148 THE X-RAYS. 



of place. First of all it should be noted that the determinations are 

 rendered much more difficult because of the unsteadiness of the sources 

 of radiation. The tubes are sensitive to each irregularity of the inter- 

 ruption of the primary current; and such frequently occur with the 

 Dupret and especially with the Foucault form of interrupter. Many 

 repetitions of each measurement are therefore necessary. Second, I 

 may indicate upon what factors the brightness of a given fluorescent 

 screen depends which is acted upon by a shower of X-rays so rapid 

 that the observer's eye can not detect the intermittent character of 

 the illumination. This brightness depends (1) upon the intensity of 

 the radiation proceeding from the platinum plate of the discharge 

 tube; (2) very probably upon the nature of the rays falling on the 

 screen, for, as will be shown, different kinds of radiation are not 

 equally active in exciting fluorescence; (3) upon the distance of the 

 screen from the source of the rays; (4) upon the absorption which the 

 rays experience in their journey to the fluorescent screen; (5) ujion 

 the number of discharges per second; (G) upon the duration of each 

 separate discharge; (7) upon the duration and the strength of the after 

 light of the barium-ijlatinum cyanide, and (8) upon the radiation to the 

 screen from the surrounding bodies. In order to avoid errors, one 

 must bear in mind that he has to do with a matter which is as if he 

 were conducting experiments to compare by the aid of fluorescence two 

 intermittent light sources of ditterent colors and was obliged to carry 

 on his experiments within an absorbing shell and in a turbid or fluor- 

 escing medium. 



3. According to section 12 of my first communication, the source of 

 the X-rays is the place where the cathode rays meet the discharge 

 tube and the X-rays radiate outward in all directions. It is now of 

 interest to investigate how the intensity of the rays varies with the 

 direction. 



For this investigation the spherical form of discharge apparatus with 

 a well-i)olished platinum plate inclined at an angle of 45 degrees to 

 the direction of the cathode rays is best adapted. Without other aid, 

 there appears evidence in the uniformly bright fluorescence which is 

 visible in the hemispherical glass wall above the platinum plate that 

 there are no great differences in the intensity of the illumination, so 

 that Lambert's law can not hold. However, this fluorescence may be 

 largely excited by the cathode rays. 



For the j)urpose of more exact investigation experiments were made 

 with the photometer ujion the intensity of the rays emanating from 

 several tubes in various directions, and I have besides exposed photo- 

 graphic films which were bent in a half circle to a radius of 25 centi- 

 meters about the platinum plate as a center. In both experiments the 

 varying thickness of the glass at ditterent parts of the tube walls 

 entered as a serious disturbing factor, because the X-rays were thus 

 unequally absorbed in the various directions. It was, however, pes 

 sible to make the thickness of the glass approximately uniform by the 

 interposition of thin plates. 



