474 A. M. Mayer — Researches on the Rontgen Rays. 



paper ; but as lie does not give any photometric measures, or 

 estimates, of the intensity of the fluorescence in the geometric 

 image of the slit, and outside of this image, one cannot form an 

 opinion of the amount of the diffusion he describes. The facts 

 of diffusion described by Professor Pupin are opposed to those 

 discovered by so distinguished and experienced a physicist as 

 Rontgen, who, in section 10 of his first paper, writes : "I find, 

 using a Weber's photometer, that the intensity of the fluorescent 

 light varies nearly as the inverse squares of the distance between 

 screen and discharge tube. This result is obtained from three 

 very consistent sets of observations at distances of 100 and 

 200 mm . Hence air absorbs the X-rays much less than the 

 cathode rays. This result is in complete agreement with the 

 previously described result, that the fluorescence of the screen 

 can be observed at 2 metres distance from the vacuum tube." 

 Rontgen does not mention any diffusion observed by him in 

 these experiments, and it is certain that he would have men- 

 tioned the existence of diffusion in experiments made to deter- 

 mine a law of radiation, and which diffusion necessarily would 

 have invalidated the law of inverse squares. 



The radiation from the Crookes tube, used in the experi- 

 ments described in this paper, came from a calcined shell sup- 

 ported by the anode wire and placed opposite the cathode. 

 The tube was not sensibly heated during the experiments and 

 the actinic power of its radiation remained constant. It was 

 proved by taking a pin-hole photograph of the naked tube, 

 with an exposure of two hours, that by far the larger propor- 

 tion of the X-raJrs emanated from a 4 mm square surface of the 

 shell which was acted on by the cathode rays. The walls of 

 the glass tube, as was proved by other independent photo- 

 graphic experiments, furnished a small percentage of X-rays 

 but not enough to make their presence known in the pin-hole 

 photograph, which had furnished quite a dense image of a por- 

 tion of the shell. Furthermore the X-rays from the glass wall 

 itself were cut off and prevented from reaching the photo- 

 graphic plate by a diaphragm of lead with a circular opening 

 of one-half inch, and at one inch distant from the radiant 

 source. It is also to be remarked that this second source of 

 the X-rays was nearer to the photographic plate only by t 7 q- inch 

 minus -06 inch, the thickness of the tube, hence any effects due 

 to them may be neglected, as indeed the results in the experi- 

 ments on the inverse squares show. 



The experiments described in this paper were made in the 

 private laboratory of Professor Rood in Columbia University. 

 Professor Rood not only gave me the use of his apparatus, 

 which he had made the subject of a special investigation before 

 investigating with it, but he also gave me the advantage of the 

 experience obtained during his researches on the X-rays. 



