March 5, 1896] 



NATURE 



413 



The term " anode rays " for the rays discovered with so much 

 eclat by Prof. Rontgen, whether they be the same as those pre- 

 viously discovered by Dr. Lenard or not, is suggested by re- 

 marks from Mr. A. W. Porter at a recent meeting of the Royal 

 Society. They certainly do not start from the kathode, but from 

 some opposed surface, a surface which may be an actual anode, 

 and which always has some anodic properties. From each point 

 of such a surface rays start in all directions ; this is proved by the 

 shadows they cast of slits, holes, and wires. 



Oliver J. Lodge. 



I MAY state that in a lecture which I gave here on the 

 evening of Tuesday last, the 25th ult., I showed to a large 

 audience, by means of a sheet of Imrium platino-cyanide, 

 rendered fluorescent by the Rontgen rays from a Crookes' tube, 

 all the things referred to by Mr. Campbell Swinton in his letter 

 in the last number of Nature. The shadows of coins in a 

 purse, and of a hand, were distinctly visible to the audience 

 when placed behind screens perfectly opaque to ordinary light, 

 and, though more dimly, even through a book of eight or nine 

 hundred pages. 



I must confess that I cannot see why, after Prof. Rdntgen's 

 account of his own work, the success of such experiments as 

 those made by Mr. Swinton or myself should be regarded as 

 surprising, or accounts of them received with incredulity. They 

 seem to me to be merely a variation of Prof. Rontgen's own ex- 

 periments, or at most to be a matter of the most obvious inference 

 from these experiments. 



The statements that have appeared to the effect that Signor 

 Salvioni has devised a method of rendering the retina of the 

 human eye sensitive to Rontgen rays, and that by his method 

 objects are directly seen through planks of wood, sheets of 

 aluminium, t\:c. , are simply absurd. The fluorescent light 

 produced is entirely distinct from the Rontgen rays, and affects 

 the retina like ordinary light ; and of course parts of the sheet 

 which do not fluoresce, liecause they are shielded by the opaque 

 objects liehind Irom the Rontgen rays, appear dark. Seeing 

 such shadows can no more l)e said to be seeing the objects 

 themselves by means of the Rontgen rays, than a man can be 

 said to see himself when he looks at his shadow thrown by an 

 ordinary gas-lamp on the street. 



Prof. Rontgen discovered the fluorescence of the barium 

 ))latino-cyanide under the rays now called by his name, and the 

 transparency of ordinarily opaque matter to these rays, and the 

 discoveries of Signor Salvioni and others, so far, at any rate, as 

 they have been described in Natire^ and other journals I have 

 seen, amount to nothing more. It is only just that in accounts 

 I if verifications of Rontgen's discoveries an attempt should be made 

 lu show clearly that such observations are only verifications, so as 

 to prevent the credit of discovery which is Prof. Rontgen's due 

 from any appearance, however unintentional, of indirect 

 diminution. 



Scientific accounts of verifications, as far as I have seen them 

 in Nature or elsewhere, are in themselves unexceptionable ; but 

 extra precaution seems necessary in order that the public should 

 not be led by newspaper paragraphists, retailing such accounts 

 at second-hand, to regard as extensions of Rontgen's work what j 

 are only direct and obvious consequences, perceived by himself, 

 of the facts which he has observed. Andrew Gray. 



University College of North Wales, Bangor, March I. 



I.N your last issue (p. 399), in the account of the work appear- 

 ing in the Coinptes rendu s, you state that M. de Heen " proves 

 conclusively that the X-rays proceed from the anode and not 

 the kathode." May I point out (as I did at the Royal Society, 

 in the course of the discussion on Prof. J. J. Thomson's paper, 

 February 13), that I have proved that this is undoubtedly true 

 for the bulb that I have been using throiiic;hout my experiments 

 on the X-radiation. The bulb is one in which the negative 

 electrode is concave, and the negative stream is thereby focussed 

 to a point on the anode, which is a platinum disc placed near 

 the centre of the bull). By measuring the positions of different 

 parts of a rarliograph of a series of concentric zones of tinfoil 

 placed in a measured position, I have shown that the actinic rays 

 diverge from the anode disc. 



