X-RA V DETECTOR. 36 



field of the fluorescent screen. If the two on the disk are in 

 coincidence with the stationary rod, it indicates that the \'iew of 

 the screen inside has been completely shut off. There is also a 

 rod one centimeter long on the disk in that position which cor- 

 responds to the half-moon effect referred to in the last para- 

 graph. 



The device for changing the view of the screen of the instru- 

 ment is for the purpose of giving the experimenter a means of 

 determining whether the screen, which is supposed to be under 

 the influence of X-rays, is really fluorescing or not. For, 

 although the screen may appear to be luminous, the effect of 

 vision may be only an optical delusion. 



The contrivance showai in fig. lo is used to test observations 

 as follows : If there seems to be a fluorescence of the screen, 

 the experimenter can attempt to set the cross-bar, J, fig. lo, 

 over the center of the luminous field. 



Then, afterwards, the real position of the bar can be deter- 

 mined with absolute certainty by means of the indicating disk 

 on the outside of the instrument. If the luminous effect was 

 caused by light coming from the screen, and the setting of the 

 cross-bar made correctly, a coincidence must be found between 

 a certain one of the rods on the disk and the stationary rod on 

 the casing of the instrument, as previously explained. The 

 relative position of these rods is determined by the sense of 

 touch with the forefinger of the right hand. When the right 

 coincidence is found to exist, the conclusion must be that the 

 screen is fluorescing. 



Thus we have a checking device on the observations of the 

 fluorescence, which is perfectly free from any personal equation. 



As it is often desirable to work with Rontgen rays completely 

 in the dark, Professor Rood suggested the use of the small 

 pointed rods on the brass indicating disk instead of marks, in 

 order that the investigator might remain in darkness during 

 experiments and determine the position of the revolving check- 

 ing device (shown in figure lo) by the sense of touch. 



The means of eliminating the personal equations just de- 

 scribed, is important, because a conscientious observer may be 



