38 X-RAY DETECTOR. 



appears to be luminous is measured. The ratio of the squares 

 of the two distances will show the relative fluorescent values of 

 the substances, at least from an optical standpoint. In this test, 

 the checking device which is shown in fig. lo is used to deter- 

 mine the correct distances. The screen holder, screen and 

 screen cover are drawn in detail in fig. 1 1 . 



The " X-ray Detector" is provided with a base, //, fig. 8, 

 which is an advantage, because it is desirable that experiments 

 should be conducted under steady conditions. This stand has 

 been made very firm, and has a device by which motion in three 

 planes is possible, when observations are required with the '' X- 

 ray Detector" in different positions with respect to the source of 

 the Rontgen rays. The instrument is also so constructed that 

 it can be easily freed from the base and held by a handle {G, 

 fig. 8) in the hand of the experimenter. 



The particular apparatus, described and shown in the accom- 

 panying illustrations, was made by J. Grunow, instrument 

 maker, New York, N. Y., from a model constructed by the 

 writer. 



