A SPECTROPHOTOMETRY STUDY OP FLUORESCENCE. 



19 



can be followed nearly through the spectrum. The curves of resorcin-blau, 

 fluorite, and sesculin afford other examples of this phenomenon. Where, on 

 the other hand, the maximum is in the middle of the spectrum, as in the case 

 of rhodamin, eosin, fluorescein, etc., the fluorescence soon becomes too small 

 for measurement on both sides of the maximum because of the diminution 

 of luminosity toward the ends of the visible spectrum. 



The green fluorescence described by Stokes was faintly discernible under 

 strong illumination, but its intensity would not permit of measurements. 



The question of the applicability of Stokes's law to this solution was 

 tested by observing with monochromatic light, by the method already de- 

 scribed, the longest wave-length which was capable of producing appreciable 

 fluorescence. The limiting wave-length (0.720 //), which gave faint but 

 unmistakable fluorescence (see line /, Fig. 15), lies on the infra side of the 

 maximum of the curve A. 



100 



80 



60 



40 



20 



Fig. 17.- 



.5/i 



-Canary glass. 



.6/1 



Curve .4. Fluorescence spectrum when excited by violet light, X =0.407 p. 



Curve B. Fluorescence spectrum when the exciting light lay in the green, in the region marked D. 



Curve T. Transmission spectrum for a layer 7 cm. thick. 



CANARY GLASS. 



In our experiments upon canary glass one corner of a rectangular slab 

 1 cm. in thickness and 7 cm. wide was used. This was mounted in front 

 of the slit of the Lummer-Brodhun spectrophotometer in place of and in a 

 position corresponding to the cell employed in the study of the fluorescent 

 solutions. Fluorescence was excited in the manner already described, by 

 means of light entering the glass at right angles to the axis of the collimator 

 tube. Measurements were made using the Hg line 0.407 n (see curve A, 

 Fig. 17) and green light from the spectrum of the Nernst filament (curve B). 



