542 



SCIENCE. 



[N. S. Vol. in. No. 67. 



screen (about 6 inches by 4 inclies) was 

 uniformly illuminated. There Avas evi- 

 dently a diffuse scattering of the X-rays in 

 their passage through the air. This infer- 

 ence was confirmed by other experiments 

 which will be discussed presently. Various 

 well-known devices were employed to con- 

 centrate the cathode rays along the axis of 

 the tube. So, for instance, wrapping tin- 

 foU around the tube. This, however, did 

 not diminish the gradual diffusion of the 

 image of the slit on the fluorescent screen 

 when the distance between the slit and the 

 fiuoroscope was gradually increased. Up 

 to about three inches from the slit the real 

 image of the slit could still be distinguished 

 easily from the diffuse background as a 

 band of maximum intensity. 



b. The platinum mirror was now placed 

 quite near the slit and at a convenient angle 

 to the direction of the ray, and the fiuoro- 

 scope was placed quite near the mirror. 

 There was a faint illumination of the fiuor- 

 escent screen, but it was perfectly uniform. 

 Not the slightest indication of an image of 

 the slit could be detected, although the dis- 

 tance between the slit and the mirror plus 

 the distance between the mirror and the 

 fluorescent screen was less than the dis- 

 tance at which the image of the slit on the 

 fluorescent screen appeared as a band of 

 maximum intensity when observed directly. 

 A change in the angle of the mirror pro- 

 duced but a small change in the fluorescence 

 of the screen, and then the change seemed to 

 be such as to approach a maximum when 

 the mirror and the fluorescent screen were 

 parallel to each other. The same experi- 

 ment was repeated with other metals and 

 with the same result. This experiment, 

 therefore, does not speak in favor of regu- 

 lar reflection. 



c. Turning the mirror completely around, 

 so that the face of the wooden block on 

 which the metal plate was fastened served 

 as a mirror it was found that the fluorescent 



efl'ect upon the screen was stronger than 

 with the platinum. A pad of paper of 

 about the same size as the wooden block 

 acted more strongly than the platinum or 

 any other metal. Various substances were 

 tried, like glass, vulcanite, the hand, va- 

 rious metals, and they all produced a diffuse 

 reflection of varying intensity, and at all 

 angles of inclination. In all cases the 

 maximum effect seemed to take place when 

 the broadest side of the reflecting object 

 was about parallel to the fluorescent screen. 

 But the fluorescence was verj' weak as long 

 as the slit was narrow. 



d. The slit was now made wider, and 

 the same series of experiments were re- 

 peated with various widths of the slit. The 

 fluorescence of the exploring screen in- 

 creases, of course, with the width of the 

 slit. The observations made with the nar- 

 row slit were conflrmed. In every case the 

 maximum intensity on the exploring screen 

 was obtained when the broadest side of the 

 reflecting object was about parallel to the 

 screen. Wood and transparent insulators 

 produced a stronger effect than metals. No 

 accurate quantitative comparisons have yet 

 been made. Among the insulators experi- 

 mented with, wood produced the strongest 

 effect, and among the metals aluminium 

 is the weakest for the same thickness of 

 the plate. The thickness of the reflecting 

 plate increases the effect; this increase will 

 go on until the reflecting plate is several 

 inches thick if this plate is an insulator. 

 In the case of metals, however, like sheets 

 of iron or copper, the change in the fluores- 

 cent effect due to the diffusely reflected ra- 

 diance ceases as soon as the reflecting plate 

 becomes thick enough to be practically 

 opaque to the direct ray. 



e. The human body when in the path of 

 the X-rays will act as a reflector. It is 

 quite an easy matter to detect a person 

 walking across the room in the vicinity of 

 the slit, for as soon as a person crosses the 



