0. N. Rood — Reflection of the Rontgen Rays. 177 



but hazy. The reflection experiment with X-rays above de- 

 scribed was now repeated with its aid, the exposure as before 

 being fifty hours. The result obtained was in general the 

 same as before, with the important difference that the vertical 

 lines were decidedly more blurred, and the sharp horizontal 

 lines produced by the speculum metal rarely made their appear- 

 ance. If flat metallic surfaces reflect only diffused X-rays, it 

 is impossible to explain the marked differences exhibited by 

 the negatives obtained from these two mirrors, which differed 

 only in perfection of surface. It may be remarked in passing, 

 that although no special attempt was made to measure the 

 relative amount of the X-rays reflected by the two metals, yet 

 the negative due to the platinum appeared slightly more dense, 

 indicating a somewhat superior reflecting power. 



Experiment with a cylindrical mirror. 



While it would seem nearly impossible to explain the results 

 above given, except on the assumption that X-rays reflected 

 from flat polished metallic surfaces consist of regularly reflected 

 rays mingled with others that have undergone diffuse reflec- 

 tion, yet it was decided to try another experiment that would 

 be even more conclusive, and at the same time furnish at least 

 a rough measure of the relative amounts of the two sets of 

 rays. An ordinary pear-shaped Crookes tube was used, the 

 hemispherical end of the tube opposite the cathode being the 

 source of the X-rays. Its diameter was 90"™, the fluorescent 

 portion having a diameter of about 83°^"^ ; hence the source of 

 the X-rays was a rather large surface. The rays fell at an 

 angle of 45° on a rectangular concave cylindrical mirror, its 

 dimensions being 102"°^' by 170'"°^. The distance from the 

 tube to the center of the mirror was 152'''"' ; from the center 

 of the mirror to that of the plate-holder it was 165"""^. The 

 surface of the cylindrical mirror was made of lead-foil faced 

 with a thin coating of tin ; the polish was brilliant* The 

 specific gravity of the foil was 10*87. This foil was cemented 

 on a sheet of mica, which was then bent so as to form a con- 

 cave cylindrical mirror, the radius of its central portion being 

 255"'™ ; those of the external portions were somewhat greater. 

 The mirror was placed so that the axis of the cylinder was 

 vertical. Owing to the arrangement and the size of source of 

 illumination, the beam of ordinary light reflected from the 

 mirror to the plate-holder was somewhat divergent, but less so 

 than would have been the case with a flat mirror. The lead- 

 foil mirror of course had certain deformations, generally verti- 



*This a commercial article in the United States, and is known simply as 

 " tin foil." 



