642 



PHYSICS, PROGRESS OF, IN 1896. 



exposing two plates of metal (copper and lead) to 

 an electric arc for an hour, he placed between them 

 a photographic negative and a sensitive plate, the 

 unexposed faces of the plates being inward. An 

 image resulted which M. Le Bon attributes to 

 " dark light " stored on the surface of the metal 

 plates. Without preliminary exposure to the light 

 of the arc no image was obtained. He believes 

 that there are four forms of invisible radiation the 

 " X-rays " of Rontgen, the invisible fluorescence of 

 Becquerel (see Phosphorescence above), the new form 

 of radiation just described, and a similar radiation 

 which he asserts is emitted by organized beings in 

 darkness. David E. Packer, an English astronomer, 

 has also asserted that he has photographed the solar 

 corona in broad daylight, apparently by rays simi- 

 lar to " dark light," by allowing the sunlight to filter 

 through metal foil. Other experimenters, however, 

 have been unable to repeat his work. Aschenholz 

 (Berlin Physical Society, March 27) concludes from 

 experiments that Le Bon's " dark light " has no 

 existence. He calls attention to the fact that it is 

 very difficult to exclude lateral light, even with 

 metal screens, and he exhibited photographs to sup- 

 port his view. 



Lenses. Barton (" Philosophical Magazine," Jan- 

 uary) describes a graphic method for determining 

 the focal lengths of lenses and mirrors. For a con- 

 cave mirror the distance of object and image re- 

 spectively are laid off on two co-ordinate axes, and 

 the extreme points are joined. Two separate ob- 

 servations thus give two lines that intersect in a 

 point equidistant from the two axes, whose co-ordi- 

 nates are both equal to the focal length sought. 

 Hastings (National Academy of Sciences) has dis- 

 covered by mathematical calculation a form of lens 

 for telescopes by using which the secondary chro- 

 matic aberration can be very nearly done away with. 

 He estimates that his discovery will increase the 

 power of any telescope by 10 per cent. 



Monochromatic Projection. Abney exhibited at 

 a conversazione of the Royal Society (" Nature." 

 May 14) an apparatus for throwing on a screen or 

 on a photographic plate the image of a luminous 

 object in monochromatic light. An image is first 

 thrown on the. face of a prism or a grating, and the 

 spectrum is formed as usual, the color desired being 

 allowed to pass through a slit placed directly in the 

 spectrum. A second lens close to this slit forms the 

 desired image. 



Heliostat. Mayer (New York Academy of Sci- 

 ences, May 4) has devised a heliostat that uses lenses 

 and total-reflection prisms, the advantage being 

 that it can give a powerful beam of light that prac- 

 tically emanates from a point. 



Electricity. Velocity. Blondlot (" Annales de 

 Chemie et de Physique," April) has measured the 

 velocity of an electro-magnetic disturbance along a 

 wire by discharging Leyden jars that are connected 

 with the same spark gap by two wires, a short one 

 and a very long one. The time interval between 

 the resulting two sparks was measured by throw- 

 ing their images on a photographic plate by means 

 of a rotating mirror whose speed was deduced from 

 the musical note produced by the vibrations of its 

 axle. The mean of five sets of measurements gave 

 296,400 kilometres a second, while three others in 

 which the distance traversed was 1,821 metres 

 nearly twice as great as at first gave 298,000. 



Resistance, and Conductivity. Appleyard (Lon- 

 don Physical Society, May 22) has investigated the 

 effect of temperature on dielectric resistance, and 

 finds that the capacity of a paraffin condenser varies 

 irregularly with the temperature, but that the ca- 

 pacity of a mica condenser is constant between 33 

 F. and 110 F., at least within one per cent. With 

 paraffin, when the temperature reaches a point with- 



in about 20 of the melting point, the resistance 

 falls rapidly; when melting begins there is a rapid 

 drop, but while it is in progress the resistance re- 

 mains constant. Dewar and Fleming have recently 

 shown that with pure metals the electrical resist- 

 ance shows every sign of disappearing as the abso- 

 lute zero of temperature is approached, but in the 

 case of alloys, even when the second metal is present 

 only as a slight impurity, this is by no means the 

 case. Lord Rayleigh (" Nat ure," June 18) suggests 

 that the apparent resistance of an alloy may be 

 made up of thermoelectric effects, the reverse elec- 

 tro-motive forces due to an infinite number of inter- 

 nal metallic junctions simulating a true resistance, 

 which of course would have no relation to the real 

 resistances of the metals that compose the alloy. 

 Fleming, in a lecture at the Royal Institution 

 (" Science," Aug. 7), has shown that the conduc- 

 tivity of pure metals is increased enormously by 

 the cold of liquid air, while that of alloys is only 

 slightly increased. Dewar and Fleming are of 

 opinion that pure metals would become perfect 

 conductors at the absolute zero of temperature, and 

 hence also absolutely opaque to electric radiation. 

 These experiments furnish additional proof that 

 the passage of an electric current depends on some- 

 thing that goes on outside of what we call the con- 

 ductor. They furnish also a means of testing the 

 purity of metals, that almost rivals the spectroscope 

 in delicacy. Ayrton (London Physical Society, 

 March 13) states that the conductivity of copper 

 has been steadily increasing, the result being spe- 

 cially noticeable in the process where the metal is 

 continually burnished while depositing electrolyt- 

 ically. It has been supposed that the effect was due 

 to increased density, but Prof. Ayrton says this has 

 now been proved untrue. This change invalidates 

 such resistance standards as are based on the spe- 

 cific resistance of copper. Griffiths (Manchester 

 Literary and Philosophical Society, Oct. 20) finds 

 that the resistance of a salt solution that contains 

 gelatin, and is allowed to "set" slowly at a con- 

 stant temperature, does not alter appreciably even 

 when the viscosity becomes very great. Strowd 

 and Henderson (London Physical Society, Oct. 30) 

 measure electrolytic conductivity by means of con- 

 tinuous currents, by using a balancing electrolytic 

 cell with the principal cell so that the electro-motive 

 force of polarization in each balances the other's 

 effect on the galvanometer. ReevesfLondon Physical 

 Socioty, March 13) has described an addition to the 

 Wheatstone bridge for the purpose of measuring 

 low resistances. He does not balance by varying 

 the length of standard wire between the two con- 

 tacts, but instead the distance between these con- 

 tacts, and also the length of wire that is being 

 measured, are kept constant, balance being ob- 

 tained by altering other resistances in the network. 

 By this means the resistance of metre lengths of 

 heavy copper wire can be obtained with an accu- 

 racy of one tenth of 1 per cent. One advantage 

 of the method is that it is independent of the resist- 

 ances at the contact. 



The Question of " Negative Resistance." Frith 

 and Rogers (London Physical Society, May 8) find 

 that there is a critical frequency of alternation 

 above which the resistance of an alternating-current 

 arc has a positive value that is independent of the 

 frequency and below which it has a negative value. 

 This critical frequency lies between 7'5 and 0. S. P. 

 Thompson (London Physical Society, June 26) as- 

 serts that the observations of Frith and Rogers 

 only prove that that part of the resistance of an arc 

 that is not constant is positive and varies inversely 

 as the current. The rate of increase of resistance 

 compared with current will therefore be negative. 

 Discussion showed that there were diverse opinions 



