450 



NA TURE 



[March 12, 1896 



with a metallic point connected to the ground or to the kathode. 

 The rays then are attracted to a point on the tube exactly op- 

 posite to that touched by the conductor ; the best position for the 

 latter is at a distance from the kathode equal to its radius of 

 curvature. Tubes could, of course, be constructed so that the 

 focus of the kathode rays fell normally on the wall of the tube ; 

 it is found, however, that after some time the glass loses its 

 fluorescent property, showing a dark spot at the incident point 

 of the rays. The device with the metallic point enables the 

 fluorescent spot to be changed from time to time. 



The surgical aspects of photography with Rontgen rays are 

 considered by Mr. Albert Carless in the March number of the 

 Practitioner. The conclusion arrived at with reference to 

 radiography is " that it can be of real value in a certain limited 

 number of cases there can be no doubt, but unless very con- 

 siderable improvements are made in the technique, it will be 

 but little resorted to in practical work." 



Prof. M. J. Pupin contributes to the March number of the 

 Engineering Magazine (New York) an account of his experi- 

 ments with Rontgen rays. He has obtained excellent results 

 with a pear-shaped Crookes' tube, of five inches diameter, 

 excited by a six-plate Holtz machine. He finds that a 

 cylindrical vacuum tube without internal electrodes, but with 

 external tinfoil coatings at the extremity of the tube, will do 

 very well as a substitute for a Crookes' tube. 



Our American correspondent writes, under date February 28 : 

 " The new art of ' radiography ' is still prosecuted in America 

 with unabated ardour. The most successful apparatus yet de- 

 vised seems to be that of Prof. John S. McKay, of the Packer 

 Institute, Brooklyn. It is a small tube five inches long, and an 

 inch, or rather more, in diameter, which is known and sold by 

 instrument-makers as a ' perfect vacuum ' tube. This is 

 attached directly to the terminals of the secondary coil. The 

 copper electrodes are less than an eighth of an inch apart ; but 

 the vacuum is so perfect, that the spark will leap the whole length 

 of the tube outside rather than cross this small interval inside. 

 The tube produces very little light, and is sometimes, when in use, 

 perfectly dark. This tube is light and convenient, and does not 

 become hot like the Crookes' tube. After running it continuously 

 for half an hour with a pressure of 200,000 volts, it is scarcely 

 warm to the touch. But the special advantage is that the rays 

 radiate in all directions from the centre, so that exposures may 

 be made simultaneously within a radius of two feet from the 

 centre of the tube. The best results were found at a distance of 

 one foot, and with an exposure of five minutes. Experiments 

 to determine the relative opacity of different substances show 

 that the opacity to X-rays is generally in inverse proportion to 

 the diathermancy of the substances tested. Thus rock-salt is 

 most opaque ; next comes alum, then glass, then quartz. 

 Camphor .gum, gum copal, and vulcanite are almost equally 

 transparent ; amber somewhat less so, and sealing-wax quite 

 opaque. Iceland spar, mica, and selenite are quite transparent. 

 Iceland spar seemed to give evidence of double refraction. 

 Charcoal is quite transparent, more so than wood. Anthracite 

 coal is somewhat opaque, but not so much as glass. Egg-shells, 

 like bones, are opaque. Of liquids tested, mercury, sulphuric 

 acid, glycerine, and kerosene were somewhat opaque, the 

 opacity varying about as the density. Prof. McKay has also 

 produced pictures on a sensitive gelatine film wrapped in paper 

 with some metallic object upon it, and placed in the dielectric 

 of an electric condenser, the terminals of which are connected 

 with an induction coil or Holtz machine. After rapidly charging 

 and discharging this condenser or Leyden jar for two or three 

 minutes, a distinct image of the metallic body is found to be 

 radiographed upon the sensitive film. 



