Positive Ray Spectrograph 



709' 



Possibilities of focussing positive rays. 



The very great accuracy attained in the spectrometry of 

 light depends largely on the fact that a considerable solid 

 angle of divergent rays from a point source can be brought 

 to a point image by means of a lens. It is of importance to 

 inquire if any such convergence can be applied to rays of 

 charged particles by any electric or magnetic device. 



As regards the ordinary lens the problem appears rather 

 hopeless, but electric or magnetic analogues of the cylindrical 

 lens can be made theoretically in several ways. 



Thus magnetically homogeneous rays diverging from a 

 point or slit source S (fig. 1) will be brought to a first-order 



Fio-. 1. 



focus F if the integrated intensity of the field traversed by 

 any one ray is proportional to its angular distance from the 

 line SF, e. g. by the use of parallel pole-pieces of wedge-shaped 

 section. 



Such a ' magnetic lens ' is not of much immediate value 

 as the magnetic spectrum of positive rays is very complex, 

 The electric spectrum, on the other hand, possesses one very 

 important simplicity, namely, that the distribution of intensity 

 in it is a property of the discharge rather than of the particles 

 carrying it, and is to a great extent the same for all particles: 

 in other words, the value of mv 2 giving the brightest result for 

 particles of one mass will in general give the brightest result 

 for all. 



Fis-. 2. 



A variety of electrical analogues of the cylindrical lens are 

 possible. In fio-. 2 divergent rays of constant mv* passing 

 through the field between two charged plates whose sections 

 are two concentric coaxial rectangular hyperbolas will be 

 brought to a focus on the line passing through the source 



