34 



TROWBRIDGE. 



The exact setting of the shield and cross-bar can be deter- 

 mined in the following manner : Outside of the box on the 

 brass disk. E, and near its edge several short rods, 6 cms. long, 

 are placed. These are pointed at the ends and project outward 

 from the face of the disk. They are indicated by F, figs. 8 

 and 10. One rod indicates the position of the cross-bar inside, 



Fig. io. Revolving device used in the " X-Ray Detector." 



and two rods the position of brass shield. Just beyond the 

 edge of the disk E, figs. 8 and 9, and projecting from the cas- 

 ing of the instrument is a rod similar to those on the disk. 

 The pointed rods on the disk E are so arranged, that if one of 

 them coincides with the stationary rod on the casing of the 

 apparatus, it shows that the cross-bar inside is bisecting the 



