September 3, 1896] 



NA TURE 



429 



I may say, the kathoclic rays too much in his head, and 

 attributed the whole effect on either side of the wall to the 

 kathodic rays. Really the effect is due in part to the kathodic 

 rays, and in part to the R<intgen rays, the existence of which 

 he was not aware of. They cannot be distinguished merely by 

 iheir effect on a fluorescent screen or on a photographic plate, 

 since both these recipients are affected by the rays of both 

 kinds. 



Such was the state of things when Riintgen made his re- 

 markable discovery. According to an account which I saw in 

 one of the newspapers (w-e cannot vouch for the truth of every- 

 thing we see in the newspapers), the iliscovery was made in the 

 first instance accidentally. I cannot give you more authentic 

 information than that, but he had been working with a Crookes 

 tube and he observed that a photographic plate, enclosed in the 

 usual case in which these plates are enclosed when you want to 

 protect them from light, showed on development certain 

 markings an it ; so he put the whole apparatus as it had been, 

 with a photographic plate in its case in the same position as 

 before, and the thing was repeated. That is according to 

 the account in the newspapers. A very remarkable discovery 

 was the result. He found that rays were capable of coming out 

 of some part of a Crookes tube which had the remarkable 

 property of passing through substances that are opaque to 

 ordinary light, and opaque even to the ultra-violet with which 

 we were previously acquainted. They pass freely through 

 black paper, through cork, wood, or even through the flesh of 

 the hand, though less freely through the bones, so that by simply 

 laying his hand upon the case containing the photographic 

 plate, he actually got a photograph of the bones cf his hand. 



Well, what is the nature of these rays and from whence do 

 they come ? As Riintgen said in his original paper, a slight 

 examination .shows that they have their origin in the part of the 

 Crookes tube opposite to the kathode, and which is rendered 

 phosphorescent by the discharge from the kathode. 



The rays, however, which come from this part of the tube, 

 and which appear to have their origm there, differ utterly in 

 some respects from the so-called kathodic rays. If you isolate 

 a portion of them, you find that a magnet has no action upon 

 them ; unlike the kathodic rays, they proceed onwards without 

 deflection, just as if the magnet were not there. Like light 

 they proceed in a straight course, but these rays are able to pass 

 through a variety of substances that are opaque to ordinary 

 light, while on the other hand they are stopped by other 

 substances which let light freely through. That, however, does 

 not prove that they are not of the nature of light. You may 

 have, suppose, a red glass which is opaque to green rays, but lets 

 red rays through very freely, so that as regards merely the fact 

 of the X rays being stopped by substances transparent to light, 

 while they pass more or less freely through other substances 

 which are quite opaque to ordinary light, that establishes no 

 greater distinction than exists between green and red light. Are 

 they then of the same nature as light ? 



The X rays have some very remarkable properties by which 

 they appear at first sight to differ in tolo from ordinary light. 

 They ])ass with either no refraction, or excessively small 

 refraction, through prism-shaped bodies, which we know rays of 

 light do not. They suffer hardly any, if any, regular reflection, 

 unless perhaps at a grazing incidence. 



Rbnigen himself, in his original paper, dwelt on these pecu- 

 liarities of the new rays. lie formed a prism of aluminium, 

 with which he attempted to obtain deviation of the new rays, 

 but the experiment .showed that if there were any deviation 

 at all, at any rate the refractive index could not exceed i 05. 

 lie speaks of the rays not Ijcing apparently capable of regular 

 reflection, but he brought forward experiments which show 

 that in a certain sense they appear to be capable of reflection. 



A photographic plate with the .sensitive surface downwards 

 was placed in its case under a Crookes tube, and immediately 

 under the plate, and inside the case, were placed portions of 

 different kinds of metal, which would be capable of reflecting 

 back the rayi on to the sensitive surface, if they admitted of 

 reflection ; and it was found that the plate was much more 

 darkened over certain of those metals than where the metal did 

 not exist. There was very little darkening over aluminium, and 

 a great deal of darkening comparatively over platinum. This 

 indicated that some effect was produced, though the greater 

 part of it is not one of regular reflection. He conceived the 

 eftect to be one of reflection such as you might have from a 

 turbid medium. 



