Mechanical Phosphorescence. 361 
4. The reciprocal of the square root of the emission in- 
tensity has a uniform time gradient. 
5. When the system is saturated the initial emission in- 
tensity is proportional to the intensity of the incident 
disturbance when the latter is small. 
7. Xow when a substance phosphoresces : — 
1. The emission spectrum in any neighbourhood is inde- 
pendent of the exciting light. 
2. Phosphorescence is produced by light of any frequency 
within a range which includes the emission frequency 
near its lower end. 
3. The intensity of emission is maximum for an exciting 
frequency greater than that of the emission. 
4. The reciprocal of the square root of the emission in- 
tensity when plotted against the time, gives a straight 
line during an interval in which the greater portion 
(over 00 per cent.) of the stored energy is dissipated ; 
there is then a rapid bend in the graph which quickly 
becomes straight again at a smaller inclination. 
5. The energy stored under given disturbance increases 
asymptotically to a definite limit, and when the system 
is saturated the initial emission intensity is propor- 
tional to the intensity of the exciting light, provided 
the latter is not too large *. 
There is thus a remarkable agreement between the pro- 
perties of the phosphorescent substance and those of the 
system — an agreement which is all the more striking in view 
of a very noteworthy difference, namely, that phosphorescent 
matter gives a band spectrum while the system gives a line. 
A secondary point of contrast arises in the rate of decay of 
emission ; as we have seen for the system the I - * — t graph 
is simply a straight line, while in the case of the phosphor- 
escent matter some radical change takes place when only a 
small percentage of the energy remains, and the rate of 
change of I - * is diminished. Connected with this there is a 
hysteresis effect f which has no analogue in our mechanical 
system. 
The phosphorescent bands differ very much in brilliance 
* "On the Decay of Phosphorescence in Sidot-blende and certain 
other Substances," by Messrs. E. L. Nichols and E. Merritt, Physical 
Review, vol. xxiii. (1906) p. 37. 
f Nichols and Merritt, he. cit. 
