PHYSICS: E. L. NICHOLS 
329 
cribed to the existence of two or more bands differing greatly in their 
rate of decay. This is indeed a phenomenon, common to all the phos- 
phorescent sulphides, which has been extensively studied by Lenard^ 
and others and recently by a different method by the present writer.^ 
The uranyl salts on account of their remarkable spectra afford an un- 
usual opportunity for the determination of this question but while the 
fluorescence of these substances has been examined in great detail 
little or no attention has been given to their phosphorescence. 
A new form of phosphoroscope, the synchrono-phosphoroscope, re- 
cently described by the author^ of this paper is well adapted for the ob- 
servation of these fleeting phenomena, which have a duration of only a 
few thousandths of a second, and the study of the phosphorescence of a 
typical uranyl salt, to be described in the present paper, was one of the 
first uses to which this instrument was put. 
The method, briefly outHned, is as follows. The substance, enclosed 
in a flat tube of glass about 8 cm. long and 2 cm. wide, is viewed through 
FIG. 1. FIG. 2. 
the rapidly revolving sectored disk of the synchrono-phosphoroscope. 
It is mounted vertically with its axis at right angles to the radius of the 
disk as shown in figure 1 . 
It is uniformly excited by zinc sparks 120 times a second while hidden 
by the closed sectors and is visible for 1 /240 of a second during the pas- 
sage of each of the intervening open sectors. 
A phosphorescent substance of slow decay appears under these cir- 
cumstances to be equally bright from top to bottom but if one of the 
uranyl salts, such as the double uranyl-ammonio sulphate, which was the 
substance selected for detailed study, be used, it appears a very bright 
green at the botton of the tube shading off to bare visibiHty at the top. 
The rate of decay of this substance, and of the other uranyl salts 
is so rapid that the upper end of the tube, which is seen at the intensity 
which corresponds approximately to the instant 0.003 second after ex- 
citation, has only a small fraction of the brightness of the lower end 
which is viewed about 0.0005 second after excitation. 