NO. 1 40 I, VOL. 54] 



There is, however, another mode of explanalion which .seems 

 worth considering, viz. that the Rontgen rays, falling upon 

 the metal, throw the molecules into a state of vibration, which 

 they communicate to the ether, by a sort of phosphorescence 

 or fluorescence of X light ; so that the rays which come from 

 the molecules, though perhaps not of exactly the same nature as 

 the X rays that fell upon them, still have enough of the "X" 

 quality about them, whatever that is, to enable them to get 

 through objects which are op.aque to ordinary light. 



Lord BIythswood, who has worked a great deal with the 

 Rontgen rays, has written a paper, which was communicated to 

 the Royal Society by Lord Kelvin, in which he establishes a 

 minute regular reflection of those rays from speculum metal at 

 an angle of about 45°. Two plane specula were placed side by 

 side so as to receive at that angle the X rays coming from a 

 Crookes tube, and a duly protected photographic plate was 

 placed in such a position as to receive the regularly reflected 

 rays if there should be any. The developed plate appeared to 

 show a slight indication of the junction between the mirrors ; 

 and that the appearance was not illusory was shown by Lord 

 Kelvin, who made measurements on the im.age and compared 

 the results with what they ought to be on the supposition of a 

 regular reflection. The indication was so faint that I could not 

 myself perceive it (I have not seen the negative, but only 

 positive copies), but Lord BIythswood has given me some 

 positive copies of a negative which he subsequently obtained 

 by reflection from a concave speculum at a small angle of 

 incidence, and which show for certain a minute regular 

 reflection of X rays, while at the same time they prove that 

 the quantity of X light returned by regular reflection is 

 extremely small compared with that which comes from the 

 mirror by some different process. 



Now there is another remarkable property of these rays, or 

 absence of property, if you like so to call it. Rays of light, 

 as we know, admit of diftraction. If you pass light from a 

 luminous point through a very small slit, or a small hole, the 

 riband, or the beam of light at the other side, does not follow 

 merely the geometrical projection of the slit or hole as seen from 

 the source of light, but is more or less widened, and certain 

 alternations of illumination are visible, a phenomenon referable 

 to interferences which I have not time to go into. How do 

 these X rays behave under such conditions ? It is a very 

 remarkable thing that they do not show these enlargements or 

 exhibit any sign of interference. 



The last number of the Coiiiptes reiidus contains a paper 

 by M. Gouy in continuation of a former paper, but describing 

 experiments carried out in a still more elaborate manner, 

 which proves the truth of this to a very high degree of strict- 

 ness. He makes out that if these X rays are periodical, the 

 wave-length cannot well be more than the one-hundredth part 

 of the wave-length of green light, indicating an enormously high 

 degree of frequency. 



Now, if we assume that the X rays, like rays of light, 

 and unlike the kathodic rays, are a disturbance propagated in 

 the ether, ponderable matter being concerned only in their 

 origination, not in their propagation, the question arises. 

 What is the relation between the direction of vibration and 

 the direction of propagation ? Are the vibrations normal or 

 transversal? We know that the vibrations of the air which 

 constitute sound take place in a to and fro direction, or are 

 what is called normal — that is, perpendicular to the waves of 

 sound. We have the fullest evidence that the vibrations of the 

 ether which constitute light take place in directions perpen- 

 dicular to that of propagation, or are what is called transversal. 

 To which category do the vibrations belong which constitute 

 the X rays ? 



If we could obtain polarisation, or even partial polarisation, 

 of the X rays, that would settle the question, and prove that 

 they are due to transversal vibrations. But most of those 

 who have attempted to obtain indications of their polarisation 

 have failed. This, however, does not prove that the vibrations 

 are normal, for the peculiar properties of the X rays shut us 

 out — or, at least, almost completely shut us out — from the 

 ordinary means of obtaining polarisation. There is, however, 

 one paper in the Coiuptes rciidus, by Prince Galitzine and M. 

 de Karnojitsky, in which the authors profess to have obtained 

 by a special method undoubted indications of polarisation. 

 No reasonable doubt can remain as to the abstract capacity of 

 these rays for polarisation after what has been done by another 

 physicist. I wish I had time to go into the experiments that 



