Recent Advances in Electricity. 25 



influence is now known as the "Lenard rays." He found that 

 it could affect a photographic plate, and could also produce 

 fluorescence. Shortly afterwards Rontgen found that along 

 with these ''Lenard rays" there were present other rays 

 possessing still more remarkable properties, and to these he 

 gave the name of the X rays, X being adopted as a convenient 

 designation for an unknown quantity. One may be disposed 

 at first to think that the Lenard rays and the X rays are not two 

 things, but one and the same thing investigated by two different 

 people. This, however, would be an erroneous impression. 

 There are two distinct kinds of rays mixed up together. One 

 test for distinguishing them is the influence of a magnet. 

 When lines of magnetic force run across a moveable wire 

 through which a current is passing they make the wire move 

 sideways. The visible discharge in an ordinary Geissler tube 

 is moved in the same way, and so is the cathode stream in a 

 Crookes' tube, and so also is the stream of Lenard rays outside 

 the Crookes' tube ; but the stream of X rays is unaffected — 

 the X rays cannot be deflected by a magnet. I may mention, 

 in passing, some particulars which I heard from Prof. Lenard's 

 own lips at the recent meeting of the British Association, and 

 which were new to most of his audience. The Lenard rays, when 

 they have to pass through air at atmospheric pressure, behave 

 very much like rays of light passing through a turbid medium, 

 such as muddy water or a dense fog ; but when the pressure is 

 reduced to about \ of an atmosphere they begin to show a 

 definite track, and the more the pressure is diminished the 

 more definite they become. When you have sufficiently 

 diminished the pressure to obtain a well-defined and narrow 

 stream, it is found, on bringing magnetism to bear upon it, 

 that the stream is not only deflected to one side, that is to say, 

 curved towards one side, but that it is widened out by unequal 

 deflection of its different constituents — an effect precisely 

 analogous to the widening out of a narrow beam of solar light 

 into a spectrum by unequal refrangibility. The interpretation 

 put upon this phenomenon is, that the stream is a stream of 



