THE X-RAYS. 155 



walls emit X rays, tliougb in less intensity. Cathode rays proceed in 

 all directions, but their intensity is considerable only near the axis of 

 the concave cathode mirror, and, consequently, the X-rays are strongly 

 emitted only near the point where this axis meets the platinum plate. 

 When the tube is very hard and the platinum thin, many rays ijroceed 

 also from the rear surface of the platinum plate, but, as may be shown 

 by the pinhole camera, chiefly from the spot lying on the axis of the 

 mirror. * * * 



I can confirm the observation of G. Braiides that the X-rays are able 

 to produce a sensation of light upon the retina of the eye. In my record 

 book appears a notice entered in the early part of Xovember, 1895, to 

 the effect that when in a darkened chamber near a wooden door I per- 

 ceived a weak appearance of light when a Hittorf tube upon the other 

 side of the door was i)ut in operation. Since this appearance was only 

 once observed, I regarded it as a subjective, and the reason that it was 

 not then repeatedly observed lay in the fact that other tubes were sub- 

 stituted for the Hittorf tube which were less completely evacuated 

 and not i)rovided with platinum anodes. The Hittorf tube furnishes 

 rays of slight absorbability on account of its higli vacuum, and, at the 

 same time, of great intensity on account of the employment of a platinum 

 anode for the reception of the cathode rays. * * * 



With the tubes now in use I can easily repeat the Braudes experi- 

 ment. * * * 



Since the beginning of my investigation of X-rays I have repeatedly 

 endeavored to produce diffraction phenomena with them. I obtained 

 at various times, when using narrow slits, appearances similar to 

 diffraction effects, but when modifications were made in the conditions 

 for the purpose of thoroughly proving the accuracy of this explanation 

 of the phenomena it was found in each case that the appearances were 

 produced in other ways than by diffraction. I know of no experiment 

 which gives satisfactory evidence of the existence of diffraction with 

 the X-rays. 



WiJRZBURG, Physik. Institut d. Univ., March 10, 1897. 



