CATHODE RAYS. 159 



iug- up of the glass to allow of the remaking of the bulb. I will now 

 leave the chemical effects produced by these rays and pass on to con- 

 sider their behavior when in a magnetic field. 



First, let us consider for a moment the effect of magnetic force on 

 the ordinary discharge between terminals at a pressure much higher 

 than that at which the cathode rays begin to come off. I have here 

 photographs (see tigs. 1 and 2, PI. I) of the spark in a magnetic field. 

 You see that when the discharge, which passes as a thin bright line 

 between the terminals, is acted upon by the magnetic field it is-pulled 

 aside as a stretched string would be if acted upon by a force at right 

 angles to its length. The curve is quite continuous, and though there 

 maybe gaps in the luminosity of the discharge, yet there are no breaks 

 at such points in the curve, into which the discharge is bent by a mag- 

 net. Again, if the discharge, instead of taking place between points 

 passes between flat discs, the effect of the magnetic force is to move 

 the sparks as a whole, the sparks keeping straight until their termina- 

 tions reach the edges of the discs. The fine thread-like discharge is 

 not much spread out by the action of the magnetic field. The appear- 

 ance of the discharge indicates that when the discharge passes through 

 the gas it manufactures out of the gas something stretching from ter- 

 minal to terminal, which, unlike a gas, is capable of sustaining a ten- 

 sion. The amount of deflection produced, other circumstances being 

 the same, depends on the nature of the gas. As the photographs (figs. 

 3 and 4, PI. I) show, the deflection is very small in the case of hydrogen 

 and very considerable in the case of carbonic acid. As a general rule 

 it seems smaller in elementary than in compound gases. 



Let us contrast the behavior of this kind of discharge under the 

 action of a magnetic field with that of the cathode rays. I have here 

 some photographs (PI. II) taken of a narrow beam formed by sending 

 the cathode rays through a tube in which there was a plug with a slit 

 in it, the plug being used as an anode and connected with the earth, 

 these rays traversing a uniform magnetic field. The narrow beam 

 spreads out under the action of the magnetic force into a broad fan- 

 shaped luminosity in the gas. The luminosity in this fan is not 

 uniformly distributed, but is condensed along certain lines. The 

 phosphorescence produced when the rays reach the glass is also not 

 uniformly distributed. It is much spread out, showing that the beam 

 consists of rays which are not all deflected to the same extent by the 

 magnet. The luminous patch on the glass is crossed by bands along 

 which the luminosity is very much greater than in the adjacent parts. 

 These bright and dark bands are called by Birkeland, who first observed 

 them, "the magnetic spectrum." The brightest places on the glass 

 are by no means always the terminations of the brightest streaks of 

 luminosity in the gas ; in fact, in some cases a very bright spot on the 

 glass is not connected with the cathode by any appreciable luminosity, 

 though there is plenty of luminosity in other parts of the gas. 



One very interesting point brought out by the photographs is that 



