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



cal, but they were of no consequence in the experiments that 

 followed. In a perfectly darkened room the ordinary whitish 

 green light from the end of the tube was reflected from the 

 cylindrical mirror towards the plate-holder faced with white 

 paper, and the shadows cast by vertical and horizontal wires 

 were studied. This beam of light had certain characteristics 

 impressed on it by the act of regular or specular reflection 

 from the cylindrical mirror, and it was found that the shadows 

 of the vertical wires of a netting held at a distance of a few 

 niillimeters from the plate-holder were blurred and doubled, 

 while the shadows of the horizontal wires remained perfectly 

 sharp and single. In order to study these shadows with a 

 stronger illumination, the Crookes tube was removed, and its 

 luminous end replaced by a circular transparent screen 83™"' 

 in diameter, illuminated from behind with a gas-flame, but no 

 new facts were developed. 



The apparatus being properly arranged, the tube and mirror 

 were cemented fast to their supports, and the plate-holder was 

 provided with cemented guides, so that after removal it could 

 be returned to its original position. In the first experiment 

 the sensitive plate was shielded only with the draw-slide that 

 had proved impervious to an exposure of seven hours of sun- 

 light ; on it was fastened a coarse wire netting, the average 

 distance between the wires being 12'3"'™ ; the thickness of the 

 wire was 1-34°'°'. This plate was exposed for forty-six hours 

 to the reflected X-rays, and furnished a fine negative. Lest it 

 should be urged that it was partly due to ordinary ultra violet 

 light, a second plate was exposed for ninety hours, the strips 

 of aluminium foil and aluminium plate above mentioned being 

 used. These were fastened on the plate-holder diagonally, so 

 as to interfere as little as possible with the image. There was 

 no difference between the 46-hour and the 90-hour negatives, 

 except in the matter of density ; the foil left no trace of its 

 presence, but the portions under the aluminium plate were a 

 little less strong. 



As was expected, the vertical lines in both negatives were 

 broadened and blurred, their diffused image in some cases 

 extending half-way across the open squares, while on the other 

 hand the horizontal lines were sharply defined and scarcely 

 broader than the wires that produced them. The mirror was 

 now covered up with chalked paper neatly fitted to its surface, 

 and a negative taken with an exposure of one minute, the 

 whitish green light from the end of the Crookes tube being 

 employed as the source of illumination. Its appearance was 

 totally different from that obtained by the use of the X-rays, 

 the shadows of both horizontal and vertical wires having a 

 tolerably uniform breadth of about 6"""". This negative, taken 



