NOVEWBEE 4, 1898.] 



SCIENCE. 



603 



though most elaborately and with many 

 optical refinements by Oliver Lodge at the 

 Oxford meeting in 1894. But not until 

 Signer Marconi induced the British post- 

 office and foreign governments to try large 

 scale experiments did wireless signalling 

 become generally and popularly known or 

 practically developed as a special kind of 

 telegraphy. Its feasibility depends on the 

 discovery of a singularly sensitive detector 

 for Hertz waves — a detector whose sensi- 

 tiveness in some cases seems almost to com- 

 pare with that of the eye itself. The fact 

 noticed by Oliver Lodge in 1889, that an 

 infinitesimal metallic gap subjected to an 

 electric jerk became conducting, so as to 

 complete an electric cii'cait, was rediscov- 

 ered soon afterwards in a more tangible 

 and definite form and applied to the detec- 

 tion of Hertz waves by M. E. Branly. 

 Oliver Lodge then continued the work, and 

 produced the vacuum filing-tube coherers with 

 automatic tapper-back, which are of ac- 

 knowledged practical service. It is this 

 varying continuity of contact under the in- 

 fluence of extremely feeble electric stimulus 

 alternating with mechanical tremor which, 

 in combination with the mode of producing 

 the waves revealed by Hertz, constitutes 

 the essential and fundamental feature of 

 ' wireless telegraphy.' There is a curious 

 and widely-spread misapprehension about 

 coherers, to the effect that to make a co- 

 herer work the wave must fall upon it. 

 Oliver Lodge has disproved this fallacy. 

 Let the wave fall on a suitable receiver, 

 such as a metallic wire, or, better still, on 

 an arrangement of metal wings resembling 

 a Hertz sender, and the waves set up oscil- 

 lating currents which may be led by wires 

 (enclosed in metal pipes) to the coherer. 

 The coherer acts apparently by a species of 

 end-impact of the oscillatory current, and 

 does not need to be attacked in the flank 

 by the waves themselves. This interesting 

 method of signalling — already developing 



in Marconi's hands into a successful prac- 

 tical system which inevitably will be largely 

 used in lighthouse and marine work — pre- 

 sents more analogy to optical signals by 

 flash-light than to what is usually under- 

 stood as electric telegraphy, notwithstand- 

 ing the fact that an ordinary Morse instru- 

 ment at one end responds to the movements 

 of a key at the other, or, as arranged by 

 Alexander Muirhead, a siphon recorder 

 responds to an automatic transmitter at 

 about the rate of slow cable telegraphy. 

 But although no apparent optical apparatus 

 is employed, it remains true that the im- 

 pulse travels from sender to receiver by 

 essentially the same process as that which 

 enables a flash of magnesium powder to 

 excite a distant eye. 



The phenomenon discovered by Zeeman, 

 that a source of radiation is afiected by a 

 strong magnetic field in such a way that 

 light of one refrangibility becomes divided 

 usually into three components, two of which 

 are displaced by diffraction analysis on 

 either side of the mean position and are op- 

 positely polarized to the third or residual 

 constituent, has been examined by many 

 observers in all countries. The phenomenon, 

 has been subjected to photography with 

 conspicuously successful results by Professor 

 T. Preston in Dublin and by Professor 

 Michelson and Dr. Ames and others in 

 America. 



It appears that the differeut lines in the 

 spectrum are differently afi'ected, some of 

 them being tripled with difierent grades of 

 relative intensity, some doubled, some quad- 

 rupled, some sextupled and some left un- 

 changed. Even the two components of the 

 D lines are not similarly influenced. More- 

 over, whereas the polarization is usually 

 such as to indicate that motions of a nega- 

 tive ion or electron constitute the source 

 of light, a few lines are stated by the ob- 

 servers at Baltimore, who used what they 

 call the ' small ' grating of five inches' width 



