Febeuaey 14, 1896.] 



SCIENCE. 



229 



point to a refractive index 1.05. No devia- 

 tion can be observed by means of tlie fluo- 

 rescent screen. Investigations witli the 

 heavier metals have not as yet led to any. 

 result, because of their small transparency 

 and the consequent enfeebling of the trans- 

 mitted rays. 



On account of the importance of the 

 question it is desirable to try in other ways 

 whether the X-rays are susceptible of re- 

 fraction. Finely-powdered bodies allow 

 in thick layers but little of the incident 

 light to pass through, in consequence of re- 

 fraction and reflection. In the case of the 

 X-rays, however, such layers of powder are 

 for equal masses of substance equally trans- 

 parent with the coherent solid itself. Hence 

 we cannot conclude anjr regular reflection 

 or refraction of the X-rays. The research 

 was conducted by the aid of finely-powdered 

 rock salt, fine electrolytic silver powder, 

 and zinc dust, already many times em- 

 ployed in chemical work. In all these 

 cases the result, whether by the fluorescent 

 screen or the photographic method, indi- 

 cated no difference in transparency between 

 the powder and the coherent solid. 



It is, hence, obvious that lenses cannot be 

 looked upon as capable of concentrating the 

 X-rays; in effect, both an. ebonite and a 

 glass lens of large size prove to be without 

 action. The shadow photograph of a round 

 rod is darker in the middle than at the 

 edge ; the image of a cylinder filled with a 

 body more transparent than its walls ex- 

 hibits the middle brighter than the edge. 



8. The preceding experiments, and 

 others which I pass over, point to the rays 

 being incapable of regular reflection. It is, 

 however, well to detail an observation which 

 at first sight seemed to lead to an opposite 

 conclusion. 



I exposed a plate, protected by a black 

 paper sheath, to the X-rays, so that the 

 glass side lay next to the vacuum tube. 

 The sensitive film was partly covered with 



star-shaped pieces of platinum, lead, zinc 

 and aluminium. On the developed negative 

 the star-shaped impression showed dark 

 under platinum, lead, and, more markedly, 

 under zinc ; the aluminium gave no image. 

 It seems, therefore, that these three metals 

 can reflect the X-rays ; as, however, another 

 explanation is possible, I repeated the ex- 

 periment with this only difference, that a 

 film of thin aluminium foil was interposed 

 between the sensitive film and the metal 

 stars. Such an aluminium plate is opaque 

 to ultra-violet rays, but transparent to X- 

 rays. In the result the images appeared as 

 before, this pointing still to the existence of 

 refiection at metal surfaces. 



If one considers this observation in con- 

 nection with others, namely, on the trans- 

 parency of powders, and on the state of the 

 surface not being efffective in altering the 

 passage of the X-rays through a body, it 

 leads to the probable conclusion that regu- 

 lar reflection does not exist, but that bodies 

 behave to the X-rays as turbid media to 

 light. 



Since I have obtained no evidence of re- 

 fraction at the surface of different media, it 

 seems probable that the X-rays move with 

 the same velocity in all bodies, and in a 

 medium which penetrates everything, and 

 in which the molecules of bodies are em- 

 bedded. The molecules obstruct the 

 X-rays the more eff'ectively as the density 

 of the body concerned is greater. 



9. It seemed possible that the geometri- 

 cal arrangement of the molecules might 

 affect the action of a body upon the X-rays, 

 so that, for example, Iceland spar might ex- 

 hibit different phenomena according to the 

 relation of the surface of the plate to the 

 axis of the crystal. Experiments with 

 quartz and Iceland spar on this point lead 

 to a negative result. 



10. It is known that Lenard in his inves- 

 tigations on cathode rays has shown that 

 they belong to the ether and can pass 



