PHYSICS: E. L. NICHOLS 
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example indeed of such bands or systems of great complexity of structure 
in the case of the uranyl salts. 
The evidence that, in general, the spectra of the phosphorescent 
sulphides contain more than one band or complex has already been 
mentioned, e.g., the marked changes of the color of phosphorescence 
with temperature and during the process of decay. 
The Decay of Phosphorescence in Different Parts of the Spectrum. — 
To obtain the curve of decay for a restricted region of the spectrum Drs. 
Howes and Hodge used a spectrophotometer in combination with the 
synchrono-phosphoroscope. In this way a set of curves corresponding 
to several nearly equidistant regions within the phosphorescent spectrum 
were obtained for each of the three sulphides under consideration. 
A notable feature of all these curves is the existence of two so-called 
linear processes the first of steeper slope and therefore indicative of a 
more rapid decay of phosphorescence than the second. This form of 
curve, as is well known, is characteristic of phosphorescent substances 
in general, the only well established exceptions being those occurring 
in the case of the uranyl salts. ^ As regards the relation of the two pro- 
cesses recorded in these diagrams to what appear as the first and second 
processes in the usual study of the long time phosphorescence of such sul- 
phides, it is clear that the second process in our curves is not identical 
with the first process as observed by the usual long time methods. 
Assuming the second process to continue, the intensity after one second 
would be about 1/1000 of that at 0.01 second or roughly 1 /20,000 of its 
initial brightness whereas as is well known these substances retain an 
easily visible phosphorescence after many seconds. 
Owing to the over-lapping of the components in the spectra under 
consideration it is difficult to determine whether the group of equidistant 
bands are to be regarded as a unit, as is in the case of the uranyl salts 
or indeed whether they constitute the whole of the phosphorescence 
spectrum. To that end some method permitting of more complete 
resolution must be devised. 
The pronounced changes in the color of the phosphorescent light 
would make it seem probable that we have to do in these observations 
chiefly with components of the phosphorescence that are of rapid decay 
and that, after a few hundredths of a second, these disappear leaving 
behind other components which constitute the phosphorescence of 
long duration. These, which are probably of relatively insignificant 
initial brightness, doubtless overlap the phosphorescence of short dura- 
tion but occupy, as a whole a somewhat different portion of the spectrum. 
In that case since one has to do with a different group of bands in 
