162 



SCIENCE. 



[N. S. Vol,. III. No. 57. 



special experiments showed that diiferent 

 metals are not equally pervious if the pro- 

 duct of thickness and density is equal; the 

 perviousness of platinum 0.018 mm. thick 

 and a density of 2.15 equals that of lead 

 0.05 mm. thick, density 11.3 and that of 

 tin 0.1 mm. thick, density 7.1, and that of 

 aluminium 3.5 mm. thick and a density of 

 2.6. Aluminium may thus be 200 times 

 thicker than platinum, while its density is 

 one-tenth. 



The fluorescent efiect of the new rays is 

 not confined to barium platinocyanide, but 

 it occurs also on glass, calc-spar, rock-salt, 

 etc. Prisms and lenses do not diifract the 

 rays, nor do prisms of hard rubber or alu- 

 minium. With regard to reflection and 

 diifraction the following experiment is in- 

 teresting. It is well known that pulverized 

 substances do not let pass much light owing 

 to refraction and reflection. Eontgen found 

 with pulverized salt, calc-spar, zinc and 

 other substance that the ray pass through 

 the powder with exactly the same intensitj^ 

 as through the solid substance. Objects 

 with rough surface let it pass exactly like 

 polished ones. The shadow of a round 

 stick is in the middle darker than at the 

 edges; the shadow of a metal tube is in the 

 middle lighter than at the edges. 



With regard to the effect on photographic 

 plates, it must not be forgotten that lenses 

 do not refract the rays and therefore ordi- 

 nary photography is not possible ; the pic- 

 tures of the objects are only shadows. But 

 these shadow-pictures can be taken in the 

 closed wooden box of the camera in a 

 light room, as the sunlight of course does 

 not pass through the wood while the X-rays 

 do. In this way Eontgen took photographs 

 of a set of metal weights in a wooden box 

 and of a thick wire wound as a spiral 

 around a wooden stick ; the wood was 

 pervious, the metal of that thickness not, 

 and' so the shadows of the weights and of 

 the wire are seen in the photograph, those 



of the wood scarcely at all. In the same 

 manner he took the picture of a compass 

 needle in the closed box. The door between 

 two rooms did not hinder the chemical 

 effect. 



With regard to the nature of the X-ray& 

 it seems too early to say anything definite- 

 Eontgen emphazises the fact that they show 

 no refraction and probably therefore move 

 in all substances with equal velocitj^ and 

 are transmitted by a medium which exists 

 evei-ywhere and in which are the molecules 

 of the substances. That is they are ether 

 rays, but not transverse ether waves like 

 the visible or the ultra red or ultra violet 

 invisible light ; Eontgen supposes that they 

 are longitudinal ether waves, the existence 

 of which has for a long time been suspected 

 by physicists. Eesearches regarding many 

 other qualities of the new rays are in pro- 

 gress, and their results may clear up the' 

 theoretical interpretation. 



It may be that the practical importance 

 of the discovery is equal to the theoretical. 

 It is well known throughout the world that 

 the physical laboratories of Germany have 

 no windows looking towards the patent 

 ofi&ce. The hunting for practical inventions 

 is not usually important for theoretical 

 science, but the progress of theory usually 

 has practical applications. One practical 

 result in this case is already clear, as the 

 new rays pass boards but not thick metal 

 plates, so they pass the organic substances 

 of the human body, such as skin, muscles, 

 etc., but not the bones. As the metal 

 weights in the wooden box can be photo- 

 graphed, so can photographs of the human 

 bones be taken. Eontgen has put liis hand 

 between the tube and the dry plate in the 

 closed camera; the photograph shows clearly 

 all the bones of the hand without the flesh 

 and skin, and the gold rings seem to hang 

 in the air. The value of such a method 

 for medical diagnosis is clear. Fractures 

 and diseases of bones can be examined by 



