1897.] Cathode Bays. 419 



WEEKLY EVENING MEETING, 



Friday, April 30, 1897. 



' Sir Frederick Bramwell, Bart. D.C.L. LL.D. F.R.S. Honorary 

 Secretary and Vice-President, in tlie Chair. 



Professor J. J. Thomson, M.A. LL.D. Sc.D. F.R.S. 



Cathode Bays. 



The first observer to leave any record of what are now known as 

 the Cathode Rays seems to have been Pliicker, who in 1859 observed 

 the now well known green phosphorescence on the glass in the 

 neighbourhood of the negative electrode. Pliicker was the first 

 physicist to make experiments on the discharge through a tube, in a 

 state anything approaching what we should now call a high vacuum : 

 he owed the opportunity to do this to his fellow townsman Geissler, 

 who first made such vacua attainable. Pliicker, who had made a 

 very minute study of the efifect of a magnetic field on the ordinary 

 discharge which stretches from one terminal to the other, distin- 

 guished the discharge which produced the green phosphorescence 

 from the ordinary discharge, by the difference in its behaviour when 

 in a magnetic field. Pliicker ascribed these phosphorescent jmtches 

 to currents of electricity which went from the cathode to the walls of 

 the tube, and then for some reason or other retraced their steps. 



The subject was next taken up by Pliicker's pupil, Hittorf, who 

 greatly extended our knowledge of the subject, and to whom we owe 

 the observation that a solid body placed between a pointed cathode 

 and the walls of the tube cast a well defined shadow. This observa- 

 tion was extended by Goldstein, who found that a well marked, 

 though not very sharply defined shadow was cast by a small body 

 placed near a cathode of considerable area ; this was a very important 

 observation, for it showed that the rays casting the shadow came in 

 a definite direction from the cathode. If the cathode were replaced 

 by a luminous disc of the same size, this disc would not cast a shadow 

 of a small object placed near it, for though the object might intercept 

 the rays which came out normally from the disc, yet enough light 

 would be given out sideways from other parts of the disc to jDrevent 

 the shadow being at all well marked. Goldstein seems to have been 

 the first to advance the theory, which has attained a good deal of 

 prevalence in Germany, that these cathode rays are transversal 

 vibrations in the ether. 



The physicist, however, who did more tlian any one else to direct 

 attention to these rays was Mr. Crookes, whose experiments, by their 

 beauty and importance, attracted the attention of all physicists to this 



