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Sir J. J. Thomson on 



could be produced between the pole-pieces. Even with these 

 short pole-pieces there was a very considerable production of 



secondary rays, as is shown by the photograph reproduced in 

 fig. 8 (PL I.). It ought, however, to be mentioned that the 

 pressure in this case was not very low, as by an accident there 

 was a little grease inside the tube which gave off an appreciable 

 amount of vapour. The photograph is interesting as it shows 

 how intense this production of secondary radiation may be. 

 To test the point further, the length of the magnetic field 

 was reduced to 1 mm. ; this was done by replacing the iron 

 rods A and B in fig. 7 by thin iron plates, and increasing the 

 strength of the electric and magnetic fields sufficiently to 

 produce appreciable magnetic and electric deflexions even 

 though the path of the rays through these fields was only 

 1 mm. long. In this case all the curves of type (2) com- 

 pletely disappeared, and the only curves on the plate were 

 the parabolas of type (1). A photograph taken with this 

 apparatus is reproduced in fig. 9 ; it will be seen that there 

 is nothing on the plate nearer the vertical than the heads of 

 the bright parabolas. 



We should also get rays of type (2) if some of the primary 

 rays, after passing through the cathode, lost their charges as 

 they were passing through the electric and magnetic fields. 

 In this case the parts of the curves near would be due to 

 rays which had lost their charges near the beginning of the 

 electric and magnetic fields, while in the case we have just 

 been considering this part of the curves would be due to rays 



