I9I0.] ON PHOSPHORESCENCE AND FLUORESCENCE. 277 



The study of the fluorescence of these and other substances 

 shows that the relation of fluorescence to phosphorescence is not so 

 simple as at first appears. Observations with the spectrophotometer 

 bring out various complexities in the fluorescence spectrum. 

 Bands that seen with the spectroscope appear to be single are found 

 to consist of tw^o or more components more or less closely over- 

 lapping. Changes of temperature affect all wave-lengths of a 

 single band in the same manner and, in some cases, neighboring 

 bands are similarly affected. In the fluorescence spectrum of com- 

 mercial anthracene for example, there are besides the blue and 

 violet bands of the pure substance, two bright bands having their 

 crests at .502 yu, and .538/1, and a much fainter band with a maxi- 

 mum at .575 i^i. At — 185° all three of these bands are reduced in 

 intensity in nearly the same proportion (see Fig. 10). In the case 

 of all three moreover the diminution is greater on the side towards 

 the violet so that there is a slight shift of all three bands towards 

 the red. The positions of the crests are now .504 /x, 538 /a and 577 )«■ 

 respectively. At the temperature of liquid air moreover the bands 

 are narrower and the yoke between them is much lower. 



Observations upon the crests of the two brighter bands, as the 

 substance is gradually cooled, show that the two crests rise and fall 

 in intensity together preserving very nearly the same relative heights 

 as indicated by the curves in Fig. 11. 



This displacement of fluorescence bands towards the red on cool- 

 ing is of frequent occurrence. In the case of the single band in the 

 red-yellow of the fluorescence spectrum of solutions of resorufin 

 in alcohol for instance (see Fig. 12) we find upon reducing the tem- 

 perature from +20° C. to — 95° C. that the intensity at the crest is 

 reduced to about one half and the band is narrower than at room 

 temperature. There is however no appreciable shift. At — 165° 

 C. the whole band is shifted to the red and there is further narrow- 

 ing which shows itself in the steeper slope of the curve on the violet 

 side. The intensity however is the same as at — 95° C. Further 

 cooling to — 185° C. greatly increases the intensity without further 

 shift. Fig. 13 shows the variations in the brightness of the crest 

 of this band throughout the ransre from -f 20° to — 18;°. It 



