JANUAEY 31, 1896.] 



SCIENCE. 



161 



museums, but since they cannot, the person 

 who has formed a private collection can 

 miost successfully manage one for the use of 

 the public, since he better than anyone else 

 is able, in considering the needs of tl^e 

 museum visitor, to keep in mind that say- 

 ing which is so useful a guide in museum 

 practice — ' Put yourself in his place.' 



G. Beown Goode. 



THE X-RAYS. 



Helmholtz, Hertz and Kundt, the three 

 greatest physicists of modern Germany, 

 have died within two years, and the friends 

 of German science feared that this loss 

 would be followed by a standstill in physics, 

 or at least by a lack of really important 

 ■discoveries. But now we have Professor 

 W. Rontgen's investigations in the physical 

 laboratory of the University in Wiirzburg, 

 the importance of which does not stand be- 

 hind the famous electrical discoveries of 

 Hertz in Bonn. Eontgen has found a new 

 kind of rays — he calls them the X-rays — 

 which, though invisible to the eye, affect 

 the photographic plate ; which produce 

 fluorescent phenomena ; which pass through 

 wood, metal and the human body ; which 

 are neither broken by prism and lenses nor 

 reflected. 



The chief facts about the X-rays are the 

 following : It is well known that the dis- 

 charges of a large Euhmkorff induction coil 

 produce in a vacuum tube, such as Crookes' 

 or Hittorf's, colored rays which go in 

 straight lines from the cathode to the glass 

 of the tube. These cathode rays, which 

 have been much studied, are visible to the 

 eye and are well characterized by the 

 fact that the magnet changes their direc- 

 tion ; they do not pass thick cardboard, 

 wood, etc. The place where these cathode 

 rays reach the glass of the tube is the 

 centre of Eontgen's X-rays. They are 

 not visible and are not turned aside 

 by a magnet ; in short, they are not 



cathode rays, but are produced by them. 

 If in a dark room we cover the tube 

 by thin, black cardboard, nothing can be 

 seen at all, even if we bring the eye in 

 the direct neighborhood of the tube during 

 the electric discharges. But if we now bring 

 a card covered with barium platinocyanide 

 near it the paper flashes up with every dis- 

 charge, and this fluorescent effect is visible 

 even if the paper is distant 2 meters from 

 the tube, and it does not matter whether 

 the varnished or the other side of the paper 

 is directed towards the tube. The X-rays 

 thus go through the black cardboard which 

 is opaque to sunlight, and the same effect 

 follows when a bound volume of a thousand 

 printed pages is put between the tube and 

 the fluorescent paper. We can measure 

 the perviousness of the different substances 

 to the new rays by the intensity of the light 

 on the paper, comparing the effect with and 

 without objects between the tube and the 

 fluorescent surface. But there is also an 

 objective way possible to study the pervious- 

 ness, as the raj'S produce an effect upon 

 photographic dry plates, which, of course, 

 remains and allows us to control the sub- 

 jective comparisons. Both methods show 

 that wood is not much less pervious than 

 paper; boards 3 cm. thick absorb very little. 

 Hard rubber disks several centimeters thick 

 do not stop the rays, and even aluminium 

 plates 15 mm. thick do not make the fluor- 

 escence entirely disappear. Glass plates 

 vary with the lead in them, those contain- 

 ing lead being less pervious. Platinum is 

 slightly pervious, if the plate is not thicker 

 than 0.2 mm., silver and copper can be 

 a little thicker; lead plates 1.5 mm. thick 

 are no longer pervious. All substances 

 become less pervious with increasing thick- 

 ness, a fact which is nicely demonstrated 

 by photographs taken through tinfoils of 

 gradually increasing number. The pei'vious- 

 ness of substances of equal thickness seems 

 chiefly dependent on the density, but 



