CHAPTER III. 



THE LUMINESCENCE OF SIDOT BLENDE. 1 



Among the substances whose luminescence is sufficiently bright for 

 spectrophotometric study, Sidot blende or phosphorescent zinc sulphide 

 seems especially well suited to bring out the relationships that doubtless 

 exist between different types of luminescence ; for not only is this substance 

 excited to luminescence by all known exciting agents light, Roentgen 

 rays, radium rays, cathode rays, etc. but the stimulating effect of heat 

 and the property possessed by the red and infra-red rays of suppressing 

 phosphorescence are exhibited in unusual degree. It is for this reason that 

 we have chosen this substance for spectrophotometric study. 



The material used in these experiments was in the form of a screen, of 

 the kind frequently used in exhibiting the properties of radium and for 

 numerous experiments in which it is desirable to have a fluorescent sub- 

 stance in convenient form. The experimental methods were similar to 



Fig. 38. 



The uppercurve shows the luminescence 

 spectrum of Sidot blende during ex- 

 citation by Roentgen rays. The lower 

 curve shows the phosphorescence 5 

 seconds after excitation. 



0.46 



0.50 



54 



0.58 



0.62 JU 



those described in Chapters I and II, a Lummer-Brodhun spectrophoto- 

 meter, with an acetylene flame as a comparison source, being used to measure 

 the intensity in different parts of the luminescence spectrum. In certain 

 portions of the work special devices were required which will be described 

 in their proper place. 



LUMINESCENCE EXCITED BY ROENTGEN RAYS. 



The screen was placed in front of the collimator slit of the spectrophoto- 

 meter at a distance of only a few centimeters, while the Queen self-regulating 

 tube used for excitation was about 20 cm. behind the screen. Roentgen 

 rays of moderate "hardness" were used. No systematic experiments to 

 determine the effect of varying hardness upon the form of the luminescence 

 spectrum have yet been made, but a few preliminary tests indicate that 

 the effect is not great. Corrections due to fluorescence excited in the 

 glass of the spectrophotometer, which we had at first thought would be 

 necessary, were found to be negligible. 



The upper curve in Fig. 38 shows the luminescence spectrum observed 

 during excitation, while the lower curve shows the distribution of intensity 



J An account of the experiments described in this chapter was presented to the American Physical Society 

 at the Philadelphia meeting December 30, 1904. Physical Review, XXI, p. 247; XXII, p. 279; XXIII, p. 37. 



41 



