May 26, 1898] 



NA TURE 



91 



kathode are fixed, but in which there is a movable conical 

 glass shield which can be brought up from behind the kathode 

 so as to impede the access of the atoms which, as we have seen, 

 come in round the edges of the kathode, to any desired extent. 

 This tube regulates just as did the adjustable kathode tube. 



In order to produce sharply-defined Rontgen photographs, it 

 is of course of the utmost importance that the rays should be 

 given off from a very small area. The sharpness of definition 

 varies considerably with different tubes, and a ready means of 

 judging as to their quality in this respect is very useful. 



The best and most accurate method is by means of pin-hole 

 photography. Seeing that the Ronlgen rays are not refracted, 

 photography with a lens is, of course, out of the question ; 

 but with a pin-hole, very accurate and distinct images can be 

 obtained. It is only necessary to place a sheet of lead, pierced 

 by a pin-hole, near the tube, and then to examine the rays 

 coming through the hole with a fluorescent screen, placed some 

 way behind the lead sheet, in order to see exactly the size and 

 shape of the active area of the anti-kathode ; or, instead of the 

 screen, a photographic plate may be employed and the effect 

 recorded. Fig. 4 shows three pin-hole photographs of the 

 anti-kathode taken in this way, giving the effect produced with 

 three different distances between the kathode and anti-kathode. 

 The largest figure is produced with the greatest distance, and 

 ■vice versa. It will be observed that, owing to the anti-kathode 

 being placed obliquely to the kathode, the figures are all oblique, 

 though somewhat imperfect, conic sections ; further, that when 

 the distance between kathode and anti-kathode is great, we 

 have a section of the divergent cone giving a hollow ring with 

 a central spot. The ring gets smaller and smaller, and finally 



the most ultra-violet wavesi hitherto known that they pass 

 between the molecules of matter, and are consequently neither 

 refracted nor easily absorbed or reflected by any media. 

 Lastly, there is the theory, first suggested to the writer early in 

 1896 by Prof. George Forbes, and recently independently 

 enunciated and elaborated by Sir George Stokes, which imagines 

 them to be frequently but irregularly repeated, isolated, and 

 independent disturbances or pulses of the ether, each pulse 

 being similar, perhaps, to a single wave of light, and consisting 

 of a single transverse wave or ripple, but the pulses following 

 one another in no regular order, or at any regular frequency, as 

 do the trains of vibration of ordinary light. 



Then, again, there is the question of the mechanism by means 

 of which the Rontgen rays are produced. They are generated 

 by the impact of the kathode stream upon the anti-kathode, and 

 it is now becoming more and more certain that the kathode 

 stream consists of negatively charged atoms travelling at 

 enormous velocity. If we accept this view, there are obviously 

 several methods by which we may imagine the Rontgen rays 

 being generated by the impact of the travelling atoms upon the 

 anti-kathode. Each kathode-ray atom carries a negative charge, 

 while the anti-kathode is positively charged, so that when the 

 two come into contact an electrical discharge will take place 

 between them. An electrical oscillation will thus take place in 

 the atom just as in the brass balls of a Hertz oscillator, and 

 transverse electromagnetic waves will be propagated through 

 the ether in all available directions. As the electrostatic 

 capacity of the atom must be exceedingly small, the periodicity 

 of oscillation and the wave frequently will be enormous, while 

 at the same time the oscillation will probably die out with 



I 



disappears as the distance between the electrodes is reduced, 

 and the focus approaches the anti-kathode. It will also be 

 noticed that where in the ring portion of the figures the kathode 

 rays strike most normally — that is to say, at one of the two 

 points of greatest curvature of each ellipse — the Rontgen rays 

 are produced more actively than in the remaining portion 

 where the kathode rays impinge on the anti-kathode more on 

 the slant. 



By some it is imagined that because the Rontgen rays are so 

 very penetrating, therefore they are of the nature of an in- 

 visible light of great intensity, which, though not affecting the 

 human retina, acts upon photographic plates very powerfully. 

 This is quite erroneous, and, as a matter of fact, the photographic 

 effect of Rontgen rays is relatively very feeble. The author 

 has investigated this by exposing two photographic plates, re- 

 spectively, to a very powerfully excited Rontgen-ray tube, 

 screened by black paper to remove the visible luminosity, and 

 to the light of a single standard candle By adjusting the dis- 

 tances and exposures so as to obtain a precisely equal effect in 

 both cases, he has found that the photographic power of the 

 particular Rontgen-ray tube investigated was about one-sixtieth 

 of one standard candle. 



With regard to the true nature of the Rontgen rays, there 

 have been many theories. There is the original suggestion of 

 Rontgen himself, that they may possibly consist of longitudinal 

 svaves in the ether. Others have thought that they were pos- 

 sibly ether streams or vortices. There is a theory that they 

 consist of moving material particles similar to the kathode 

 rays. There is the more generally received doctrine, that they 

 are simply exceedingly short transverse ether waves, similar in 

 all respects to the waves of light, only so much shorter than 



NO. 1 49 1, VOL. 58] 



sufficient rapidity to admit of only one or two complete 

 periods. At the same time the greater the difference of poten- 

 tial between atom and anti-kathode at the moment of impact 

 the greater will be the amplitude of oscillation, and the more 

 vigorous and far-reaching the etheric disturbances. 

 ! Or we may imagine a more purely mechanical origin for the 

 Rontgen rays. It is believed that the velocity of the kathode 

 rays is enormous, being, as recently measured by J. J. Thomson, 

 over 10,000 kilometres per second ; and though Lodge, in his 

 well known endeavours to detect a movement of the ether by 

 dragging a material body through it obtained only negative 

 results, of course he could not possibly obtain any velocity at 

 all comparable to this. Assuming that at the velocity of the 

 kathode-ray atoms these do appreciably drag the ether with 

 them, there may be some other effect produced, analogous to the 

 atmospheric effect that is noted as the crack of a whip or a 

 clap of the hands, as each atom hits the anti-kathode and 

 rebounds. 



Since this paper was written, the author's attention has been 

 called to Prof. J. J. Thomson's suggestion in the Philosophical 

 Magazine for February, that the Rontgen rays consist of ver>^ 

 thin and intense electromagnetic pulses produced in the ether 

 by the sudden stoppage by the anti-kathode of the electrified 

 particles of the kathode stream. 



Or, again, it is conceivable that the phenomenon is merely one of 

 heating, and that the kathode stream atoms are, by impact with the 

 anti-cathode, raised to such an enormous temperature, that they 

 give ofT for a short space of time super-ultra-violet light. Taking 

 a velocity for the atoms of io» centimetres per second, as found by 

 J. J. Thomson to be the minimum velocity of the kathode stream, 

 and calculating the temperature to which a nitrogen atom would 



