ARTICLES 215 



charged plates Pi, P2. As the charged particles pass through 

 the electric field, they are each deflected and so spread out 

 into an " electric spectrum," the deflections being inversely- 

 proportional to the energy of the particles. The rays now 

 behave as though radiating from a virtual source, z, and a 

 group of them is selected by the diaphragm d, and allowed to 

 pass between the poles of a powerful electromagnet. These 

 poles are circular, and as a magnetic field deflects the particles 

 inversely as their momentum, it can be shown that if the angle 

 through which they are bent in this case is more than double 

 the foregoing electric deflection, and in the opposite direction, 



Fig. I. — Positive-ray spectrograph. 



Reproduced by kind permission of the Philosophical Magazine. 



rays of constant mass will come to a focus f as indicated in the 

 diagram. Also, if a photographic plate^is placed at gf in 

 the line passing through z, it will be approximately in focus 

 for all rays whatever their masses. The position of the focal 

 points along such a plate depends only upon the masses-— or, 

 to be precise, the ratios of mass to charge — of the particles 

 causing them, so the result obtained is called a " mass-spec- 

 trum." Just as in the ordinary spectrum of a gas the image 

 of the slit is deflected according to the wave-lengths of the 

 light, giving the familiar appearance of bright lines, so in 

 the mass-spectrum the image caused by one of the slits col- 

 limated by the other will appear at different points, each point 

 representing a definite mass. 



The exact mathematical relation between the positions of 

 the focal points or lines on the mass-spectrum and the masses 

 causing them is by no means simple. But, fortunately, it is not 



