444 Mr Crowther, On the Transmission of fi-rays. 



may be transmitted through the system along paths such as acb 

 or a'c'h' , even when the field is too strong or too weak to deflect 

 the rays, incident normally, along the path AGB. In this way 

 even if the rays are of uniform velocity to begin with, there will 

 be a definite range of field strength throughout which the rays 

 can pass through the system. Similarly, if the rays are not 

 homogeneous to begin with, if we are dealing with the whole of 

 the ^-rays from radium for example, there will be for every 

 mao-netic field a finite range of velocities which the yS-rays may 

 possess and still be able to pass through the system ; and if the 

 size of the apertures is considerable compared to the radius of 

 the path, and if the incident beam is of a fairly wide angle, this 

 difference of velocities may be very considerable. We shall return 

 to this point later. 



In addition to the broadening of the curve due to the finite 

 width of his apertures, there was a second possible explanation of 

 the shape of Schmidt's curves, namely that the rays from the 

 radium E were not homogeneous. 



It was natural to assume that the ;8-rays from a single homo- 

 creneous substance were homogeneous. In this case the fact that 

 the maxima of the different curves always occurred for the same 

 magnetic field, however much aluminium had been traversed, 

 would prove that the rays kept their velocity unaltered during 

 their passage through the fire. 



In a recent paper W. Wilson* has given considerable indirect 

 evidence to show that the /8-rays from a single radio-active 

 substance are not homogeneous, and in this case Schmidt's curves 

 could only be explained on the assumption that there was a definite 

 decrease in velocity as the rays passed through increasing thick- 

 nesses of absorbing material. In brief, if the finite width of 

 Schmidt's curves are due to the rays from the radium E having 

 a definite range of velocities, as the faster rays are proportionately 

 less absorbed than the slower rays, for any given thickness of 

 aluminium, the maximum should gradually move out in the 

 direction of the stronger magnetic fields, and the higher velocities, 

 in the absence of any compensating change of velocity in the rays. 



From these considerations Wilson is able to deduce the change 

 in velocity of /3-rays for different thicknesses of aluminium. He 

 calculates thus that the velocity of certain of his rays changed 

 from 2-78 x lO^" cms. per sec. to 2-69 x 10^" cms. per sec. in passing 

 through 0-489 mm. of aluminium. 



It must not be forgotten, however, that the experiments of 



Schmidt, whichever way we regard them, do conclusively prove 



one important fact, namely that the /S-rays from a single radio- 



p,-coive siiuaw.ce are absorbed in such a way that the distribution 



* Pn.Roy. Soc. A, Vol. lxxxii. 1909, p. 612. 



