4 6 



STUDIES IN LUMINESCENCE. 



was cut off. In this way the time required for the phosphorescent light 

 to fall from its initial value to any given final value could be conveniently 

 measured. 



The determination of the instant when the rapidly changing phospho- 

 rescence became equal to the constant comparison light would at first 

 appear to be a difficult observation, and one not capable of great accuracy. 

 As a matter of fact these observations were fully as reliable as ordinary spec- 

 trophotometrie settings, and at these low intensities far less trying to the eye. 



The most serious difficulty encountered in these measurements arose 

 from the unsteadiness of the exciting light. When constancy is essential 

 the arc, even under the best of conditions, leaves much to be desired. 

 After numerous unsuccessful attempts to obtain steady conditions we 

 abandoned all special efforts to keep the excitation constant, and arranged 

 to make observations only when the exciting light, either by adjustment 

 or by accidental fluctuation, had reached a certain definite intensity. In 

 order to accomplish this a second spectrophotometer (S2 in Fig. 39) was 

 used. Light from the luminescent screen was thrown into the collimator 

 by means of a mirror as shown in the figure. The comparison light came 

 from the acetylene flame F after reflection from a mirror and a block of 

 magnesium carbonate not shown in the figure. One observer at the eye- 

 piece of S2 waited until the intensity of fluorescence reached such a value as 

 to give equality of the two fields, and then, with suitable warning, dropped 

 the shutter C. The second observer at Si then made a chronograph record 

 as before described. Numerous determinations, often twenty or more, were 

 made in this way for each point of the curves described below. 



Table i 1 . 



The observed time, in seconds, during which the phosphorescence fell from 



its initial intensity to the intensity at the top of the column. 



A preliminary set of measurements in which the duration of excitation 

 was varied from 3 seconds to 30 seconds showed no variation in the time 

 of decay. We conclude that the full effect of the exciting light is produced 

 in less than 3 seconds. 1 In the subsequent experiments the duration of 

 excitation was never less than 5 seconds. 



The results of one set of determinations made in this way are given in 

 Table 1 1 and are plotted in Fig. 42. In curve / the width of the comparison 



'Later experiments (see Chapter IV) show that the long-time phosphorescence of Sidot blende is very 

 noticeably influenced by a change in the duration of excitation. 



