62 



KNOWLEDGE. 



[March 2, 1896. 



glitter was visible whichever side of the painted paper 

 was next the tube —that with the coating of birium 

 platino-cyauide upon it or the other — and the appearance 

 was produced when the paper was held several feet from 

 the tube. In a similar way tluorescence was caused in 

 many other substances, such as uranium, glass, roci<salt, 

 etc. On interposing a thick book with a thousand pages 

 between the discharge tube and the fluorescent screen, the 

 fluorescence was still generated, the discharge or the 

 effects it gave rise to being evidently able to pierce not 

 only the blackened sheet next the glass, but many folds of 

 paper. 



On repeating these and similar experiments, using 

 instead of a fluorescing screen a sensitive photographic 

 film or dry plate, Prof. Eimtgen got impressions or shadow 

 pictures formed on the sensitive surface. Thus photographs, 

 if we may call them such, could be taken while the 

 sensitive plate was kept inside a wooden box, and the 

 operations could be performed in daylight, the plate never 

 being exposed to light, but preserved throughout in a 

 wooden or cardboard box. It also became necessary to 

 guard the plate from the effect of the discharge when it 

 was not in use, shutting it up in a box being no protection. 



Further researches showed that pine wood was easily 

 penetrable by the rays. They may be called nu/s. for their 

 effects on sensitive films are similar to those of the ultra- 

 violet actinic rays, though whether these effects are due to 

 the direct action of the rays, or to secondary phenomena 

 resulting from the production of fluorescence and chemical 

 change at the plates surface, is not yet settled. Not only 

 are paper, wood, leather, and suchlike materials transparent 

 to these rays proceeding from a vacuum tube in which a 

 discharge is going on, but metals and almost all substances 

 are so to a certain degree. The amount of obstruction 

 placed in the way of the rays by different sorts of material 

 varies very much. Amongst metals, aluminium is com- 

 paratively transparent, while lead is in comparison very 

 opaque. Ebonite and india-rubber allow easy passage to 

 the rays, while some varieties of glass obstruct them much — 

 lead glass especially ; though from the fact that the tubes 

 used for the discharge are of glass, it is evident that the 

 rays are by no means cut off' by that material. 



From the relative opaqueness of lead, gold, and other 

 heavy metals, the investigator was led to compare the 

 obstructive effects of substances as related to their density ; 

 and whilst it was found that the density or quantity of 

 matter contained in unit volume played the principal idle 

 in causing stoppage of the rays, the amount of this 

 obstruction or opacity was by no means proportional to 

 the product of thickness and density. 



Eontgen next went on to try if these rays were refracted 

 or bent out of their course, as ordinary light rays are, on 

 passing from a rare to a dense medium. The result 

 obtained was striking ; no appreciable refraction was 

 obtained when prisms of glass or ebonite were used. 

 When light was passed through the same prism consider- 

 able deviation in the beam was obtained, the comparison 

 showing a remarkable difference in behaviour of light rays 

 and the new rays. Similarly the rays cannot be concen- 

 trated by means of a lens. Experiments made to test 

 reflection gave not quite decided results. Crosses or 

 stars cut out of metal and placed above a sensitive photo- 

 graphic plate indicated their position by a dark trace or 

 shadow. This apparently pointed to reflection at the 

 surface of the metal, but Prof. Eontgen inclines to the 

 belief that what takes place when the rays pass through a 

 soUd or liquid is more like the passage of light through 

 a cloudy medium consisting of discrete particles. 



The rays seem to traverse a medium separate from 



matter and contained between the molecules of all varieties 

 of matter. Substances such as glass when powdered 

 obstruct the passage of light, as is well known, owing to 

 reflection at the manifold surfaces of the particles arranged 

 at all angles. A powdered body placed in the path of 

 Uontgen's rays does not obstruct tliein more than an 

 ordinary transparent solid. This last experiment seems 

 to indicate the absence of reflection. 



What, then, can these rays be which thus behave so 

 differently from light rays ? It has been suggested that 

 they are waves of 

 light of very short 

 wave-length far 

 beyond the 

 known ultra-vio- 

 let rays. If the 

 wave-length is 

 very minute and 

 comparable with 

 the magnitude of 

 a material mole- 

 cule's diameter, 

 we might expect 

 quite different 

 effects from those 

 of ordinary re- 

 fraction. Theab- 

 sence of refrac- 

 tion in Eontgen 's 

 new rays — the .c 

 rays as he calls 

 them for short- 

 ness — seems to 

 show that the 

 velocity of these 

 rays is the same 

 in all kinds of 

 matter. 



Experiment clearly shows that the vibrations in the 

 ether to which the phenomena of light are due are 

 transverse to the direction of propagation of the waves. "^^ 

 Now there may also be longitudinal vibrations, and 

 some physicists consider that there must be such. It 

 becomes a most interesting question : — Are these new rays 

 manifestations of the possible longitudinal vibrations in the 

 ether'' On the answer which may be given to this question 

 the principal scientific interest in Eontgen's discovery turns. 

 The discoverer himself is inclined to answer the question 

 in the affirmative. Sir G. G. Stokes, our greatest 

 authority on light, considers that it is probable the 

 jihenomena may be explained without bringing in longi- 

 tudinal waves in the ether. In the present open state of 

 the question it is interesting to remember the remarks 

 made by Lord Kelvin in his course of lectures at Baltimore 

 in 1884. He said, referring to longitudinal or condensa- 

 tional waves in the ether : " We ignore this condensational 

 wave in the theory of light. We are sure that its energy 

 is very small in comparison with the energy of the 

 luminiferous vibrations we are dealing with. But to say 

 that it is absolutely null would be an assumption that we 

 have no right to make." Later in the same address he 

 went on to say : " But that there are such waves I believe ; 

 and I believe that the velocity of propagation of electro- 

 static force is the unknown condensational velocity that 

 we are speaking of." 



The velocity of propagation of such longitudinal waves 

 is shown by mathematical calculation to be enormously 



• See article on ''The Luminiferous Ether" in Knowiedgb, 1894. 



Ruutgeu Raj' Sliadow-pioture of a Liviug 



Human Foot. Exposure 8 minutes, 



tliroufili ordinary cardboard. 