I am of opinion, however, that in this respect this bulb differs 



I A translation of Prof. Salvioni"s paper will be found in another part of 

 lTi;s issue. — Eu. Natuki:. 



from those which have been employed by others. In these latter, 

 judging from the published accounts, the negative stream 

 impinges directly on the glass ; and for bulbs of this kind, it has 

 been shown conclusively by Prof. J. J. Thomson that the seat 

 of the origin of the rays is the glass itself. The proof is that a 

 sensitive plate placed inside the bulb in the path of the negative 

 stream is not acted upon. I venture to think that, in the case of 

 my bulb, a sensitive plate placed inside would be acted upon 

 provided it lay in the hemisphere of the bulb in which the 

 kathode lies. I intend to test this conjecture experimentally. 

 Should it prove true, the behaviour of both varieties of bulb will 

 probably be capable of description by the following single state- 

 nient : — 



The seat of the origin of the X-rays is where the n^ative 

 stream first impinges against a solid, and gives up, or partially 

 gives up, its negative charge. Alfred W. Porter. 



University College, London, February 28. 



NO. 1375, VOL. 53] 



In your " Notes" of last week you refer to a communication 

 of M. de Heen, stating that the X-rays proceed from the anode. 

 Some experiments, which I made at the beginning of last month, 

 bearing on this point, may be of interest to your readers. It is 

 of course not the case that the X-rays proceed from the anode in 

 general, but they may be made to do so by placing a small disc 

 as anode facing the kathode. The kathode streams impinge on 

 the former, and the X-rays being generated there radiate from 

 it. The experiment was made by placing a lead plate (4 cm. by 

 10 cm.), with a rough circular hole in it, at a distance of 10 cm. 

 above the photographic plate, and the tube (a small one with a 

 curved kathode facing a small disc anode) io"5 cm. above the 

 lead. After development the negative was replaced exactly in 

 its former position. Several interesting facts showed themselves, 

 the most striking being that the image of the hole consisted of a 

 well-defined circle showing even individual splinters on the 

 edge, and in addition diffused elongation on two sides. On 

 placing the eye so that the hole exactly covered its well-defined 

 image, it was necessary to put it in the position occupied by the 

 anode. 



The diffused images in the same way were seen to be due 

 to the fluorescent parts of the glass sides of the tube — a kind of 

 pin-hole photograph, in fact. 



The rays leave the anode as if they were the splash of a jet of 

 water occupying the position of the kathode stream. In other 

 words — supposing the plane of the anode vertical and the 

 kathode to the right, then no rays appear to the left of the plane 

 of the anode, whilst on the right the space is exceedingly rich. 

 The negatives show, in every case tried, two well-defined regions, 

 viz. nothing to the left of the intersection of the plane of the 

 anode with the negative, and a dense deposit on the right, the 

 richest part apparently being close to this line. It is difficult to 

 account for this on the supposition that the rays are due to waves 

 generated at the point of impact. We should expect in this case 

 the action on the plate to increase with the visual angle of the 

 anode disc as seen from points on the plate. On the other hand, 

 certain further experiments seem to show that the action is not 

 in all respects similar to the splash of a jet. Whether the effect 

 is due to the fact that the place of impact of the kathode stream 

 is an anode, or simply an internal obstacle, I have not yet 

 determined, but experiments in progress will, I hope, settle this 

 point. 



For photographic purposes, the best kinds of tube are those 

 with a curved kathode converging the streams on a small plane 

 anode, remembering, of course, that the strong field is on the 

 kathode side of the anode plane. This behaves very approxim- 

 ately as a radiating point. With this I have obtained, with 

 comparatively short exposures, and a 2^" to 3" spark length, 

 strong negatives of remarkable definition, certainly finer than 

 any I have yet seen. W^ M. HiCKS. 



Firth College, March I. 



I was interested to see in your last issue a letter from Mr. 

 Swinton describing his reproduction of Prof Salvioni's experi- 

 ments with phosphorescent screens. Mr. Swinton uses a piece 

 of blotting-paper impregnated with platino-cyanide of barium. 

 I have tried this method, but have obtained better results with a 

 screen prepared with the same salt, as follows : 



A piece of fairly stout black paper, free from pinholes, is coated 

 with gum containing a little glycerine, and, as soon as it has 



