STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 



121 



and time in seconds as abscissas. Fig. 127, which gives the decay curve 

 for cadmium sulphate with sodium bromide, and exhibits the two straight 

 lines merging into each other, is a typical example. 



SUMMARY. 



The most important points brought out by the experiments here de- 

 scribed may be briefly stated as follows : 



1. The decay curve when plotted with the values of /"- as ordinates 

 and corresponding values of t as abscissas consists of two straight lines 

 gradually merging into each other. In this respect the short-time and 

 long-time phosphorescent compounds seem to be similar. 



2. The transition from fluorescence to phosphorescence is gradual, i. e., 

 the curve shows no sign of discontinuity. 



3. The shape of the decay curve and the intensity depend upon the time 

 of excitation. 



4. The effect of heat treatment is such as to change both the intensity 

 and the rate of decay of phosphorescence. 



5. The effect of infra-red on short-time phosphorescence, if it exists at 

 all, is very slight; but its effect on the initial decay of Sidot blende is quite 

 marked. 



6. The experiments indicate that at ordinary temperatures all portions 

 of the phosphorescence band decay at the same rate. 



THE EXPERIMENTS OF MR. CARL ZELLER. 



The experiments described below deal with the phosphorescence of three 

 groups of compounds, namely, (1) the aniline dyes in their solid form; (2) a 

 group of manganese compounds of known percentage concentration and a 

 group of cadmium compounds, both prepared by C. W. Waggoner; (3) a 

 group of four phosphorescent sul- 

 phides furnished by Leppin and Ql 

 Masche, and prepared by the method 

 of Lenard and Klatt. 



The phosphoroscope described on 

 page 109 was used in connection with 

 a special form of photometer, the 

 arrangement of which is indicated in 

 Fig. 128. 



In the diagram D is the revolving 



disk of the phosphoroscope, S the ( 



spark gap, C the phosphorescent sub- -| / n \ \ 



stance, M the revolving mirror. The 



photometer was made of two brass 



tubes Z and X set at right angles, Fig. 128. 



with a mirror A mounted at 45 at 



the point of intersection of their axes. The mirror was a piece of microscopic 



slide glass cut in the form of an oval with a hole in the center and then 



silvered. This, when viewed from E, gave a contrast field with a patch 



of phosphorescent light in the middle surrounded by a disk of light 



'Carl A. Zeller, Physical Review, xxxi, p. 367. 



