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



tive. Only the rays beyond B can be excluded from the num- 

 ber of those which produce fluorescence ; at least I know of no 

 substance which is fluorescent in that portion of the spectrum. 

 But when we consider that the light emitted by fluorescence is 

 less refrangible than the exciting rays, we can well understand 

 how it is that fluorescence excited in the extreme red is not 

 visible to our eyes. 



As regards the greater or less extent of fluorescence in the 

 spectrum, great differences are found between different sub- 

 stances. There are cases in which fluorescence commences in 

 the violet, after the line G — for example, fluor spar, and a 

 solution which I first studied and which must contain bisulph- 

 anthrachinon ; while in other cases it extends nearly through 

 the spectrum, as takes place with ethereal and alcoholic solution 

 of chlorophyl, alcoholic, solution of naphthaline-rose and thio- 

 melic acid. In the direction of the extreme violet, fluorescence 

 always extends beyond the group H. 



In respect of intensity in the different parts of the spectrum 

 this remarkable fact presents itself — that in a great number of 

 cases several different maxima of fluorescence are observed, sepa- 

 rated by bands of relative minimum. These maxima are not 

 all equally marked ; the difference of brightness between them 

 and the minima likewise varies. 



The number of these maxima also is very variable. Thus, 

 for example, seven are observed with fresh solution of chlorophy], 

 five with solution of lampblack in alcohol or turpentiue, three 

 with naphthaline-rose, alcoholic solution of turnsole, purpurine 

 in alum, and uranium glass, two with the alcoholic solution of 

 guaiacum resin, and only one with solution of morine alum and 

 tbiomelic acid and with the solutions of sulphate of quinine, 

 sesculine, fraxine, photene or anthracene, petroleum, and nitrate 

 of uranium. 



Correlation of Fluorescence and Absorption.— -The fact that 

 wherever incident light produces fluorescence, absorption exists 

 at the same time, is a consequence of the law of the conserva- 

 tion of vis viva which it was easy to foresee. In my observa- 

 tions I continually saw absorption accompany fluorescence; 

 often the absorption-spectrum could even serve to determine 

 more precisely the maxima of fluorescence. What is still more 

 remarkable is the inverse fact recognized by Stokes, that, in 

 fluorescent substances in general, absorption is always accom- 

 panied by fluorescence, which was not necessarily to be fore- 

 seen ; for in other coloured liquids there is often absorption of 

 light without fluorescence. On this last point in particular, 

 although my researches have fully confirmed the connexion 

 already known between the two, yet I have found cases in which 



