Sf:PTEMBER 3. 1896] 



NA TURE 



427 



in the distance separating each pair of stars, a value of the 

 constant of aberration, and a second part of the work was to 

 make a comparison of the measured and computed distances, 

 which would give the corrections to be appUcd to the refrac- 

 tion tables. A series of subsidiary investigations, the results of 

 which are given on page 203, was also completed. The result 1 

 of the whole investigation furnishes as a definite result : Constant I 

 of Aberration =20 •443 + o" 010, which diftersonly very slightly j 

 from the commonly accepted value obtained by Struve, and this 

 within its own limit of probable error. The volume is accom- 

 panied by some excellent illustrations of the instrument and the 

 novel dome which protects it. The second part of this volume 

 Contains the observations of the right ascensions of the stars 

 observed with the prism apparatus made by Mr. Albert S. 

 Flint. 



New Feature on M.\rs. — A telegram from Kiel announces 

 the observation of a bright prominence on the terminator of the 

 planet, by Messrs. Hussey and Holden, at the Lick Observatory 

 on Wednesday last, August 27. The planet is well situated for 

 observation at midnight, being at present some five or si.\ degrees 

 north of a Tauri. 



THE ECLIPSE AT BODO AND NORTH 

 FINLAND. 

 TT^K give this week a reproduction of the drawing of the 

 corona made near Bodd, which accompanied Dr. Brester's 

 letter in our last issue (p. 390). 



Further particulars have been received concerning the doings 

 of the Russian Expedition under Baron Kaulbars, which ob- 

 served in Russian Finland. There was an unusually large develop- 



ment of the corona, the extensive and often oblique rays of 

 which surrounded the dark disc of the moon. One ot these rays 

 reached a length double that of the sun's diameter. Some of 

 the rays crossed each other, and Baron Kaulbars writes to the 

 St. I'elersbiirger Zei/tiiig " that the remarkable proportions of 

 the corona coincide with the opinion according to which this 

 phenomenon is only very little developed with a miniiiium of 

 sun-spots, for he had been able to see only very insignificant 

 spots on the sun at rare moments during observations extending 

 over several weeks." 



Other expeditions to the Maritime Province of the Amur 

 appear to have been very successful. 



NO. 1401, VOL. 54] 



ON THE RUNTGEN RA YS.^ 



\\J\\0 would have dreamt at the last annual meeting of the 

 ' * Victoria Institute, that before a year was out, we should be 

 able to see on a screen, to receive on a photographic plate, 

 which is afterwards developed, the skeleton, or a portion of the 

 skeleton of a living man, or at least a living child ? And as the 

 modes of exciting these rays improve, we shall probably go on, 

 step by step — indeed already, I believe, the whole body of a 

 full grown man has been penetrated by these rays, the discovery 

 of which we owe to Dr. Rontgen. 



I feel some difiidence in bringing this subject before you, 

 because I have never, myself, made experiments with the 

 Rontgen rays. Nevertheless I have read a good deal about 

 them, following what others have done, more especially where 

 it connected itself with the subject of light, to which I have paid 

 a good deal of attention. So I cannot but have a tolerably 

 definite idea in my own mind as to the nature of these Rontgen 

 rays which has been a matter in dispute and, I may say, is still 

 in dispute, although I think opinions are generally coming round 

 to that which I will bring before you in the end. 



Now before I go to the Rontgen rays direct, I must touch on 

 previous work which gradually led up to them. 



For a very long time it has been known that an electric dis- 

 charge passes more readily through tolerably rarefied air, than 

 through air of greater density, and so with other gases. If we 

 have a longish closed tube, provided with electrodes at the ends 

 by means of platinum wires passing through the glass, if the air 

 be tolerably exhausted from it, an electric discharge passes, 

 comparatively speaking, freely through it, forming a beautiful 

 skein of light, if I may so speak, and under certain circumstances 

 that skein of light is divided into strata in a very remarkable 

 manner. These strata fill the greater part of the tube from the 

 positive electrode, or anode, as it is called, till we get nearly, 

 but not quite, to the negative electrode, or kathode. There is a 

 dark space separating the end of the positive discharge which, 

 as I said, under suitable conditions and sufficiently high ex- 

 haustion, shows stratification, from a blue glow enveloping the 

 negative electrode or part of it. The luminosity about the 

 kathode is somewhat indefinitely bounded on the side of the 

 stratification. 



When, however, the exhaustion is carried still further, at the 

 same time the strata become wider apart, and the luminosity 

 recedes from the kathode and expands, forming a sort of glowing 

 halo much more sharply defined on the inside than the outside ; 

 in that respect resembling the ordinary luminous halo — not the 

 corona — occasionally seen round the moon. We have here, 

 ihen, these two dark spaces, one outside the halo, where the 

 luminosity gradually fades oflT, and another dark space on the 

 inside, where the luminosity is more sharply defined, and which 

 reaches to the negative electrode. 



Now it is the phenomena in connection with this second dark 

 space that I have more particularly to bring before you. As 

 the exhaustion is rendered higher and higher, the inner dark 

 space gets wider and wider until at a sufficiently high exhaustion 

 it fills the whole tube or bulb. Mr. Crookes has worked more 

 especially at this subject, and, indeed, the tubes which are now 

 used for the production of the Rontgen rays, are generally called 

 " Crookes tubes." I have seen in some of the foreign periodicals 

 the word "Crookes" used to signify one of these tubes. Mr. 

 Crookes' researches in very high vacua led him up to that most 

 remarkable instrument, the radiometer, the nature of which led 

 us to form clearer conceptions, than we had hitherto done, of 

 the nature of the motion of molecules in gas ; or rather, when 

 the theory of the radiometer was made out, presented us, as I 

 may say, with a visible exhibition of the thing in actual 

 working. 



NoH these researches, which led Mr. Crookes to improve his 

 v.acuum, naturally led him to examine the electrical phenomena 

 produced by excessively high vacua. 



I have said that it was with the second or inner dark space 

 that I had chiefly to do. When the exhaustion is sufficient, that 

 fills the whole tube. 



Now what takes place in this dark space ? Suppose we inter- 

 pose a screen, such as a plate of mica with a hole in it. A 

 portion of the discharge from the negative electrode goes 

 through that hole and continues onwards in a straight course 

 until it reaches the wall of the tube. When it reaches the wall 



1 .\n e.\tr.->ct from the .'Vnnu.ll .\dilress to the Victoria Institute, by Sir 

 C. 6. Stokes, F.kS., the President. 



