ON PHOSPHOROSCOPES. 241 



hand of the observer. A series of such comparisons, together with the initial 

 time, formed a series of points for a decay curve. It is clear that such a 

 device is only suitable for the study of decay which endures for several minutes. 



E. H. Kennard 1 devised a phosphoroscope of this type with shutters actuated 

 by the magnetic release of phosphor-bronze springs. Three shutters (Si, 82, 

 S 3 , of fig. 11) were used, Si to admit exciting light from the mercury arc A 

 to the specimen P, S 2 and 83 to limit the time during which the phosphorescence 

 was allowed to produce photo-electric action on the cell C. The latter adapta- 

 tion to phosphorescence work is unique. In his preliminary work the times 

 between opening and closing of shutters were determined from the known 

 curve of a ballistic galvanometer, the passage of a shutter opening and closing 

 shunts which allowed a definite quantity of electricity to pass into the gal- 

 vanometer. In his later work the shutters were magnetically released by the 

 swing of a seconds pendulum across mercury cups set at convenient positions 

 along the path of the bob. The photo-electric current, for low intensities, is 

 proportional to the intensity of phosphorescence and was measured by a 

 quadrant electrometer. 



It is to be inferred from the preceding summary on phosphoroscopes that 

 there may be no single machine which is well adapted to the study of a par- 

 ticular phosphorescent specimen under investigation. As in the study of 

 phosphorescence in the past, the investigator has often devised one or more 

 machines for the study of the various types of phosphorescence; so in the 

 future the machine must be adapted to the behavior of the specimen. Then, 

 too, the precision with which the time of observation is desired makes it 

 necessary that the modern phosphoroscope be equipped with an accurate and 

 if possible a direct-reading speed register. The plotting of decay curves by 

 Nichols and Merritt, Trowbridge, Ives, and others made it imperative that 

 both times and excitations be well known. The lack of constancy of excita- 

 tion has been recognized and observations corrected. The importance of 

 obtaining the same degree of saturation before decay begins is paramount if 

 results are to be considered comparable. In the use of the continuously 

 excited type the importance of this factor becomes evident. The use of the 

 interrupted excitation type for eclipses of 16 or less a second should be entirely 

 avoided, because of the behavior of the eye when flicker is noticeable. The 

 human eye should not be fatigued beyond instant recovery during the process 

 of observing decay, neither should it be dark-adapted before beginning a set 

 of observations. The necessity of adequate screening is of great importance 

 when it is considered that luminescence radiation is several thousand times 

 less than the photo-excitation of approximately the same wave-length. Stray 

 luminescence may add to the selected portion of the luminescence beam and 

 produce errors. These factors are fundamental considerations for the future 

 student of phosphorescence. 



1 Kennard, Physical Review (2), vol. 4, p. 278. 1914. 



