PHYSICS: E. L. NICHOLS 
331 
X 
L.B. 
case of the early and late stages of phosphorescence, some of the fainter 
bands being invisible but changes such as might be looked for, i.e., those 
due to the greater persistence of certain series, could scarcely have es- 
caped notice. The significance of these observations is two-fold: 
On the one hand we find that for the only examples of luminescence 
which admit of such detailed inspection, the spectrum of phosphorescence 
is identical with that of fluorescence and since there are no indications to 
the contrary in the case of other classes of substances thus far studied it 
is probable that the above statement will apply to all phosphorescent 
materials. 
On the other hand we find that in spite of its great complexity, the 
luminescence spectrum of a uranyl salt is to be regarded as a unit, all 
its components decaying at the same rate after the cessation of excitation. 
With this instrument the change of intensity of phosphorescence with 
the time may likewise be readily determined and as no one appears to 
have studied the uranyl 
salts in this respect, it o — — --— o 
seemed of interest to ob- 
tain the curve of decay — 
of one of the salts under 
observation, i.e., the ura- 
nyl-ammonio sulphate. 
To this end a simple 
form of photometer pre- 
viously used in a study of the phosphorescence of Kunzite^ was mounted 
in front of the sectored disk. A lateral strip of the phosphorescent salt 
1 cm. wide was excited by sparks from a single spark gap between zinc 
terminals and measurements of the brightness were made at various 
times after the close of excitation. The necessary conditions were at- 
tained by shifting the disk successively through small angles so as to 
vary the interval between excitation and observation. The time could 
be estimated with sufficient accuracy by noting the instantaneous posi- 
tions of the disk for each adjustment, as given by the strictly syn- 
chronous illumination due to the spark. 
The arrangement of the apparatus is shown in figure 4 in which P 
is the phosphorescent surface, DD the sectored disk, L. B. the Lummer- 
Brodhun cube of the photometer, E the eye-piece, S a color screen and 
matte translucent plate, C the comparison lamp which travelled along 
the track of an optical bench. The cross at Z indicates the position of 
the spark gap. 
FIG. 4. 
