CHAPTER IX. 



A SPECTROPHOTOMETRIC STUDY OF CERTAIN CASES OF 

 KATHODO-LUMINESCENCE.* 



The spectrophotometric study of luminescence has thus far been confined 

 almost entirely to cases of photo-luminescence. It is of interest to inquire 

 whether exciting agents other than light will give luminescence spectra of 

 the same type, and to what extent the laws that have been found to hold 

 in the case of photo-luminescence possess a more general application. The 

 experiments on kathodo-luminescence described in the present chapter are 

 of interest chiefly because of the bearing of the results upon these and 

 similar questions. During the course of the work preliminary data have 

 also been obtained with regard to the de- 

 pendence of kathodo-luminescence upon 

 discharge potential and current strength. 



The form of the vacuum tube used is 

 shown in Fig. 144, the substance to be 

 tested being placed at S. In order to pre- 

 vent disturbance from the fluorescence of 

 the glass the specimen in some instances 

 was placed in a metal inclosure which 

 protected the glass from excitation by the 

 kathode rays. The kathode, K, was a flat - 

 aluminum disk. The fluorescent light was I,' 



reflected directl v into the slit of the Lummer- 

 Brodhun spectrophotometer by a small J 



mirror at the side of the tube. An acetylene 

 flame was used as a comparison source. 

 Current was furnished by a motor-driven 

 Hoi tz machine capable of giving a current of 

 0.6 milliampere. A galvanometer placed Fig. 144. 



in the circuit next to the grounded pole of 



the machine served to indicate the constancy, or lack of constancy, of the 

 current through the tube. In most cases relative values only of the current 

 have been recorded. The potential difference between the terminals of the 

 tube was measured by a Kelvin electrostatic voltmeter and served as a 

 sensitive indicator of changes in the vacuum. "- 



Considerable difficulty was encountered in keeping the discharge through 

 the tube steady. Sudden changes occurred in the intensity of the fluores- 



The results contained in this chapter were in part presented to the American Physical Society, at the 

 meeting held in Washington, April 24, 1908, and were published in the Physical Review, xxvm, p. 349. 



2 Since the absolute values of the potential used are of 110 great significance in the present work, we have 

 plotted voltmeter readings rather than the actual values of the potential in the curves contained in this 

 chapter. If the potential in volts is desired thi-; may be found bv multiplying the voltmeter deflection by 

 385. 



137 



