598 



3Ir. Alan A. Camplell Stcinton 



[Feb. 4, 



becoming more and more of a line when viewed nearer and nearer 

 towards tbe plane of the anti -cathode. 



The best and most accurate way of investigating tbe area of the 

 anti-cathode from which the I.'ontgen rays proceed is by means of 

 pin-hole photography. Seeing that the Eontgen rays are not re- 

 fracted, photography with a lens is of cours-e out of the question, but 

 with a pill-hole very fairly accurate and distinct images can be 

 obtained. It is only necessary to place a sheet of lead, pierced by a 

 p n-h()]e, near the tube, and then to examine the rays coming through 

 the hole with a fluorescent screen placed some way behind the lead 



Fig. 22. 



-Runtgen ray pliotogrsiph of a wire screen, taken almost in the 

 plane of the auti-catliode, showing astigmatic effect. 



sheet, in order to see exactly the size and shape of the active area of 

 the anti-cathode ; or instead of the screen a photographic plate may bo 

 employed and the effect re corded. 



Fig. 23 shows four pin-hole photographs of the anti-cathode taken 

 in this way, giving the effect produced with four different distances 

 between the cathode and anti-cathode. The largest figure is jjroduced 

 with the greatest distance, and vire versa. It will be observed that 

 owing to the anti-catln do being placed obliquely to the cathode the 

 figures are all obli([ue, though somewhat imperfect, conic sections ; 

 further, that when the distance between cathode and anti-cathode is 

 great, we have a section of the divergent cone giving a hollow ring 

 with a central spot, just as was visible with the carbon anti-cathode. 

 The ring gets smaller and smaller, and finally disappears as the dis- 

 tance between the electroiles is reduced and the focus api^roaclics the 

 anti-cathode. It will also be noticed tliat where in the ring porti(m 

 of the figures the cathode rays strike most normally, that is to say, 



