730 



PHYSICS, PROGRESS OP, IN 1891. 



of light emitted by phosphorescent bodies and 

 the duration of illumination. The results agree 

 very closely with his father's experiments. 



Intensity. Dr. A. Richardson (" Philosophical 

 Magazine," September) has devised a new method 

 of measuring the intensity of rays of high re- 

 frangibility. It depends on the sensitiveness 

 of chlorine to light. 



Photography. Lord Rayleigh (" Philosophical 

 Magazine," February) finds that a pin-hole may 

 replace a lens under certain conditions, but to 

 obtain the definition of a lens of four-inch aper- 

 ture, a focal distance of five miles would be 

 necessary. 



Stereoscopy. C. J. Woodward exhibited to 

 the London Physical Society, on May 22, Dr. 

 Schobbens's lantern stereoscope. The two pictures 

 are superposed on the screen, red glass being 

 placed in front of one lantern lens and green in 

 front of the other. The spectators wear spec- 

 tacles with one green and one red glass. Each 

 eye, therefore, selects its own picture, and the 

 usual stereoscopic effect results. 



Liquoscope. M. Sonden, of Stockholm (" Na- 

 ture," Sept. 17), has devised what he calls a 

 "liquoscope" for optical comparison of trans- 

 parent liquids. Two hollow prisms holding the 

 liquids are separated by a partition at right 

 angles to the refracting edge, and are placed in 

 a vessel of glycerin with plane parallel glass 

 sides. The deflection of the rays through the 

 prisms is thus compensated. A straight mark 

 viewed through the instrument appears as two 

 disconnected halves, the relative displacement 

 of which gives a measure of the difference of the 

 refractive powers of the two liquids. 



Electricity. Its Velocity. The velocity of an 

 electric disturbance through a wire, as found by 

 Wheatstone, was half as great again as that found 

 by Kirchhof. It has been proved by I. Stefan 

 (Vienna " Berichte," April 23) that the dis- 

 crepancy is due to the fact that Wheatstone 

 used a coiled wire. Stefan compared by Hertz's 

 method the waves in a straight wire with those 

 in a coil like that of Wheatstone, and found a 

 similar difference in velocity. 



Electric Waves. (See " Annual Cyclopae- 

 dia," 1889, p. 694 ; 1890, p. 715). Prof. Lodge 

 (' Nature," July 16) objects to the expression 

 " electric resonance " (used of responsiveness to 

 electric radiation), as not conveying an exact 

 meaning. Dr. Arthur Myers suggests '' syntony " 

 (Greek <riWoj/os), with its derivatives " syntonize " 

 and "syntonic." "Symphonic" has also been 

 suggested, but is objectionable, as it has already 

 a different meaning. Prof. Lodge (London 

 Physical Society, June 11) has yet found in in- 

 sulators no trace of opacity to electro-magnetic 

 radiation, but the thinnest film of metal is quite 

 opaque. Even cardboard rubbed with graphite 

 is a complete screen. He has devised what he 

 calls a " graduated electric eye," or " harp," con- 

 sisting of strips of tinfoil of different lengths, 

 which is sensitive to a wide range of radiation. 

 Prof. Lodge considers that the opacity of ebon- 

 ite (the most conspicuous exception to the rule 

 that insulators are transparent) is due to inter- 

 nal reflections, like that of ground glass. Prof. 

 Klemencic (Vienna " Berichte," Feb. 19), in ex- 

 periments on the reflection of electro-magnetic 

 waves from plates of metal and sulphur, finds 



that the results in general agree with Fresnel's 

 formulae for intensity, but that there are some 

 noteworthy exceptions, probably due to the large 

 size of the ray compared to that of the plate. 

 F. T. Trouton (' Philosophical Magazine," July) 

 finds that as the reflector is smaller the distance 

 of the node becomes greater, and that the dis- 

 tance also varies according as the long dimen- 

 sion of the mirror is parallel to the magnetic or 

 to the electric component of the wave. He con- 

 siders this due to the fact that the beam rapidly 

 decreases in intensity as it leaves the mirror. F. 

 Von Dobrzynski claims to have photographed 

 electro-magnetic waves in air, the time of ex- 

 posure being three hours. He concludes that 

 waves of from 0'6 to 20 centimetres in length 

 are photographically effective. Electric waves 

 in conductors also have been further investi- 

 gated. E. Sarasin and L. de la Rive (Paris 

 Academy of Sciences, March 31) conclude that 

 the velocity of such waves is sensibly the same 

 as in air. A. Elsass (" Annales de Physique," 

 Nov. 12, 1890) detects waves on a wire in open 

 circuit by hanging a telephone on it at success- 

 ive points. The source of electricity was a 

 single Daniell cell. Prof. Lodge (London Phys- 

 ical Society, June 12), in experiments on " reso- 

 nant Leyden jars," the discharge of one of which 

 precipitated that of others, " timed " wires to 

 respond to the oscillatory discharge of the jars. 

 When a thin wire was connected to the knob of 

 a jar and a parallel wire was joined to the outer 

 coating, the points of greatest vibration in the 

 wires were plainly shown by a glow. The iden- 

 tity of electro-magnetic radiation with light has 

 received new confirmation in the experiments of 

 L. Arons and H. Reubens, who find that Max- 

 well's relation, n 2 = /x, holds for four fluids (n 

 being the ratio of the velocity of the wave in 

 vacua to that in the given substance, and p being 

 its dielectric constant). This relation is a con- 

 sequence of the electro-magnetic theory of light. 

 C. V. Boys has continued his trials to make an 

 electric-radiation meter (" Annual Cyclopaedia," 

 1890, p. 716). With the aid of Prof. Fitzgerald 

 (" Philosophical Magazine," January) he showed 

 that the heating of a wire exposed to the radia- 

 tion was equal to that caused by the current 

 from one Daniell cell through an external resist- 

 ance of 115 ohms and a working resistance of 38 

 ohms, or about -01 of an ampere. Hertz himself 

 (" Lumiere Electrique," March 28) has studied 

 the mechanical action of his waves, the electric 

 force being measured by a cylinder of gilt 

 paper, and the magnetic force of a circular ring 

 of aluminum wire. He concludes that the mag- 

 netic and electric vibrations are at right angles, 

 and that the nodes of each coincide with the 

 antinodes of the other. 



Photo-electricity. Prof. G. M. Minchin (Lon- 

 don Physical Society, Jan. 16) has explained 

 at length the construction of his photo-elec- 

 tric cells ("Annual Cyclopanlia," 1890, p. 717). 

 In unsuccessful efforts to photograph distant 

 objects, he discovered that an electric current is 

 produced by the action of light on silver plates 

 coated with a collodion or gelatin emulsion of 

 a haloid silver salt, or with certain aniline dyes, 

 when immersed in a liquid, one plate only being 

 illuminated. The electromotive force was rarely 

 greater than one twentieth of a volt. Uncleaned 



