﻿418 Sir E. Rutherford and Dr. J. Chadwick on the 



the element and on the velocity of the incident a particle. 

 Under the conditions of the experiment, these H nuclei 

 could only arise from a disruption of the atomic nucleus by 

 the action of the a particles. 



Attention was also drawn to the remarkable fact that 

 in the case of the one element examined, viz. aluminium, 

 the particles were liberated in all directions relative to the 

 incident a particles. 



In the present paper we shall give an account of ex- 

 periments to throw further light on these points and to 

 test whether any evidence of artificial disintegration can 

 be observed in the case of other light elements. 



Magnetic Deflexion of the Particles, 



In the course of this work, the microscope used for the 

 counting of scintillations has been further improved. For 

 the present experiments it was essential, in order to obtain 

 a sufficient number of scintillations per minute, that the area 

 of zinc-sulphide screen under observation should be greatly 

 increased without diminution of the light-gathering power 

 of the microscope system. Following the suggestion of 

 Dr. Hartridge, a modified form of Kellner eyepiece was 

 constructed. A planoconvex lens of about 7 cm. focal 

 length was placed so as to render the rays of light from the 

 objective approximately parallel, and the image so formed 

 was viewed through an eyepiece consisting of a similar lens 

 and an eye-lens of 4 cm. focal length. Used in conjunction 

 with the old objective, Watson's Holoscopic of 16 mm. 

 focal length and *45 numerical aperture, this system gave a 

 field of view of a little more than 6 mm. diameter. A 

 rectangular diaphragm was placed in the eyepiece ? limiting 

 the field of view to an area 6 mm. x 4'9 mm. Our previous 

 system had a field of view of 83 sq. mm. area, so that the 

 new microscope, under similar conditions, gave about three 

 times the number of scintillations of the old. 



The precautions adopted in counting were similar to those 

 described in our previous paper. 



The method of measuring the magnetic deflexion of the 

 long particles was very similar to that described by one * 

 of us in the Bakerian Lecture of 1920. The experimental 

 arrangement is shown in fig. 1. 



The source of a. rays was placed at R and was inclined at 

 an angle of 20° to the horizontal. The lower edge was 

 level with the face of a brass plate S which acted as a slit. 



* Rutherford, he. cit. 