" A remarkable application of Prof. McKay's apparatus 

 has been made by Edward P. Thompson, an electrician, 

 who has devised a fluorescent screen on which shadows 

 may be thrown showing the action of the bones in motion, 

 as of the hand, and he hopes to show the motion of the 

 bony skeleton of a bird in flight. It has been stated that 

 the great drawback to aerial locomotion is our ignorance 

 of the exact way in which a bird flies. Hence the in- 

 ventor attaches much importance to his apparatus, which 

 he calls the ' kinetoscotoscope.' It will show, among other 

 things, the motion of the parts of a broken bone, indicating the 

 locality and nature of the fracture, as the bones may be moved 

 or bent back and forth before the screen, thus opening and 

 closing the crack. The taking of pictures is not the design of 

 this apparatus. The fluorescent screen is prepared by pulveris- 



NO. 1376, VOL. 53] 



ing barium platino-cyanide to a fine powder, and pouring upon 

 a draughtsman's tracing cloth a small quantity of varnish, or of 

 a mixture of oil and turpentine, stirring the powder in with it, 

 and drying." 



With regard to seeing Rontgen shadows by means of a 

 fluorescent screen, Mr. Swinton informs us that he finds that 

 instead of employing blotting-paper saturated with barium 

 platino-cyanide, it is much better to make a hot emulsion of the 

 barium platino-cyanide in gelatine and water, and apply this in 

 a thick coat to a piece of glass On cooling, the barium platino- 

 cyanide crystallises out, and the gelatine protects it from 

 abrasion. A thick and uniform coating is what is wanted. 

 The glass being rather opaque to the X-rays, but transparent 

 to light, the plate should be obviously placed with the gelatine 

 side next to the Crookes' tube, and the glass side next the eye. 



ON THE GENERATION OF LONGITUDINAL 



WAVES IN ETHER} 

 T N a short note published in Nature of February 6, 1 suggested 

 -'■ an arrangement of four insulated and electrified spherical 

 conductors with their centres in one line, giving rise to ethereal 

 waves in the surrounding atmosphere, of which the disturbance 

 in the line of centres is essentially longitudinal. But at any 

 finite distance from this line there must also be laminar or dis- 

 tortional waves of the kind expressed in Maxwell's equations. 

 The object of my present communication is to show an arrange- 

 ment by which a large space of air is traversed by pressural dis- 

 turbance, or by waves essentially longitudinal, or by condensa- 

 tional-rarefactional vibrations ; with but a very small proportion, 

 practically evanescent, of laminar waves. 



Let AA be a plane circular metal plate msulated within a 

 metal case ccc'c', as indicated in the drawing. Let D be a 

 discharger which can be pushed in so as to make contact with A. 



Let A be charged to begin with, positively for instance as indi- 

 cated by the letters YVpp ; nn«« showing negative electricity 



induced by it. Let now the discliarger be pushed in till a spark 

 passes. The result, as regards the space between aa and the 

 roof RR over it, will be either an instantaneous transmission of 

 commencement of diminution of electrostatic force, or a set of 

 electric waves of almost purely longitudinal displacement, accord- 

 ing as ether is incompressible or compressible. 



Hence, if the theory of longitudinal waves, suggested by 

 Rontgen as the explanation of his discovery (for the consideration 

 of which he has given strong reasons), be true, it would seem 

 probable that a sensitive photographic plate in the space between 

 AA and RR should be acted on, as sensitive plates are, by 

 Rontgen rays. Either a Wimshurst electrical machine or an 

 induction-coil, adapted to keep incessantly charging aa with 

 great rapidity so as to cause an exceedingly rapid succession of 

 sparks between D and A, might give a practical result. In 

 trying for it, the light of the sparks at D must be carefully 

 screened to prevent general illumination of the interior of the 

 case and ordinary photographic action on the sensitive plate. 



The arrangement may be varied by making the roof of sheet 

 aluminium, perhaps about a millimetre thick, and placing the 

 sensitive photographic plate, or phosphorescent substance, on 

 the outside of this roof, or in any convenient position above it. 

 When a photographic plate is used there must, of course, be an 

 outer cover of metal or of wood, to shut out all ordinary light 

 from above. This arrangement will allow the spark gap at D to 

 be made wider and wider, until in preference the sparks pass 

 between aa and the aluminium roof above it. The transparency 

 of the aluminium for Rontgen light will allow the photographic 

 plate to be marked, if enough of this kind of light is produced in 

 the space between the roof and aa, whether with or without 

 sparks. 

 1 A paper by Lord Kelvin, read before the Ro) al Society on February 13 



