IO 



STUDIES IN LUMINESCENCE. 



of Lommel. They are indeed in every respect analogous to those obtained 

 with fluorescein and lead to the same conclusions. Each solution was 

 excited by three distinct regions of the spectrum, one lying as far toward the 

 red as practicable, one toward the blue, and one at an intermediate wave- 

 length. Curves showing the distribution of intensities in the three spectra 

 thus produced are shown in the figures and it will be noted that, as in the 

 corresponding curves for fluorescein, the position of the maximum is entirely 

 independent of the wave-length of the exciting light and that the general 

 character of the curve remains unchanged. In the case of these two solu- 

 tions, as in that of fluorescein, it was possible to obtain a measurable amount 

 of fluorescence by the use of light of wave-length greater than that of the 

 maximum of the fluorescence spectrum. The form of fluorescence curve is 

 very similar for these three substances, but each has its own place in the 



5/' 



Fig. io. Naphthalin-roth. 



Fluorescence spectra observed when the exciting light lies in different regions 

 of the spectrum. Curve A was obtained when the exciting light was 

 confined to the region marked A, etc. Vertical scales arbitrary. 



spectrum. The maximum for fluorescein (Fig. 3) is at 0.5 1 7 fi, that for eosin 

 at 0.580JU, and that for naphthalin-roth at 0.594 /x. The position of the max- 

 imum of these three curves with reference to the absorption band appears 

 to vary with the different substances. The maximum for eosin coincides 

 approximately with the infra edge of the absorption band; that for fluo- 

 rescein lies slightly (about 0.05 /j.) toward the violet, while the maximum for 

 naphthalin-roth is much farther displaced toward the short wave-lengths. 



Lommel's contention that it is possible to excite fluorescence in eosin 

 by means of the light of the sodium flame is fully confirmed by the data 

 plotted in Fig. 9, from which it will be seen that the exciting light by means 

 of which curve A was obtained had a mean wave-length almost precisely 

 equal to that of the sodium lines. Since it was found possible by means of 

 light, all of which was of greater wave-length than the maximum of the 



