X-RA V DETECTOR. 33 



some other strongly fluorescent substance. These crystals are 

 uniformly distributed over the area of a circle 5.5 centimeters in 

 diameter. 



The black paper screen is held securely in place by a cap, IJ, 

 which is circular in form, and which fastens over the ring on the 

 screen holder, C, in such a manner that it is impossible for light 

 to enter the instrument from that portion. The construction of 

 these parts is such, however, that the cap, D, can be very easily 

 removed and the screen taken out and another substituted in its 

 place. 



A device which is used by the X-ray Detector for purposes 

 which will be presently explained is shown in fig. 10. Its position 

 in the apparatus can be seen in figs. 8 and 9. In these figures 

 j5" is a disk of brass 7.5 cms. in diameter and .15 cms. in thick- 

 ness, fitted on a small shaft .5 cms. in diameter, that passes 

 through the casing of the instrument from one side to the other, 

 6 centimeters back of the screen holder C, but below a line join- 

 ing the aperture in C and the eye -protector B (Fig. 8) ; so that 

 a view of the screen from the eye end is not crossed by the 

 shaft. On this shaft, but inside the instrument, is fastened a metal 

 shield, as shown by L, in fig. 10, which measures approximately 

 6.0 X 6.0 centimeters, and extends radially outward from the 

 shaft. 



Diametrically opposite to the shield L, two metal posts sup- 

 port a cross-bar y, fig. 10, 6.0 cms. long, and 1.6 cms. in thick- 

 ness, parallel to the shaft, and such a distance from it that when 

 the shaft is turned to one position, this bar will cross the center 

 of the field of the fluorescent screen. 



The entire device can be made to revolve by turning a milled - 

 head E' , fig. 10, attached to the brass disk E, and is so con- 

 structed, that if the shaft be turned to certain positions by 

 means of this milled-head, the view of the screen, as seen from 

 the eye-protector, may be partly or entirely shut off by the 

 metal shield L, fig. 10, or it may be bisected by the cross-bar 

 /, fig. 10. Thus the view of the fluorescent screen may be 

 changed, in the different positions of the shaft, from a full to a 

 half-moon effect, to perfect occupation, and to the field of a cir- 

 cular screen crossed by a bar. 



