Media iiiad Phosp7torescen.ee, 363 
with Sidot blende as phosphorescent substance*. The waves 
in this region have the effect of greatly retarding the storage 
of energy when incident along with the exciting disturbance; 
and in general they also diminish the total store and rate of 
emission if applied during phosphorescence. There is an 
exception to be noted, however, in the case of Sidot blende ; 
one particular band of the phosphorescence spectrum is rendered 
more intense throughout its decay by application of the 
infra-red during emission. This effect is consistent with the 
property enunciated above for the mechanical system, but 
further experiment is necessary to determine the range of 
(infra-red) frequency which produces the brightening. If 
the range were found to contain, or lie near, half the phos- 
phorescence frequency, a further point of resemblance would 
be established between the mechanical system and the phos- 
phorescent substance. 
The general infra-red effects, on the other hand, do not 
appear to come within the analogue, possibly on account of 
the difference referred to in the preceding section. 
Another point which is worthy of experimental investi- 
gation is the variation in the rate at which energy is stored 
under given intensity of excitation for different frequencies 
within the effective range. It has been shown in § 3 that 
the initial rate of energy absorption in the mechanical system 
is maximum for a frequency rather greater than the central 
value of the range, and gradually diminishes towards zero at 
the ends of the range. 
If the excitation intensity is not too great it is found that 
an appreciable time f is required for the saturation of some 
phosphorescent substances, and there does not appear to be 
any insuperable difficulty in the way of the experiment. 
The inquiry would be of special interest in determining the 
saturation limit for different frequencies throughout the 
range. 
Manchester, November 1907. 
* " The Influence of the Red and Infra-red Rays upon the Photo- 
luminescence of Sidot Blende.'' Phvsical Review, vol. xxv. (1907) 
p. 362. 
+ About five minutes in the only case recorded by Nichols and Merritt, 
p. 45, in the paper first mentioned. For trustworthy results it would be 
necessary to make measurements of the short duration phosphorescence 
before the bend of the I~* — t graph. 
