224 



Mr. A. A. C. Swinton. 



trative value of the X rays could be increased or decreased as desired 

 within the limits of the focus on the one hand and the travel of the 

 anti-cathode on the other. 



Again, I have found that the penetrative value of the X rays can 

 be altered by employing cathodes of different diameters. 



Having noticed that focus tubes with small cathodes give X rays 

 of any given penetrative value at a much lower exhaustion than do 

 exactly similar tubes having cathodes of larger diameter, the tube, 

 fig. 2, was constructed, in which there are two cathodes, C and C, 



A 



both focussing upon opposite sides of the same platinum anti-cathode. 

 B. The two cathodes were of dissimilar diameter, C being 0*375 in. 

 diameter, and C 1125 in. diameter. Both had the same radius of 

 curvature, namely, 0*75 in. 



On exhausting this tube with the smaller cathode, C, in use, and 

 the other not connected, a degree of vacuum was soon reached when 

 X rays began to be produced plentifully. Pumping was then sus- 

 pended, and the negative connection changed from the smaller 

 cathode, C, to the larger, C. Under these conditions, no X rays could 

 be obtained, the vacuum being evidently much too low for the larger 

 cathode. On continuing the exhaustion to the point when, with the 

 larger cathode in operation, X rays were produced, but of a low 

 penetrative value, the negative connection was removed from the 

 larger cathode, and the smaller cathode put into use. Immediately 

 it was found that the penetrative value of the X rays had increased 

 to an extent that could only be obtained with the larger cathode with 

 a very much higher vacuum. 



Throughout the experiments with this tube it was further found 



