — that, namely, 

 which is immediately above F, and corresponds to the first band of 

 absorption. 



Curiously, also, the solution in turpentine, which seems to give but 

 the faintest trace of a band at this same place, shows also only a very 

 faint maximum at the corresponding point; and yet, if the thallene 

 be recovered from the turpentine and dissolved in benzole, this line 

 of absorption comes out as strongly as ever. 



By arranging a slit in the screen, E (see Fig. 3), and placing a lens 

 at F, and the objects to be examined at G, we can, with the spetro- 

 scope, H, study their deportment in the various rays of the spectrum. 



Fig. 3. 



