Rays of Positive Electricity . 233 



which had been produced near the end of these fields ; by 

 the beginning of the field is meant ihe part o£ the field 

 nearest the cathode, and by the end of the field the part 

 nearest the photographic plate. This consideration leads to 

 a method by which we can distinguish whether the curves of 

 type (2) originate in one or other of these ways. For 

 suppose that the electric and magnetic fields, instead of being 

 coterminous, are phiced one after the other, let us suppose 

 that the magnetic field is the one nearest to the photographic 

 plate. Then if the curves are due to rays produced by the 

 passage of the undeflected rays through the gas, some of 

 these will be produced after the undeflected rays have left 

 the electric and are passing through the magnetic field, the 

 rays so produced will suffer magnetic but not electric de- 

 flexions, and the curve they will trace on the photographic 

 plate will be somewhat like that shown in fig. 6 (p. 231). 

 Curves of this kind can be seen on the photographs repro- 

 duced in figs. 5 (PI. I.). If, however, the rays, instead of 

 being produced between the cathode and the photographic 

 plate, are losing their charges as they travel through the 

 electric and magnetic fields, some of them will lose their 

 charges whilst they are in the electric and before they have 

 reached the magnetic field ; these will experience an electric 

 but not a magnetic deflexion, and the curves they trace on 

 the photographic plate will be of the kind shown in fig. J 1 



(PL I.). 



Fi"-. 10, 



The concavities of the curves in figs. 10 and 11 are turned 

 in opposite directions, and this criterion will enable us to 

 decide as to the way the rays originate. For example, let 

 lis take the most prominent curve of type (2) when the rays 

 pass through helium. Fig. 11 represents the curves for 

 helium when the electric and magnetic fields are nearly 

 coterminous; it will be seen that the curve marked with a x , 

 which is the one under consideration, is concave to the hori- 

 zontal axis. When, however, the fields are separated and the 

 magnetic field placed nearer to the photographic plate 1 than 

 the electric, the shape of the curve is as in the photograph 



