1898.] on Some New Studies in Cathode and Bontgen Radiations. 597 



from a very small area or point. 

 The sharpness of definitioii 

 varies considerably with differeut 

 tubes, and a ready means of 

 judging as to their quality in 

 this respect is very useful. I 

 have here a very pretty arrange- 

 ment for this purpose which is 

 the idea of Mr. Mackenzie David- 

 son. It consists simply of a 

 square wooden frame over which 

 are stretched at equal distances 

 a number of parallel wires. There 

 are two sets of wires crossing 

 one another at riglit angles. By 

 holding this screen near the tube 

 and examining the shadows cast 

 by the wires upon a fluorescent 

 screen at different distances, it is 

 easy to see whether the definition 

 of the tube is good or b.id. Here 

 are three Eontgen photographs 

 of the wires, all taken at the 

 same distance but with different 

 tubes. As will bo observed, they 

 vary very considerably as regards 

 distinctness, showing that the 

 tubes were very unequal in 

 defanitlon. 



Fig. 22 shows a photograph 

 of the wires taken almost in the 

 plane of the anti-cathode, the 

 shadow of which is visible at 

 the right of the picture. As 

 will be observed, the shadows 

 of the wires jiarallel to the 

 plane of the anti-cathode be- 

 come less and less distinct the 

 iurther they are from the Litter, 

 while the wires that are at right 

 angles to the anti-cathode plane 

 are exceedingly indistinct. This 

 is of course due to the Eontgen 

 ra3's being given off' from a spot 

 of considerable area in the par- 

 ticular tube with which this 

 photograph was taken, and to 

 the projection of the active area 



Vol. XV. (No. 92.) 



Fig. 21. 

 Tube with adjustable glass shield. 



2 R 



