142 Trowbridge — Phosphorescence produced by Canal Rays. 



line- of the solenoid thus passed through the core. Fig. 1 shows 

 the arrangement ; A represents the circular iron terminal with 

 its central orifice perforated by a glass tube ; S the solenoid ; 

 L the 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 Z, and the apparatus 

 can be called in popular 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 held. Immediately on striking the layer of lithium 

 chloride the red phosphorescence appears at the center 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 n is the number of cathode par- 

 ticles, m their mass, v their velocity ; and n' the number of 

 positive particles, ml their mass, v' their velocity, that the 

 equation 



nmv*=n'm'v' a 

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

 between the color produced by the cathode rays and the canal 

 rays disappear. 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. Caesium 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 blue color ; the red, 

 however, is much less bright than in the case of lithium chlor- 

 ide. All of these salts are quickly, decomposed. Calcium 

 tungstate recovers from fatigue very quickly and is not decom- 

 posed appreciably even after long exposures. Its use for X-ray 

 screens is, therefore, substantiated by these experiments. 



Jefferson Physical Laboratory, Harvard University. 



