12 



STUDIES IN LUMINESCENCE. 



always of less refrangibility than the longest wave-length capable of exciting 

 fluorescence. We later extended our measurements to various other fluo- 

 rescent substances, determining in each case the location and character of 

 the absorption band with which fluorescence is associated and the form of 

 the curve of relative intensities in the fluorescence spectrum. 



RHODAMIN AND RESORCIN-BEAU. 



Of the substances examined rhodamin and resorcin-blau belong to the 

 same group as those previously examined, being organic dye-stuffs which 

 show fluorescence in solution. The fluorescence of these solutions was of 

 sufficient intensity to permit of the use of the method already described. 

 Two nearly monochromatic regions of the spectrum were selected in each 

 case, one lying well within the absorption band of the solution and the other 

 as far toward the red as it was possible to go without reducing fluorescence 

 to an extent which rendered measurements inexact. The fluorescence 

 spectrum was observed as before by means of a Lummer-Brodhun spectro- 



100 



80 



60 



40 



20 



B W 



Fit] 



.611 



1 1.- 



-Rhodamin. 



Curve .1. Fluorescence spectrum when excited by green liht in region marked A 

 Curve B. Fluorescence spectrum when excited by blue light in region marked B. 

 Curve T. Transmission spectrum. Layer i.i cm. thick. 



photometer from a direction at right angles to that of the exciting ray. The 

 solution was placed in a glass cell with plane faces, the exciting light being 

 introduced through a slit and passing through the layer of liquid lying next 

 to the wall of the cell through which the fluorescent light passed. 



Figures 1 1 and 1 2 give in graphical form the results of these measure- 

 ments. The curves of the fluorescence spectra are of the same type as those 

 of fluorescein, eosin, and naphthalin-roth already cited. They rise from 

 within the region of the absorption band to a well-defined maximum, which 

 is located, generally speaking, near the infra edge of the absorption band; 

 beyond the maximum they fall away rapidly with increasing wave-length. 

 The comparison spectrum in these, as in the previous cases, was that ob- 

 tained from the light diffusely reflected by a block of magnesium carbonate 

 illuminated by an acetylene flame. 



