140 THE X-RAYS. 



refracted portion lay from 10 to 20 centimeters distant from that not 

 refracted. With prisms of hard rubber and aluminum of about 30 

 degrees refracting angle I obtained exposures on a photographic plate 

 which perhaps indicated a slight refraction. This is, however, very- 

 doubtful and the deviation is, if present, so small that the index of 

 refraction for X-rays in these substances can not exceed 1.05. I could 

 not observe with the fluorescent screen any deviation in these cases. 

 Experiments with prisms of the denser metals have so far yielded no 

 certain results on account of the slight transmissibility and the conse- 

 quent decrease of the intensity of the transmitted ray. 



In consideration of these results on the one hand, and on the other 

 of the importance of the question whether or not the X-rays in passing 

 from one medium to another undergo refraction, it is very gratifying 

 that this question may be investigated by other means than by the help 

 of prisms. Finely pulverized bodies in suitable layers allow but little 

 light to pass, in consequence of refraction and reflection. If now the 

 X-rays are transmitted equally well through powder as through the 

 coherent substance, equal masses being presupposed, it is proved that 

 neither refraction nor regular reflection is present in any marked 

 degree. This experiment was jDerformed using finely pulverized rock- 

 salt, finely divided silver, obtained by electrolysis, and the zinc dust 

 so frequently utilized in chemical processes. In no case was any differ- 

 ence in transmissibility between the powder and the coherent sub- 

 stance detected either by the use of the fluorescent screen or the pho- 

 tographic plate. 



It follows of course from the results thus obtained that the X-rays 

 can not be concentrated by the use of lenses; and, indeed, a great hard 

 rubber lens and a glass lens actually proved without effect. The 

 shadow of a round rod is darker in the middle than at the edges, while 

 that of a tube which is filled with some substance more transmissible 

 than the material of which the tube is composed is darker at the edges 

 than at the center. 



8. The question as to the reflection of X-rays is so far settled by the 

 experiments already described that no marked regular reflection was 

 to be found with any of the substances examined. Other experiments 

 which I will here pass over lead to the same results. 



Nevertheless an observation should be mentioned which indicated at 

 first glance an opposite result. A photographic plate shielded from the 

 action of light rays by a black paper was exposed to X-rays so that the 

 glass side was toward the discharge tube. The sensitive film was par- 

 tially covered with bright plates of platinum, lead, zinc, and aluminum, 

 arranged in a star-shaped figure. Upon development it was observed 

 that the darkening of the film under the platinum, the lead, and espe- 

 cially the zinc, was distinctly greater than in the other parts. Xo such 

 effect was produced by the aluminum. Thus it seemed as if the three 

 metals mentioned reflected. However, there were other causes to be 



