36 TROWBRIDGE. 



led by the imagination to believe that a fluoroscopic screen is 

 fluorescing when it is really not. There are certain phenomena 

 pertaining to human sight and recognized in physiological 

 optics, giving the effect of a vision of dim grayish lights which 

 occur when the eyes are closed or in the dark. Such effects, 

 together with after-images, might often be a cause of deception 

 when an instrument is used which is unprovided with a means 

 for verifying observations of faint luminosity. 



These phenomena are sometimes so vivid that they may 

 readily cause a person to believe that a pale light is coming to 

 the eyes from without, when the effect is really subjective and 

 in the eyes of the individual who is experimenting. 



The name *' X-ray Detector " was considered suitable for the 

 instrument and was adopted, because the apparatus was designed 

 particularly for the study of the fluorescence which is caused 

 by Rontgen rays, and for the reason that it is possible to de- 

 termine with it whether a screen of fluorescent material, which is 

 supposed to be under the influence of the X-rays, is actually 

 giving forth a perceptible amount of light, or whether the effect 

 which is apparently observed is due only to an optical delusion. 



For most experiments of a scientific nature, a decided con- 

 trast between a luminous screen and the dark boundary sur- 

 rounding it is always desirable. A good contrast is obtained by 

 the employment of a comparatively small screen, such as is used 

 in " X-ray Detector." This is the case for two reasons : In the 

 first place, that portion of a Crookes tube which is the main 

 source of Rontgen rays is generally quite small, seldom being 

 of greater extent than a few square centimeters. 



If, therefore, a large screen is placed close to the source of 

 these rays, it will exhibit uneven fluorescence, one part showing 

 intense luminosity while the other portions appear faded out. 

 Such an effect tends to give little contrast between the fluoresc- 

 ing screen and the dark border surrounding it. On the other 

 hand, a small screen is fairly evenly fluoresced, its entire border 

 has equal definition, and all the contrast which is possible is 

 obtained. 



Secondly : When a flat surface is observed at a short distance 



