402 PROCEEDINGS OF THE AMERICAN ACADEMY. 



I have succeeded in producing the red phosphorescence by the cath- 

 ode rays, thus annihilating the distinction, in this case, between the 

 two kinds of rays. The method adopted seems to have a general 

 application in the study of phosphorescence and is as follows : 



The vacuum tube was of cylindrical form. Figure 1 shows the arrange- 

 ment. A represents the circular iron terminal with its central orifice 

 perforated by a glass tube ; S, the solenoid ; L, the ground-glass stopper 

 with the layer of lithium chloride at its end. 



When the solenoid is excited, the cathode rays can be brought to a 

 sharp focus on the layer at L, and the apparatus can be called in pop- 

 ular language a magnetic lens. A very intense cathode beam can be 

 made to converge at L by suitably adjusting the solenoid. The rays seek 

 the weakest part of the magnetic field. Immediately on striking the 

 layer of lithium chloride the red phosphorescence appears at the centre 

 of the focus, and is surrounded by the blue phosphorescence ; either the 

 red or the blue can be produced at pleasure. 



It seems, therefore, that if w is the number of cathode particles, m their 

 mass, V their velocity, and ;/ the number of positive particles, m^ their 

 mass, i'' their velocity, that the equation 



holds on the unit of area, and that the distinction, in this case between 

 the color produced by the cathode rays and the Canalstrahlen disap- 

 pears. The production of the two colors is a question of energy on the 

 unit of area. 



I have examined the phosphorescence of the other metals of the same 

 groUp as lithium chloride. Caesium chloride gives a very bright blue 

 color for both the cathode and the canal rays, and the blue lines of the 

 spectrum appear with the application of the cathode beam. Rubidium 

 gives both a red and a blue color ; the red, however, is much less bright 

 than in the case of lithium chloride. All of these salts are quickly de- 

 composed. Calcium tungstate recovers from fatigue very quickly, and 

 is not decomposed appreciably, even after long exposures. Its use for 

 X-ray screens is therefore substantiated by these experiments. 



Application of a Longitudinal Magnetic Field to X-ray Tubes. 



In the article on the Magnetic Field and Electric Discharges,^ I 

 stated that the application of a longitudinal field at the anode might 

 form a useful method of concentrating the cathode rays. Since this 



' These Proceedings, 28. 



