1 6 STUDIES IN LUMINESCENCE. 



The curve T gives the transmission of the solution as measured by 

 the method already described. It was not found possible, owing to 

 the fact that this solution remains transparent almost to the limits of 

 the visible spectrum, to go very far into the absorption band; but the 

 observations suffice to locate the infra edge of the band very closely. To 

 gain further information concerning the nature of the absorption band a 

 series of photographs of the transmission spectrum were taken by means of 

 a Rowland grating with sunlight as the source of illumination. The infra 

 edge of the band, as shown by the disappearance of theFraunhofer lines, was 

 found to lie in the region indicated by the spectrometric measurements 

 shown in Fig. 14. Absorption became almost complete in the neighborhood 

 of the 77 lines and continued throughout the ultra-violet, at least up to the 

 point where glass becomes opaque. 



In order to determine if possible the position of the ultra edge of the band, 

 the quinine solution was placed in a cell with quartz walls and photographs 

 were taken, using an arc light into which zinc had been introduced as a 

 source. Lines due to this metal were distinguishable in the comparison 

 spectrum as far as wave-length 0.2558 /x, to which point the opacity of the 

 solution of quinine sulphate was found to be complete. 



It will be seen from the curves A and B (Fig. 14) that the fluorescence 

 spectrum of quinine sulphate is of the same type as that of the various 

 fluorescent dye-stuffs. It consists of a single band with a sharply defined 

 maximum at 0.437 fi. Since the curves obtained with daylight and with 

 monochromatic light from the mercury arc are identical as regards the 

 position of the maximum and in general form, and since they are of the 

 same type as the various other fluorescence spectra already described, it is 

 evident that quinine sulphate belongs to the same class as fluorescein, eosin, 

 etc. It is true that in the case of this substance it is possible to trace the 

 fluorescent light throughout nearly the entire spectrum, a point which is to 

 be considered further in a subsequent paragraph, but beyond 0.5/1 intensities 

 are very small. 



Contrary to the view expressed by Lommel, the fluorescence of quinine 

 sulphate appears to be independent of the wave-length of the exciting 

 light. Since, however, this is one of the substances which has been cited 

 in support of Stokes's law, it was deemed important to determine as care- 

 fully as possible the longest wave-length of monochromatic light capable of 

 exciting fluorescence. This is a matter of considerable difficulty on account 

 of the weakness of the effect. By means of the method of double dispersion 

 already described, the electric arc being used as a source, monochromatic 

 light thoroughly free from all stray radiation was obtained. When this 

 was used for excitation the last trace of observable fluorescence was found 

 to disappear at wave-length 0.420 yu. 



In no other case have we found this limiting wave-length to lie so close 

 to the ultra edge of the fluorescence band. It will be noted, however, that 

 in making measurements by daylight readings were obtained at wave- 

 length 0.41 n and the fluorescence under strong excitation is traceable still 

 further into the violet, certainly almost, if not quite, to 0.40 /z. While 

 quinine sulphate then approaches more nearly to conformity with Stokes's 

 law than the other substances that we have studied, the evidence is dis- 



