X-RAV DETECTOR. 



37 



from the eyes (20—30 centimeters), only a small portion of it is 

 distinctly seen at one time. Thus it follows, that a small screen 

 will be more clear than one that is large, because it lies more 

 nearly within the area of distinct vision, and its border has better 

 definition than that of the large screen. 







C M 





/ 



/ 



/ 



CAP OFF CAP ON 



Fig. II. Device for holding fluorescent screen in place, showing the cap off and 



the cap on. C — Screen holder. H — Black paper screen. O — Fluorescent 

 crystals. D — Cap. P — Screws for securely fastening the cap. 



By the construction of the screen holder of the " X-ray De- 

 tector," C, fig. 8, so that screens can be easily changed, a means 

 is obtained by which comparative tests and examinations of dif- 

 ferent fluorescent substances can be made. One method is as 

 follows : If it is desired that two fluorescent materials be com- 

 pared ; screens of these substances are prepared, and then the 

 farthest distance from an active Crookes tube at which each screen 



