80ME OF THE LATEST ACHIEVEMENTS OF SCIENCE. 145 



more than one meeting- of the l^ritish Association, thouo'h most elabo- 

 rately and with many optical retinements by Oliver Lodge at the 

 Oxford meeting in 189-i. But not until Sig. Marconi induced the 

 British post-ofhce and foreign governments to try large scale experi- 

 ments did wireless signaling become generally and popularly known 

 or practically developed as a special kind of telegraphy. Its feasi- 

 bility depends on the discovery of a singularly sensitive detector for 

 Hertz waves — a detector whose sensitiveness in some cases seems 

 almost to compare 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 cir- 

 cuit, was rediscovered soon afterwards in a more tangible and definite 

 form and applied to the detection 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 acknowl- 

 edged practical service. It is this varying continuity of contact 

 under the influence of extremely feeble electric stiir.ulus alternating 

 with mechanical ti'emor which, in combination with the mode of pro- 

 ducing the waves revealed by Hertz, constitutes the essential and 

 fiuidamental feature of ''wireless telegraphy." There is a curious 

 and widely spread misapprehension about coherers to the effect that to 

 make a cohei-er work the wave must fall upon it. Oliver Lodge has 

 disproved this fallacy. Let the wave fall on a suital^le receiver, such 

 as a metallic wire, or, better still, on an arrangement of metal wings 

 I'esembling a Hertz sender, and the waves set up oscillating currents 

 which may be led b}^ wires (inclosed in metal pipes) to the coherer. 

 The coherer acts apparently by a species of end impact of the oscilla- 

 tory current, and does not need to l)e attacked \n the flank l)y the waves 

 themselves. This interesting method of signaling — already developing 

 in Marconi's hands into a successful practical system which inevitably 

 will be largely used in light-house and marine work — presents more 

 analogy to optical signals by flash light than to Avhat is usually under- 

 stood as electric telegraphy, notwithstanding the fact that an ordinary 

 Morse instrument at one end responds to the movements of a key at 

 the other, or, as arranged b}' 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 impulse 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 ))v Zeeman. that a source of radiation 

 is affected by a strong magnetic field in such a way that light of one 

 refrangil)ility becomes divided usually into three components, two of 

 which are displaced by diffraction analysis on either side of the mean 

 position and are oppositely polarized to the third or residual con- 

 SM 99 10 



