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Prof. Thomson, The nature of the y rays. 
The nature of the 7 rays. By J. J. Thomson, M.A., F.R.S., 
Cavendish Professor of Experimental Physics. 
[ Read 10 February 1908.] 
Two views have been advanced as to the nature of the 7 rays. 
One that they are pulses of electromagnetic disturbance propagated 
through the ether, the other that they are doublets made up of 
a positive and negative charge, travelling with great velocity and 
possessing energy and momentum. There is thus much the same 
difference between these theories as between the undulatory and 
emission theory of light. The two theories approximate closely to 
each other, if we suppose, as the author has suggested, that the 
electromagnetic disturbance in the pulse is not distributed uni- 
formly over the wave front but is concentrated in patches, as in 
each of these patches there are electric and magnetic forces, the 
patches will have energy and momentum in the direction in which 
the rays are propagated, the patches thus resemble the doublets 
in many of their properties. The two theories would be differen- 
tiated (1) by the velocity of propagation of the rays which on the 
pulse theory would be that of light and on the doublet theory 
might be anything, and (2) by the polarization in the secondary 
rays produced when the primary rays come into contact with 
matter. Professor Barkla’s experiments show that this polarization 
is in accordance with the pulse theory, and such investigations as 
have been made on the velocity of propagation indicate that this 
is equal to that of light. 
