January 23, 1896] 



NATURE 



275 



I have not been able to show experimentally that the 

 X-rays give rise to any calorific effects. These, however, 

 may be assumed, for the phenomena of fluorescence show 

 that the X-rays are capable of transformation. It is also 

 certain that all the X-rays falling on a body do not leave 

 it as such. 



The retina of the eye is quite insensitive to these rays : 

 the eye placed close to the apparatus sees nothing. It 

 is clear from the experiments that this is not due to want 

 of permeability on the part of the structures of the eye. 



(7) After my experiments on the transparency of 

 increasing thicknesses of different media, I proceeded to 

 investigate whether the X-rays could be deflected by a 

 prism. Investigations with water and carbon bisulphide 

 in mica prisms of 30^ showed no deviation either on the 

 photographic or the fluorescent plate. For comparison, 

 light rays were allowed to fall on the prism as the 

 apparatus was set up for the experiment. They were 

 deviated 10 mm. and 20 mm. respectively in the case of 

 the two prisms. 



With prisms of ebonite and aluminium, I have obtained 

 images on the photographic plate, which point to a 

 possible deviation. It is, however, uncertain, and at 

 most would point to a refractive index i "05. No deviation 

 can be observed by means of the fluorescent screen. 

 Investigations with the heavier metals have not as yet led 

 to any result, because of their small transparency and the 

 consequent enfeebling of the transmitted rays. 



On account of the importance of the question it is 

 desirable to try in other ways whether the X-rays are 

 susceptible of refraction. Finely powdered bodies allow 

 in thick layers but little of the incident light to pass 

 through, in consequence of refraction and reflection. In 

 the case of the X-rays, however, such layers of powder are 

 for equal masses of substance equally transparent with 

 the coherent solid itself. Hence we cannot conclude any 

 regular reflection or refraction of the X-rays. The 

 research was conducted by the aid of finely-powdered 

 rock-salt, fine electrolytic silver powder, and zinc dust 

 already many times employed in chemical work. In all 

 these cases the result, whether by the fluorescent screen 

 or the photographic method, indicated no difference in 

 transparency between the powder and the coherent solid. 



It is, hence, obvious that lenses cannot be looked upon 

 as capable of concentrating the X-rays ; in effect, both 

 an ebonite and a glass lens of large size prove to be 

 without action. The shadow photograph of a round rod 

 is darker in the middle than at the edge ; the image of 

 a cylinder filled with a body more transparent than its 

 walls exhibits the middle brighter than the edge. 



(8) The preceding experiments, and others which I pass 

 over, point to the rays being incapable of regular reflec- 

 tion. It is, however, well to detail an observation which 

 at first sight seemed to lead to an opposite conclusion. 



I exposed a plate, protected by a black paper sheath, 

 to the X-rays, so that the glass side lay next to the 

 vacuum tube. The sensitive film was partly covered with 

 star-shaped pieces of platinum, lead, zinc, and aluminium. 

 On the developed negative the star-shaped impression 

 showed dark under platinum, lead, and, more markedly, 

 under zinc ; the aluminium gave no image. It seems, 

 therefore, that these three metals can reflect the X-rays" ; 

 as, however, another explanation is possible, I repeated the 

 experiment with this only difference, that a film of thin 

 aluminium foil was interposed between the sensitive film 

 and the metal stars. Such an aluminium plate is opaque 

 to ultra-violet rays, but transparent to X-rays. In the 

 result the images appeared as before, this pointing still 

 to the existence of reflection at metal surfaces. 



If one considers this observation in connection with 

 others, namely, on the transparency of powders, and on 

 the state of the surface not being effective in altering the 

 passage of the X-rays through a body, it leads to the 

 probable* conclusion that regular reflection does not 



NO. 1369, VOL. 53] 



exist, but that bodies behave to the X-rays as turbid 

 media to light. 



Since I have obtained no evidence of refraction at the 

 surface of different media, it seems probable that the 

 X-rays move with the same velocity in all bodies, and 

 in a medium which penetrates everything, and in which 

 the molecules of bodies are embedded. The molecules 

 obstruct the X-rays, the more effectively as the density 

 of the body concerned is greater. 



(9) It seemed possible that the geometrical arrange- 

 ment of the molecules might affect the action of a body 

 upon the X-rays, so that, for example, Iceland spar might 

 exhibit different phenomena according to the relation of 

 the surface of the plate to the axis of the crystal. Experi- 

 ments with quartz and Iceland spar on this point lead to 

 a negative result. 



(10) It is known that Lenard, in his investigations on 

 kathode rays, has shown that they belong to the ether, and 

 can pass through all bodies. Concerning the X-rays the 

 same may be said. 



In his latest work, Lenard has investigated the absorp- 

 tion coefficients of various bodies for the kathode rays, 

 including air at atmospheric pressure, which gives 4' 10, 

 3"4o, 3*io for i cm., according to the degree of exhaustion 

 of the gas in discharge tube. To judge from the nature 

 of the discharge, I have worked at about the same pres- 

 sure, but occasionally at greater or smaller pressures. I 

 find, using a Weber's photometer, that the intensity of the 

 fluorescent light varies nearly as the inverse square of the 

 distance between screen and discharge tube. This result 

 is obtained from three very consistent sets of observations 

 at distances of 100 and 200 mm. Hence air absorbs 

 the X-rays much less than the kathode rays. This 

 result is in complete agreement with the previously 

 described result, that the fluorescence of the screen 

 can be still observed at 2 metres from the vacuum tube. 

 In general, other bodies behave like air ; they are more 

 transparent for the X-rays than for the kathode rays. 



(11) A further distinction, and a noteworthy one, 

 results from the action of a magnet. I have not succeeded 

 in observing any deviation of the X-rays even in very 

 strong magnetic fields. 



The de\iation of kathode rays by the magnet is one of 

 their peculiar characteristics ; it has been observed by 

 Hertz and Lenard, that several kinds of kathode rays exist, 

 which differ by their power of exciting phosphorescence, 

 their susceptibility of absorption, and their deviation by 

 the magnet ; but a notable deviation has been observed 

 in all cases which have yet been investigated, and I think 

 that such deviation affords a characteristic not to be set 

 aside Hghtly. 



(12) As the result of many researches, it appears that 

 the place of most brilliant phosphorescence of the walls 

 of the discharge-tube is the chief seat whence the X-rays 

 originate and spread in all directions ; that is, the X-rays 

 proceed from the front where the kathode rays strike 

 the glass. If one deviates the kathode rays within the 

 tube by means of a magnet, it is seen that the X-rays 

 proceed from a new point, i.e. again from the end of the 

 kathode rays. 



Also for this reason the X-rays, which are not deflected 

 by a magnet, cannot be regarded as kathode rays which 

 have passed through the glass, for that passage cannot, 

 according to Lenard, be the cause of the different deflec- 

 tion of the rays. Hence I conclude that the X-rays are 

 not identical with the kathode rays, but are produced from 

 the kathode rays at the glass surface of the tube. 



(13) The rays are generated not only in glass. I have 

 obtained them in an apparatus closed by an aluminium 

 plate 2 mm. thick. I purpose later to investigate the 

 behaviour of other substances. 



(14) The justification of the term "rays," applied to 

 the phenomena, lies partly in the regular shadow pictures 

 produced by the interposition of a more or less permeable 



