G2 M. E. Hagenbach's Experiments on Fluorescence. 



with sharply defined edges ; while with fluoreseeine, which gives 

 a fluorescence identical in appearance with that of the former 

 solution, similar intermissions are hardly to be distinguished. 



It might be supposed that the interruptions in the spectrum 

 projected upon the surface" of a fluorescent substance correspond 

 to the intermissions of its fluorescence-spectrum, the one being 

 the determining cause of the other : there are cases in which we 

 might believe such a coincidence to exist ; solutions of lamp- 

 black, for instance, give five maxima in the spectrum thrown 

 on their surface, and likewise five maxima in the fluorescence- 

 spectrum. But what is observed in a great number of other 

 cases demonstrates that in no instance have we here to do with a 

 simple relation. Fresh solution of chlorophyl gives seven 

 maxima in the spectrum projected upon its surface, but only two 

 in its fluorescence-spectrum ; naphthaline-rose gives three very 

 pronounced maxima in the first case, and no intermission in 

 the second; on the contrary, nitrate of uranium, which gives no 

 interruption in the spectrum thrown on its surface, shows two 

 very marked maxima in the spectrum of its fluorescence. 



Intermission in the projected spectrum, as well as in that of 

 fluorescence, may in some cases, as those of the solutions of 

 guaiacum, purpurine, orchil, and turnsol, be explained by the 

 fact that they are mixtures of several colouring -matters. 

 Nevertheless in other cases, where the colouring substance is a 

 pure crystallizable body, one can no longer see in mixture the 

 cause of the intermission. 



Stokes's Law. 



We have already mentioned, at the opening of this article, 

 the law of Stokes, which states that the light emitted by fluo- 

 rescence is never more refrangible than the exciting rays. 



M. Lommel has recently contested the accuracy of this law, 

 on the ground of a series of observations made by him on naph- 

 thaline-rose and chlorophyl. My own experiments have con- 

 trarily led me to recognize the perfect correctness of Stokes's 

 law in all possible cases. The other laws which it has been 

 thought might be introduced into the domain of fluorescence 

 have not found their verification in these researches. 



Influence of the Solvent. 

 The liquid in which the fluorescent substance is dissolved 

 exerts sometimes an influence on the nature of the light emitted; 

 but here, again, no precise rule can be established. The influ- 

 ence of the solvent manifests itself in many cases by displace- 

 ment of the bands of maximum in the projected spectrum ; thus 

 the solutions of the amide of phthalic acid, of chlorophyl, pur- 

 purine, &c. are distinguished from the others by;the bands being 



