Mr Crowther, On the Transmission of ^-rays. 445 



of velocities in the beam remains the same whatever the thickness 

 of absorbing material traversed. 



Under these circumstances it was felt that some more direct 

 experimental evidence on the point was desirable. 



The method employed consisted, briefly, in forming a beam of 

 homogeneous /S-rays by means of the magnetic deflection method 

 described above, and measuring the velocity of the rays so obtained 

 by means of a second similar system of screens. Sheets of different 

 absorbing materials could be placed in the path of the rays be- 

 tween the two systems, and the velocity of the ra.ys before and 

 after passing through the absorbing medium could be directly 

 determined. 



As has been already explained, the magnetic deflection method 

 only gives a truly homogeneous beam of rays when the apertures 

 are infinitely small. On the other hand only a limited quantity 

 of /3-rays at the best can be radiated by any radio-active substance 

 through a given area. The dimensions of the apparatus used, 

 therefore, represent a compromise between these opposing factors. 



A further point has also to be considered, namely the possible 

 scattering of the rays during their passage through the air. 

 Recent experiments* show that if the path is at all long, the 

 scattering of the rays by the air may be very considerable. It is 

 possible that this effect may not seriously affect the purity of 

 the rays transmitted, but in cases where it is desired to work 

 with a nearly parallel pencil of rays (as in experiments to be 

 described later) it is certainly of very considerable importance. It 

 was decided, therefore, that the whole operation of deflecting the 

 rays and forming them into a pencil should take place in vacuo. 



The form of apparatus finally decided upon is shown in section 

 in fig. 2. The portion EFGH is placed between the poles of an 

 electromagnet, so that the edges of the pole pieces lie along EF, 

 and FQ. The tube DA projects from the magnetic field and is 

 carefully screened by a thick block of soft iron, so as to cut off as 

 far as possible any strong field from this part of the path. 

 Windows of thin aluminium foil (-002 cm.) close the apertures at 

 A and B. 



The radius of curvature of the path of the rays is 4 cms. and 

 each of the apertures A, D, G, B is 0-5 cm. in diameter. The 

 distance AD is 35 cms. and the depth of the box at right angles 

 to the plane of the paper is 1-5 cms. The different apertures are 

 bevelled, and the screens and sides of the box coated with 

 aluminium foil to avoid as far as possible any secondary radiation. 

 The apparatus could be exhausted by means of a water pump to 

 a pressure of about 12 mm. of mercury, which was found to be 



* J. A. Crowther, Proo, Camb. Phil. Soc, YoJ. xv. 1909, p. 273. 

 VOL. XV. PT. V. 29 



