302 



NATURE 



[July 30, 1896 



the corona, or at differenl heights ; further, they most frcquenlly 

 do not correspond to elements known upon the earth. 



Special interest attaches to the investigation of the rotation of 

 the corona by observing or photographing the displacement of 

 lines in the spectrum at some distance from the limb on each 

 side of the equator. No photographic impression was secured 

 with the fourth order spectrum of a diffraction grating, adjusted 

 for H and K, and, although the eclipse occurred at a maximum 

 of sun-spots, 1474 K was too feeble in the second order spectrum 

 to permit any trustworthy measures to be made visually. A suc- 

 cessful photograph of the II and K lines was obtained, however, 

 with a 3-prism spectroscope attached to a 6-inch refractor, one 

 half of the slit being exposed on the west and the other on the 

 east side of the corona. The measured velocity of 6'8kni. per sec. 

 has led M. Deslandres to conclude that the equatorial part of 

 the corona moves very nearly with the same angular velocity as 

 the phutosphcrc. This result must be received with caution 

 until confirmed by further researches, as the photographs taken 

 at the same moment by Mr. Fowler give no indications of the 

 presence of H and K in the true coronal spectrum. It is pointed 

 out that this research may be simplified in future by making 

 only one exposure, placing the slit radially, so that the velocities 

 may be determined from the inclination of the lines, as in the 

 recent researches on Saturn's rings. 



In the last chapter of the report, various hypotheses as to the 

 nature of the solar atmosphere are reviewed, and an electrical 

 theory is propounded. It is pointed out that, notwithstanding 

 the diversity of appearances, there is really a great similarity 

 between the solar and terrestrial atmospheres, and the report 

 ends as follows : "Terrestrial meteorology and solar physics, 

 which are separated by the necessity for the division of work, 

 are in reality connected sciences, which, by the nature of things, 

 ought to be studied together." 



THE RONTGEN RA YS> 



pROF. RONTGEN, of Wurzburg, at the end of last year 

 -*■ published an account of a discovery which has excited an 

 interest unparalleled in the history of physical science. In his 

 paper read before the Wurzburg Physical .Society, he announced 

 the existence of an agent which is able to affect a photographic 

 plate placed behind substances, .such as wood or aluminium, 

 which are opaque to ordinary light. This agent, though able to 

 pass with considerable freedom through light substances, such as 

 wood or flesh, is slopped to a much greater extent by heavy 

 ones, .such as the heavy metals and the bones ; hence, if the 

 hand, or a wooden box containing metal objects, is placed between 

 the source of the Rontgen rays and a photographic plate, photo- 

 graphs such as those now- thrown on the screen are obtained. 

 This discovery, as you see, appeals strongly to one of the most 

 powerful passif)ns of human nature, curiosity, and it is not 

 surprising th.at it attracted an amount of attention quite dispro- 

 portionate to that usually given to questions of physical science. 

 Though appearing at a time of great political excitement, the 

 accounts of it occupied the most prominent parts of the news- 

 papers, and within a few weeks of its discovery it received a 

 practical application in the pages of Punch. The interest this 

 discovery aroused in men of science was equal to that shown by 

 the general public. Reports of experiments on the Rontgen 

 rays have poured in from almost every country in the world, 

 and quite a voluminous literature on the subject has already 

 sprung up. 



In view of the general interest taken in this subject, I thought 

 that the Rontgen rays might not be an unsuitable subject for the 

 Rede Lecture. 



Before discussing these rays themselves, I think it may perhaps 

 make matters clearer if I call your attention to one or two of the 

 phenomena which accompany the discharge of electricity through 

 gas at a low pressure. I have here a bulb from which the air 

 has l>een taken until the pressure has been reduced to about 

 l/ioooo part of the atmospheric pressure. When the electric 

 discharge passes through this bulb you see that there is con- 

 siderable lumino.sity in the gas in the bulb, except in a region 

 round this terminal — the negative one; this region, where the 

 luminosity is so deficient, is called the negative dark space. In 

 this bulb there is no phosphorescence on the glass, and I may 



1 The Rede Lecture, given at the University of Cambridge, on June lo, 

 by Prof. J. J. Thomson, K.R.S. 



