24 STUDIES IN LUMINESCENCE. 



transmission curve T is similar to that of the substances previously examined. 

 On the infra side, as has been pointed out by Lommel, it is possible to 

 trace the fluorescence throughout the entire spectrum ; but this, as in the 

 instances already considered, is due to the greater luminosity as we move 

 toward the middle of the spectrum rather than to any unusual maintenance 

 of the intensity of the fluorescence in that direction. The band falls off 

 quite sharply on that side, differing not at all in this respect from the bands 

 of other substances, such as quinine sulphate and fluorite. The measurement 

 of the transmission showed a very dilute solution, in which the maximum 

 of absorption is scarcely more marked than in the case of the bands of the 

 specimens of fluorite already described. 



From the location of the absorption band with reference to the maximum 

 of fluorescence it would be safe to predict for this substance, on the basis 

 of previous experience, a wide divergence from the law of Stokes. Deter- 

 mination of the limiting value of the exciting light is somewhat difficult 

 to make. It was found, using the spectrum of the arc lamp as a source, 

 that the fluorescence due to light of wave-length 0.45 /x could still be 

 discerned. 



It is not possible, with the spectrophotometer, to make a complete 

 exploration of the absorption band of aesculin, since the band extends into 

 the ultra-violet. Photographs of the absorption spectrum of this solution, 

 made by Miss Eleanor Burns at our suggestion, show this band to be com- 

 paratively narrow, its ultra edge lying at about 0.34 n, beyond which there 

 is no absorption capable of detection by photographic means. 



There is nothing in these measurements which would lead one to place 

 sesculin in a class different from that of the foregoing substances. It is 

 distinguished, as regards the character of its fluorescence spectrum, from 

 quinine sulphate and fluorite, the other two substances on our list the fluo- 

 rescence of which lies in the blue, only in that the maximum lies at a some- 

 what greater wave-length. The curve A, Fig. 21, was obtained by the use 

 of the direct light of the arc, undispersed. Since the curve corresponded 

 in every form to the general type it did not seem necessary to repeat the 

 observations with exciting light of other composition. 



SUMMARY. 



From the measurements described in this chapter the following conclu- 

 sions may be drawn : 



1. Eosin, naphthalin-roth, fluorescein, rhodamin, resorcin-blau, quinine 

 sulphate, chlorophyll, canary glass, green fluorspar, white fluorspar, and 

 sesculin all exhibit fluorescence of the same type. 



2. The fluorescence spectrum in general consists of a single band situated 

 near the infra edge of the absorption band with which fluorescence is 

 associated. 



3. The position of the maximum of the fluorescence band is in all cases 

 independent of the wave-length or composition of the exciting light. 



4. The distribution of intensities in the fluorescence spectrum is inde- 

 pendent of the wave-length of the exciting light. 



5. Stokes's law holds for none of the numerous fluorescent substances 

 thus far examined. 



