594 Mr. Alan A. Campbell Swinton [Feb. 4, 



shaking the tube its distance from the cathode can be varied. In 

 this case the nearer the anti-cathode is placed to the cathode the higher 

 is the resistance of the tube and the more penetrative are the Eontgen 

 rays that are generated. 



Fig. 15 shows another form of adjustable tube in which the anti- 

 cathode is stationary, and it is the cathode that is movable. The 

 cathode is here so mounted upon a sliding stem that it can be moved 

 in and out of a slightly conical annex blown upon one side of the 

 glass bulb of the tulie. I will put a tube of this descri2)tion into 

 operation, beginning with the cathode in the position shown in the 

 illustration in dotted lines, when it is outside the annex in the bulb, 

 and let you see tlie effect of gradually moving it backwards into the 

 annex. We will observe the character of the resulting Eoutgen rays 

 produced at each position with a fluorescent screen. The tube UFcd 

 has a small piece of iron attached to the cathode so that we can move 

 the latter by means of a magnet according to the suggestion of 

 Dr. Dawson Turner and others. 



You observe tliat, to commence with, with the cathode right out in 

 the bulb, we get E5ntgen rays which can do little more than pene- 

 trate the black paper backing of tlie screen. My hand throws a dark 

 shadow on the fluorescent surface, but you can see no bones, as the 

 rays will not penetrate my hand. I now move the cathode a little 

 back towards the edge of the annex. The bones are now just visible. 

 The hand is still very black, but the bones can be seen ; now on 

 moving the cathode just inside the edge of the annex the bones 

 become very clear, and when I move it still furtlier into the annex the 

 rays become very penetrative, and even pass through the bones so that 

 their structure can be observed. 



Figs. 16, 17 and 18 show a series of three photographs of my 

 hand obtained in this manner. I'hey were all taken with the same 

 tube under identical conditions as regards vacuum, distance, exjwsure, 

 photographic plate and development. The position of the cathode 

 only was altered, and, as will be observed, the results show a marked 

 increase of penetration the further the catliode was moved towards 

 and into the glass annex. In the case of Fig. 16 the cathode was 

 right out in the bulb, in Fig. 18 it was completely in the annex. In 

 Fig. 17 it was in an intermediate position. 



Now we have studied the cause of these effects by means of a tube 

 in which positions of both anode and cathode can be altered inde- 

 pendently by a magnetic adjustment. Fig. 19 shows a portion of 

 the tube, and above it is drawn a curve representing approximately 

 the difference of potential required to cause a discharge to pass 

 through the tube with varying positions of the anti-cathode. In the 

 diagram the abscisssB represent the distance i)etween anti-cathode 

 (which also formed the anode) and the cathode, divided in tenths of 

 an inch, while the ordinates represent also in tentlis of an inch the 

 length of the alternative sparks in air between two brass balls j inch 

 in diameter. Starting with the anti-cathode in its furthest position 



