﻿Deflexion of Recoil Stream from Radium A. 257 



photographs taken which is not without interest. The 

 a-ray lines show structure, the image being denser at the 

 edges than at the centre. This is clearly brought out when 

 the line is examined under a low-power microscope. From 

 the manner in which the recoil images were obtained, it was 

 not to be expected that they would show any characteristic 

 structure. They were generally diffuse, and even in the 

 case o£ the strongest lines there was not a vast difference 

 between their density and that of the background of the 

 photographic plate, which was always slightly fogged by the 

 /3 and 7 ray radiation from the active deposit. In one case 

 it was noticed that the centre corresponding to the recoil 

 lines was displaced relatively to that corresponding to the 

 a-ray lines, i. e. one recoil line appeared to be nearer its 

 a-ray line than the other. This was apparently due to a 

 difference in the efficiency of recoil from different parts of 

 the circumference of the wire which displaced the position 

 of maximum density within the lines. The ratio of the dis- 

 tance between the a-ray lines to that between the recoil lines 

 is, however, unaltered by this dissymmetry. A second fairly 

 strong line was often found near to the a-ray line obtained 

 after reversing the field. This was due to the a particles 

 from the radium C which had grown on the wire during the 

 experiment. In many cases this radium C line was stronger 

 than the recoil line, and occasionally a faint radium C line 

 was also visible near the initial radium A line. A direct 

 comparison of the velocities of the a particles from radium A 

 and radium C can be made by this method*, and thus the 

 empirical relation between the range and velocity of a, par- 

 ticles given by Geiger can be tested for this case f . 



In measuring the photographic plates, the distances between 

 the centres of the lines were determined. Each photographic 

 plate was mounted in a frame of black paper so as to expose 

 only the essential part of the photograph and mounted in a 

 clamp at a distance of about a metre from a travelling tele- 

 scope of low magnifying power. To illuminate the plate an 

 electric lamp was placed behind a sheet of opal glass and 

 connected in series with an adjustable resistance. It was 

 found that by adjusting the intensity of illumination of the 

 plate, the contrast between the clear portions of the plate 

 and the faint recoil lines could be best brought out at a 

 certain intensity of illumination. Although the recoil 

 lines were considerably broader than the fiducial line in the 

 eyepiece of the telescope, yet when this was set on the imago 

 it was practically impossible to see the recoil line on account 



* Tunstall and Makower, infra, p. 259. 



t Geiger, Proc. Koy. Soc. A. lxxxiii. p. 505 (1910). 



Phil Mag. S. 6. Vol. 29. No. 170. Feb. 1915. S 