NO. 1396, VOL. 54] 



say no emission of Rontgen rays. If I were still further) ta 

 reduce the pressure of the gas in this bulb, this dark space would 

 expand and encroach on the luminous part of the discharge, and 

 would, when the pressure got very low, reach the walls of the 

 tube ; the expansion of the dark space diminishes the luminosity 

 in the gas, but we find that where the dark space reaches the 

 glass of the tube the glass itself becomes luminous, until finally 

 at very low pressures we get to the state of things shown by this 

 tube, where the luminosity is all on the glass, and little or none 

 is to be observed in the gas. Rontgen rays are produced by 

 this bulb, though not by the other. 



There is one feature in this tube to which I must call your 

 attention : you see that there is a shadow on part of thei tube ; 

 this sh.adow is thrown by a mica cro.ss fixeil between the negative 

 electrode and the wall of the tube, and if we observe the shape 

 of the shadow we see that any point of the tube is in shadow if 

 the line joining that point to the negative electrode passes 

 through the mica cross. We thus conclude that we have some- 

 thing .starting from the negative electrode, travelling in straight 

 lines, and producing phosphorescence when it reaches the glass, 

 and, further, that this something is stopped by the mica cross. 

 This something which travels in straight lines from the kathode 

 is called the kathode rays : these rays are of great interest in 

 relation to the subject of this lecture, for the kathode rays seem 

 to be the parents of the Rontgen rays. Let me call your 

 attention to the effect produced by a magnet on these rays : you 

 see that when the magnet is brought near, the shadow of the 

 cro.ss is displaced ; this shows that the direction of the rays 

 casting the shadow have been deflected by the magnet, thus the 

 kathode rays are deflected by a magnet. We shall see later on 

 that the Rontgen rays, on the other hand, are not so affected. 

 This is one of the most striking differences between the parent — 

 the kathode rays — and the child, the Rontgen rays. The effects- 

 of the kathode rays inside the tube were discovered more thaii 

 twenty years ago by Crookes and Goldstein ; it is only quite 

 recently, however, that any effects produced by these rays out- 

 side the tube have been observed. In 1S94 Lenard, using a tube 

 of the kind shown in the diagram (Fig. i), where the kathode 



rays struck against a window made of very thin alumi- 

 nium, found that if he placed outside the tube in front of 

 the window a screen covered with a phosphorescent sub- 

 stance, pentadecaparatolyketon, it became phosphorescent ; 

 he found, further, that a photographic jjlate placed behind the 

 window was affected — nay, that this plate was affected even 

 though he placed in front of it a plate of aluminium or a thin 

 quartz plate — in fact, he took a photograph through aluminium 

 and quartz ; he thus obtained two of the most prominent 

 phenomena shown by the Rontgen rays. In fact, we know from 

 the researches of Rontgen that the Rontgen rays must have been 

 present and played a part in these experiments. Lenard him- 

 self ascribed the effects he observed to kathode rays which had 

 penetrated the aluminium window, and indeed it would seem 

 that something in addition to the Rontgen rays must have been 

 present, as Lenard found that the |:)usition of the phosphorescent 

 patch was affected by a magnet, while the Rontgen rays them- 

 selves are, as we shall see, not influenced by such an agent. 



I now come to the consideration of the Rontgen rays them- 

 selves, and shall endeavour to repeat some of the experiments 

 by which Rontgen established their existence. The apparatus 

 consists of a lube exhausted to such a low pressure, that w hen the 

 electric discharge passes through it there is an abundant supply 

 of kathode rays ; these rays strike against a metal plate in the 

 bulb. This metal plate is not essential for the production of the 

 rays, and was not present in the btilbs used by Rontgen ; it, 

 however, considerably increases the efficiency of the bulb. 



When the electric discharge passes through this bulb, the 

 region round it is the seat of some very remarkable phenomena. 

 I have here a screen coated with a phosphorescent sul^stance. 



