CHAPTER VII. 

 STUDIES OF PHOSPHORESCENCE OF SHORT DURATION. 



By the methods described in Chapters IV and VI of this memoir it is 

 difficult to make observations of phosphorescence within less than 0.4 

 second of the close of excitation. This suffices for the determination of 

 the form of the curve of decay with the exception of the region very near 

 the origin. There are, however, many substances exhibiting brilliant initial 

 phosphorescence where the effect fades to unmeasurably small intensity 

 within a few hundredths of a second. 



It is of considerable interest and importance not only to determine the 

 curves of decay of such substances, but also, since Lenard 1 in a recent paper 

 has questioned the linear relation between I~* and time during the so-called 

 first process of decay, to study the earliest portions of the first process in 

 the case of phosphorescence of slow decay. 



B 



Fig. 116. 



Messrs. Waggoner and Zeller at our suggestion have investigated these 

 questions at some length, using a new type of phosphoroscope of especial 

 design and working independently of each other. The present chapter 

 contains a summary of their researches. 



DR. WAGGONER'S STUDIES IN PHOSPHORESCENCE OF SHORT DURATION. 2 



METHODS OF MEASUREMENT. 



The phosphoroscope used in these measurements is shown in Fig. 116. 

 It consists of a disk D, fastened to a cylinder L, rotating about a horizontal 

 axis. On the inside of L is a shaft K which rotates at the same speed as 

 the cylinder. On the end of K is a plane mirror, placed 45 to the axis of 

 the shaft, and by use of the mechanism at C the position of the mirror, 

 relative to any point on the disk D, may be shifted while the disk is rotating. 

 The disk has an opening at 0, through which the light from the spark E 

 may pass at each successive revolution and excite the specimen placed at F. 

 If the mirror is in the position shown it will reflect into the slit of a spec- 



] Lenard, Annalen der Physik, xxi, p. 641, 1910. 



= C. W. Waggoner, Physical Review, xxvu, p. 209. 



109 



