PHYSICS: NICHOLS AND HOWES 
311 
of short duration the loss of light during the initial process may go to the very 
limit of visibility within a few hundredths of a second or less, but if the 
threshold is not actually crossed the phosphorescence may remain, barely visible 
in a completely dark room, for a considerable time. Waggoner remarks (on 
page 216 of his paper) 11 It will be noted that in practically all the substances 
studied, the measurable portion of the decay is over in 0.07 seconds. Some of 
the substances may, however, be seen in a dark room for a very much longer time." 
Phosphorescence of short duration, when not of the vanishing type, may be 
considered simply as a case of the persistent type in which the rapid initial 
process reduces the intensity nearly to or beyond the threshold of visibility. 
The subsequent processes, though gradual are therefore too faint for observa- 
tion or invisible. The brightness of the first process may be as great or greater 
in cases of quick decay than where the phosphorescence continues of meas- 
urable intensity for a long time. Zeller 2 who has studied the beginnings of 
decay of the phosphorescence of various persistent compounds has made note 
H 
EFFECT* OF INFRA-RED. 
4 
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2 
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1 . 
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. 4 .° . 8 . 0SEC - 
FIG. 7 
of the fact that duration is not in any simple way related to initial brightness. 
In a group of phosphorescent cadmium compounds, in particular, some that 
were too dim for measurement with the phosphoroscope remained visible for 
a long time while those of greatest initial intensity were of very short duration. 
An interesting substance which came to our notice during these investiga- 
tions through the kindness of Mr. W. L. Lemcke is a cadmium phosphate 
prepared by Mr. W. S. Andrews. Under the iron spark it is excited to a fine 
white phosphorescence having a measurable duration of about one second. 
We determined its curve of decay with the disk phosphoroscope. As may be 
seen from figure 6 it is of the persistent type and so far as the white afterglow 
is concerned might be classed as of rather short duration; but a very faint 
ruddy phosphorescence remains for a much longer time. This specimen, as 
viewed on the rotating disk is remarkable for the succession of color effects 
which it exhibits. Pink, very fleeting, is followed by nearly pure white, then 
for an instant by blue which goes over into a very persistent pink, turning 
ruddy as decay progresses. 
