216 



Sir J . J. Thomson : Further 



The two photographs reproduced in figs. 8 and 9 (PL IX.) 

 show cases in which, by changing the disposition of the 

 magnetic field, the secondary due to the hydrogen atom has 

 been displaced from below its primary to above it. 



Fig. 7. 



w 



If the delivery-tube for the positive rays did not reach up 

 to the beginning of the plates used to produce the electro- 

 static deflexion^ and the magnetic field overlapped the 

 electrostatic at both ends, dyjcLx would be infinite at the 

 beginning and end of the curve, and the secondary curve 

 would be shaped like the one shown in fig. 10* 



Fig. 10, 



An example of this is shown in the photograph reproduced 

 in fig, 9 (PL IX.). 



We see from these considerations that a secondary line 

 may suffer considerable variation in position, and there is 

 some danger^ unless precautions are taken, that a secondary 

 which has been displaced owing to some change in the 

 distribution of the magnetic or electric fields, might be 

 regarded as a line due to a different kind of particle. When 

 the photographs of the positive rays are used to analyse the 

 gases in the discharge-tube, the lines clue to the primary 



