On the Source of the Rontgen Rays in Focus Tubes. 433 



By means of a special camera he has now been able to make further 

 investigations. The camera is illustrated in fig. 1, where A is the 

 pinhole in a removable lead disc secured by a brass cap to the brass 

 cone B, which is lined with thick lead so as to be opaque to the 

 Bontgen rays. D is a framework into which slides either the 

 fluorescent screen E, or a carrier containing a sensitive plate should 

 photographs be required. F is an observation tube for use with the 

 fluorescent screen. It is made of insulating material to avoid danger 

 of shocks. 



FIG. 1. 



IOI254S67891O 



i-i-i f 4 i i ;- > f i I -H 



cms. 



With this apparatus directed at the anti-cathode of a focus tube, 

 it is easy with the fluorescent screen in place to take accurate note 

 of the image of the active anti-cathode area which appears on the 

 screen, and to observe the variations in form, dimensions, and 

 brilliancy that take place under varying conditions. Similarly by 

 replacing the fluorescent screen by a photographic plate in a black 

 paper envelope, the Rontgen ray image can be photographed. 

 Exposures, varying from one to thirty minutes, according to condi- 

 tions, are found sufficient to impress upon the plate any effect that 

 can be seen directly with the screen. It has not, however, been 

 found possible, even with very prolonged exposures, to photograph 

 anything not directly visible with the screen, and having regard to 

 the difficulties of maintaining the vacuum and other conditions con- 

 stant for any considerable length of time, the method of direct 

 observation seems generally to be best and most convenient. For 

 direct observation, rather a large pin-hole, say about 2 mm. in 

 diameter, gives the best results ; for photography about half this 

 diameter is preferable, as it gives sharper images. 



The writer has made numerous observations and photographs with 

 this apparatus, both with focus tubes of the ordinary pattern, and 

 also with a special tube in which both the cathode and also the anti- 

 cathode (which in addition acted as anode) were independently 

 adjustable along the axis of the tube, so that the distance between 

 them could be varied from a minimum of 4 to a maximum of 

 14 cm. This special tube is illustrated in fig. 2, and during the 



