A. W. Wright — Cathode Bays and their Effects. 235 



Art. XXVI. — Experiments upon the Cathode Bays and their 

 Effects ; by Arthur W. Wright. With Plates YI-YII. 



The history of the investigations which have led to our 

 present knowledge of the phenomena of electrical discharges 

 in rarified gases, and especially of the actions which take place 

 at the negative electrode in an exhausted tube, is a long one and 

 full of interest. But while there are many memoirs of very high 

 importance, those which most immediately relate to the subject 

 here considered are two papers of Philipp Lenard, of the Uni- 

 versity of Bonn,* which marked a great advance in the study 

 of the cathode rays, and the paper of Professor W. C. Pontgen,f 

 of Wiirzburg, which has attracted immediate attention from 

 the novelty and importance of the results therein announced. 



In his first paper Lenard, by a series of very ingeniously 

 contrived experiments, showed how, by the use of a small 

 window of thin aluminium, the cathode rays could be obtained 

 in the air outside of the vacuum-tube, and their properties 

 studied by the use of fluorescent screens and of sensitized pho- 

 tographic plates. He showed that they could be traced to a 

 distance of several centimeters in air, that they penetrated 

 various materials in different degrees, determined in a large 

 measure, by the circumstances under which they are originated. 

 He proved that in air and other gases they undergo a lateral 

 diffusion or scattering, as if by the action of a turbid medium, 

 and this in different amounts depending upon the degree of 

 exhaustion of the tube in which they originated. His researches 

 led him to the conclusion that the cathode rays are phenomena 

 of the ether. Among many experiments may be cited one in 

 which he obtained a photographic picture, in an opaque metal- 

 lic box, one side of which was made of thin aluminium. In 

 his second paper he studies the relative absorption of the rays 

 by different materials, showing that, broadly, the absorption is 

 proportional to the density of the matter, and independent of 

 its kind, but that the order of the different kinds of matter in 

 this respect is to some extent dependent upon the character of 

 the rays. 



Prof. E-ontgen's paper, which is full of most interesting 

 results, greatly extends the range of the phenomena observed, 

 especially as to the remarkable power of the rays to penetrate 

 masses of matter, and brings up some very important questions 

 as to their . character. He states that as obtained outside of 

 the vacuum-tube they are not deflected by a magnet to an 

 appreciable extent, that they undergo neither refraction, refl.ec- 



*Wied. Ann., li, p. 225, 1894; lvi, p. 255, 1895, 



f Sitzungsberichte der Wiirzburger Physik-medic. Gesellschaft, 1895; transla- 

 tion in Nature, vol. liii, p. 274, Jan. 23, 1896. 



