JULV 30, 1896] 



NA TURE 



303 



potassium platinocyanide ; though this screen is opaque to 

 ordinary light, you will see that it phosphoresces when placed 

 in the neighbourhood of the bulb. This phosphorescence 

 is due to something radiating from the bulb, because when 

 I place this piece of metal between the bulb and the 

 screen, a sharp shadow of the metal is thrown on the 

 screen. The metal is opaque to these radiations. If, how- 

 ever, I |ilacc a piece of wood, about an inch thick, between 

 the bulb and the screen, you will hardly be able to see a shadow ; 

 so that this board, though opaque to ordinary light, allows those 

 rays to pass through with considerable freedom. The lighter 

 the substance the more easily is it penetrated by these rays ; thus 

 the very light metal aluminium is very transparent, as you will 

 see by the poor shadow it casts upon the screen. This property 

 has lieen used to tletect real gems from paste, as the diamond, 

 ■consisting of the light element carbon, is much more transparent 

 than an artificial one made of heavy silicates. Since light 

 ■objects are, roughly speaking, transparent, while the heavy ones 

 are opaque, if we have a mixture of hea\y and light 

 objects between the screen and the bulb, the heavy ones 

 will throw a shadow, the lighter ones will not. We can thus 

 <ietect dense objects even when surrounded by opaque 

 ones, provided the latter are light. [Experiments throwing 

 shadows of jewellery in cases, hands, &c. , upon the screen.] 

 Prof. Lodge has in this way been able to see through a yard of 

 timber. We seem here to have the realisatiim of Sam Weller's 

 aspiration after an optical arrangement which would enable one 

 to see through " a flight of stairs and a deal door." 



I will now endeavour to show that in order to have Rontgen 

 rays you must have kathode rays to start with. I will produce 

 in the bulb, which I have used for the production of the Rontgen 

 rays, a discharge of another kind, viz. an electrodeless discharge 

 in which the discharge, instead of travelling between metallic 

 terminals in the gas, travels round a closed circuit in the gas. 

 In this way we have no kathode and no kathode rays ; you see 

 that though a bright discharge passes through the bulb, far 

 brighter than in the previous case, no luminosity is produced on 

 the screen. 



One very remarkable property discovered by Rontgen of 

 ■fhese rays, is that they are not bent when they pass from one 

 medium to another. We can .show this in the following way. I 

 place in front of the bulb this thick plate of metal, in which a 

 vertical slit has been cut ; the metal stops the rays, so that we get 

 on the screen a bright luminous vertical band. Now I place 

 between the slit and the screen this wooden prism, which covers 

 up the lower, but not the upper, half of the .slit ; if the rays 

 which came through the slit were refracted, then the lower part 

 of the bright band would no longer be in the same .straight line 

 as the upper part. You see, however, that the two halves still 

 remain on the same line ; the only effect produced by the wooden 

 prism has been to make the lower half somewhat dimmer than 

 it was before. 



Again, these rays are not deflected by a magnet ; to prove this, 

 we throw the shadow of two brass tabes on the .screen, and 

 observe the shadows before and after a horse-shoe magnet has 

 been introduced into the tubes ; you see that no appreciable effect 

 is produced by the introduction of the magnet. 



The absence of refraction leads us to expect that there would 

 be little regular reflection of the Rontgen rays, and this con- 

 clusion has been confirmed by numerous experiments. At 

 grazing insistence, however, Joly of Duljlin has been able to 

 detect a .small amount of regular reflection. Though there is 

 but little regular reflection there is an appreciable amount of 

 what, to avoid any speculation as to its nature, has been called 

 by Sir George Stokes "diffuse return" of the rays; this was 

 discovered by Rontgen himself, and is rendered very evident by 

 an experiment of Lord Blythswood We do not know yet, how- 

 ever, whether the rays coming off from the metal plate are of the 

 same kind as those which fall upon it, or whether they are 

 slightly different. If they are of the same kind, then the effect 

 would re.semble the diffu.se reflection from a piece of ground 

 gla.ss ; if they are different, it would indicate that the piece of 

 metal illuminated by the.se rays became itself a source of rays 

 not quite of the same kind as tho.se which fall upon it, just as 

 when a solution of quinine is exposed to the invisible ultra- 

 violet light it emits not ultra-violet light like that which fell 

 ■upon it, but visible blue light. This point might be settled by 

 measurements of the rates of absorption of the incident and 

 ■" diffusely returned" light. 



