494 The Bight Rev. Monsignor Gerald Molloy [Feb. 15, 



come at first rays of dark heat, like those emitted by a hot metal plate, 

 and then rays of light. 



The truth of Maxwell's theory was experimentally demonstrated 

 by Professor Heinrich Hertz, of Germany, in the year 1888, by a 

 series of researches almost unrivalled for their brilliancy and 

 thoroughness. His experiments, however, were only suitable to the 

 laboratory, and various modifications were necessary in order to 

 present the results in a sensible form before an audience. 



The apparatus he had provided for this purpose might be said 

 briefly to consist of two parts. At one end of the table was an 

 arrangement intended to produce the electric waves. It consisted of 

 an induction coil, and a pair of discharging rods ending in two brass 

 knobs. This part he would call the oscillator. At the other end of 

 the table was an arrangement intended to detect and reveal the 

 presence of the electric waves. This part, which he called the 

 resonator, was somewhat more complicated. First, there was a little 

 piece of apparatus called the coherer. The coherer, as they knew, 

 was a glass tube holding some metallic filings, which in their normal 

 condition were practically a non-conductor of electricity, but which, 

 when struck by electric waves, became a fairly good conductor. It 

 was here mounted in the circuit of a small battery and an electric 

 bell. In its normal condition, it opposed the passing of the current, 

 and the bell was silent ; when the electric waves arrived, it became 

 a conductor — the current passed and the bell was rung. Thus the 

 ringing of the bell would be a signal that the waves were there. A 

 gentle tap on the coherer would restore it to its former condition, 

 and it would be ready for a new experiment. 



The lecturer now produced a spark in the oscillator, and the bell 

 at once began to ring, proving that the electric waves had gone out 

 from the oscillator and travelled through space to the resonator. A 

 tap on the coherer reduced the bell to silence ; and an assistant now 

 proceeded to carry the resonator to various parts of the theatre. In 

 every position it responded instantaneously to the spark of the 

 oscillator ; thus showing that electric waves, like waves of light and 

 radiant heat, go out in all directions from the source of disturbance. 



In the next experiment, the lecturer used two parabolic brass 

 reflectors, which had been made many years ago for experiments on 

 radiant heat. In the focus of one, he mounted a very small oscil- 

 lator, which produced waves of some six or eight inches wave-length ; 

 while in the focus of the other he mounted a coherer in circuit with 

 a small battery and bell. He then repeated with the electric waves 

 produced in the oscillator, the well known experiment of the conjugate 

 mirrors, and thus proved that the electric waves are subject to the 

 same laws of reflection as waves of radiant heat and light. 



With the same apparatus, he now tested the transparency and 

 opacity of various bodies to electric waves — plate glass, sheet iron, 

 indiarubber, wood. In particular, he called attention to the curious 

 behaviour of plate glass with respect to waves of ether : it is very 



