IIO STUDIES IN LUMINESCENCE. 



trophotometer the light which comes from the phosphorescent screen F 

 while it is being excited by the spark. By moving the rod R the mirror 

 is turned so as to reflect the light from the screen F into the spectrophoto- 

 meter some time after excitation, and in this way the intensity of the phos- 

 phorescence after successive excitations may be measured by the spectro- 

 photometer AB. The cylinder L is driven by a motor belted to it over the 

 pulley P. S is a worm driving a cog wheel, which serves as a device for 

 recording the speed by making an electric connection with a chronograph 

 once in every hundred revolutions of the disk. Current for the motor 

 was taken from a direct-current source having a special device for main- 

 taining constant voltage, and the speed of the motor was found to remain 

 almost constant. The disk D has a second half-disk fastened to the first 

 (not shown in the figure) by which it is possible to change the size of the 

 opening at O. The essential advantage of this phosphoroscope over others 

 is that the decay of the phosphorescent light may be studied without chang- 

 ing the time of excitation. Another feature of this method is that settings 

 on the spectrophotometer for any curve may be repeated as often as desired 

 and the time taken for the determination of each setting may be as long 

 as desired. The results shown in the curves that follow were determined 

 from two different settings of the spectrophotometer for each point, and 

 the plotted point represents the average. 



The source of excitation was the spark between iron terminals. To 

 produce this spark an induction coil was connected to a sourceof alternating 

 current of 60 cycles frequency, a small condenser being connected in mul- 

 tiple with the spark gap. In order to study the effect of decreased time of 

 excitation, it was found necessary to increase the frequency of the spark, 

 this being accomplished by connecting it to a source of alternating current 

 giving 140 cycles. With this higher frequency the chances of the spark 

 exciting the screen at each turn of the disk is greater ; the higher frequency 

 spark was also used as a check on the curves taken at the lower frequency. 



The light from an incandescent lamp connected to a constant potential 

 source was used for comparison in the spectrophotometric measurements. 



METHODS OF PREPARING THE PHOSPHORESCENT COMPOUNDS. 



The phosphoroscope just described, although it has a considerable range 

 as to speed, is especially adapted to compounds whose phosphorescence 

 decays rapidly. A number of such compounds were prepared and were 

 studied along with several specimens of willemite whose decay was of suit- 

 able rapidity. 



In preparing the compounds the methods given by Wiedemann and 

 Schmidt 1 and Andrews 2 were followed. The ease with which these com- 

 pounds may be prepared and the intensity of the phosphorescent light 

 given off when they are excited by the spark seem to warrant a rather 

 detailed account of their preparation. 



ZnCl<i.-\- M nSOi.* Some zinc chloride prepared from metallic zinc was 

 dissolved in a small quantity of distilled water. A small trace (usually less 



'Wied. Ann., 54, p. 604, 1895. 2 Science, xix, March 1904. 



'MnSOt was selected as an active salt in a number of cases, since it has been found by Lenard and Klatt 

 (Annalen der Physik, 15, p. 243, 1904) that with this salt the intensity of phosphorescence is less affected 

 by varying the concentration than with any other active agent. 