That the Rontgen rays are not all of the .same kind, has been 



NO. 1396, VOL. 54] 



shown in several ways, of which, however, I have only time to 

 mention one. Mr. McClelland, working in the Cavendish 

 Laboratory, found that if he took a plate of tinfoil and a layer 

 of water, and adjusted the thicknesses so that they exerted the 

 same absorption on the Rontgen rays given out from one bulb, 

 they did not necessarily produce the same absorption in the rays 

 from another bulb, showing that the rays from the one bulb 

 were not the same as those from the other. 



Rontgen discovered that the rays not only made certain 

 substances phosphorescent, but that they affected a photographic 

 plate : so that if we replaced the phosphorescent screen in our 

 experiment by a photographic plate, we should get a permanent 

 impression of the picture, which would be thrown on a phos- 

 phorescent screen placed in the position of the photographic 

 plate. To obtain these photographs all that is necessary is to 

 protect a photographic plate from ordinary light by thick card- 

 board or aluminium, and place the object to be photographed 

 between the bulb and the plate ; after an exposure varying with 

 the nature of the object and the state of the bulb, photographs 

 of the kind which are now so well known can be obtained. 



One very marked feature of these photographs is the sharp- 

 ness of the detail ; this shows that the origin of the rays must 

 be confined to a comparatively small region. If these rays came 

 from an area comparable with that occupied by the phosphor- 

 escence on the walls of the bulb used to produce the rays, the 

 luminosity from one part of the screen would throw one pattern 

 on the screen, while the rays from another portion would throw 

 another pattern ; the superposition of these patterns would 

 produce a blurred image. To illustrate this point, I have here 

 two photographs of the same thing — one taken by the Rontgen 

 rays, the other taken by an incandescent lamp with walls of 

 frosted glass, of about the same size as the bulb used to produce 

 the Rontgen rays, and placed in the same position ; you see that 

 the photograph taken by the Rontgen rays is quite sharp, while 

 that taken by the electric lamp is much blurred. This shows 

 that the Rontgen rays do not come from an area nearly so 

 extended as the phosphorescent part of the glass. We can 

 investigate the place of origin of these rays in various ways, by 

 observing the law of. diminution with the distance of the effects 

 due to these rays, by taking pin-hole photographs, by observing 

 the direction of the shadows cast by a series of opaque bodies ; 

 the result of such observations shows that Rontgen rays are pro- 

 duced when the kathode rays strike against a solid obstacle. 

 Cases have been observed by Lord Blythswood and by Rowland, 

 which seem to show that this is not the only source of these 

 rays. 



The experiments made on these ray have not led to any 

 result absolutely decisive as to their nature, but we can profit- 

 ably discuss the question whether the facts known about these 

 rays oblige us to regard them as due to a new form of energy, 

 or whether they are consistent with these rays being a variety of 

 some form of energy already known to us ; before calling in a 

 new form, we ought to be quite sure that it is neces.sary to 

 abandon the old. The rectilinear propagation of these rays, 

 their powers of producing phosphorescence and of affecting a 

 photographic plate, their insensibility to a magnet, suggest that 

 of the old forms of energy light is the one to which these rays 

 are most closely allied. We are acquainted with so many 

 varieties of light (by light I mean transverse vibrations propagated 

 with a definite velocity) with such widely different properties, 

 that we can well contemplate the existence of other kinds with 

 still different properties. We know, for example, the ultra- 

 violet light of very small wave-length, the .subject of classical 

 researches by Sir George Stokes, which, though it affects a 

 photographic plate, does not affect the retina, and passes through 

 bodies with such difficulty that the most ultra-violet kind is 

 quenched after passing through a few millimetres of air ; then 

 we have the visible light able to affect the retina, and alile to pass 

 through great lengths of some substances which are opaque to 

 the uUra-violet rays though stopped by very small thicknesses of 

 others ; then we have the longer waves of radiant heat given out 

 by a hot body below the temperature at which it becomes 

 luminou.s. These are not visible, have but little effect on a photo- 

 graphic plate, and are able to traverse substances opaque to 

 both ultra-violet and visible light. Then we have the waves 

 emitted by vibrating electrical systems, which neither affect the 

 retina nor a photographic plate, which, as Mr. Rutherford has 

 .shown, are able to traverse the walls of the houses and the 

 bodies of the inhabitants of about three-quarters of a mile of a 

 densely populated part of Cambridge, and which are so different 



