150 



THE X-RAYS. 



a body the ratio of a brightness which a fluorescent screen held behind 

 the body at right angles to the rays bears to its brightness in the absencs 

 of the interposed body, but under conditions otherwise identical. 

 Eeferring the trausmissibility to unit thickness we obtain what may 

 be called the specific trausmissibility. This will be the dth. root of the 

 trausmissibility where d is the thickness of the transmitting layer 

 measured along the direction of the rays. In order to determine the 

 transmissibility I have since my first communication made use princi- 

 pally of the photometer described above. The two parts of the fluores- 

 cent screen having been brought to equal brightness, the plate of the 

 substance to be investigated, as for instance, aluminum, tin, glass, etc., 

 was interposed before one of the tubes, and the distance of one or 

 other of the discharge tubes was altered so that the screen became 

 again uniformly illuminated. The ratio between the squares of the dis- 

 tances of the platinum plates from the screen before and after the inter- 

 position of the body under investigation gives the value of the 

 transmissibility sought. Hy interposing a second plate its transmissi- 

 bility may be found for rays which have already passed through one. 

 plate of the same kind. 



In this procedure it is assumed that the brightness of a fluorescent 

 screen is inversely proportional to the square of its distance from the 

 source of the rays, and this can only be the case on condition, first, that 

 the air absorbs or emits no X-rays, and, second, that the brightness of 

 the fluorescent light is proportional to the intensity of the radiation 

 falling upon it. The first condition is certainly not fulfilled, and it is 

 questionable whether the second is or not. I have therefore first satis- 

 fied myself, as already set forth in section 10 of my first communication, 

 that the deviations from strict proportionality are so slight as to be 

 negligible for the purposes of experiment in the case at hand. Again, 

 with reference to the fact that X-rays are secondarily radiated from 

 bodies under their influence, it may be remarked, first, that no differ- 

 ence was to be detected with the photometer between the transmissi- 

 bility of a single plate of aluminum 0.925 millimeter thick, and of 31 

 sui:)erposed plates each of 0.0299 millimeter thickness, giving a total 

 thickness of 0,927 millimeter; and, second, that the brightness of the 

 fluorescent screen was not appreciably different whether the plate was 

 placed close u}) to the screen or at a considerable distance from it. 



The results of these experiments on transmissibility are for aluminum 

 as follows : 



Transmissibility for rays falling vertically. 



The first, 1 millimeter thick, aluminum plate 



The second, 1 millimeter thick, aluminum plate. . 

 The first, 2 millimeters thick, aluminum plate . . . 

 The second, 2 millimeters thick, aluminum plate. 



0,40 

 .55 



Tube- 



.30 

 .39 



0.39 

 .54 



0.68 

 .73 

 .50 

 .63 



