8 



STUDIES IN LUMINESCENCE. 



the fluorescence of the dilute solution, A the transmission curve of a layer 

 i.i cm. in thickness, and B the computed curve for the transmission of the 

 mean layer of liquid through which the fluorescent light has to pass. The 

 correction for this absorption is indicated by means of the dotted line. 



It having been established that the fluorescence of bodies of this class is 

 independent, as regards the distribution of intensities, of the wave-length 

 of the exciting source, it would be of interest to inquire whether the fluo- 

 rescent energy for a given wave-length of the fluorescence spectrum varies 

 with the wave-length of the exciting light when the energy of the latter 

 is constant, or whether it depends only upon the energy. The rigorous 

 determination of these relations involves a knowledge of the distribution of 

 energy in the spectrum of the exciting source, a difficult matter to determine 

 with accuracy for the shorter wave-lengths of the visible spectrum. The 

 curve which is shown in Fig. 8 may, however, be of some interest in this 



100 



80 



60 



40 



20 



CO 



80 



40 



.500/' 20 



Fig. 7. Fluorescein. 



Typical fluorescence spectrum for dilute solution. 



connection. It represents the intensity of fluorescence, taken at the maxi- 

 mum of the fluorescence spectrum of the solution of fluorescein, as a func- 

 tion of the wave-length of the exciting light. Curve A was taken with 

 the Nernst filament as a source, curve B with the acetylene flame. The 

 dotted line shows the absorption band for a layer of the solution 1.1 cm. 

 thick. It will be seen that fluorescence begins approximately at the wave- 

 length at which the solution begins to absorb, and that the maximum lies 

 well within the absorption band but is shifted to the red. The longer wave- 

 lengths within the band are more effective on account of their greater energy. 

 The difference in the form of the curves .1 and B is probably ascribable to 

 the different distributions of energy in the spectra of the sources of light 

 employed. The important questions of the actual distribution of energy 

 in fluorescence spectra and of the relative effectiveness of equal quantities 

 of energy when the wave-length is varied, in producing excitation, are con- 

 sidered at length in Chapters XII and XIII of this memoir. 



