THE MODERN THEORY OF LIGHT. 445 



retina is iiicouipeteut to respond to tbese vibrations — they are too slow. 

 Tiie vibrations set np when this large jar is discharged are from a hun- 

 dred thousand to a million per second, but that is too slow for the retina. 

 It responds only to vibrations between 400 billion and 800 billion 

 per second. The vibrations are too quick for the ear, which responds 

 only to vibrations between 40 and 40,000 per second. Between the 

 highest audible and the lowest visible vibrations there has been hither- 

 to a great gap, which these electric oscillations go far to fill up. There 

 has been a great gap simply because we liave no intermediate sense 

 organ to detect rates of vibration between 40,000 and 400,000,000,- 

 000,000 per second. It was therefore an unexplored territory. Waves 

 have been there all the time in any quantity, but we have not thought 

 J about them nor attended to them. 



It happens that I have myself succeeded in getting electric oscilla- 

 tions so slow as to be audible, the lowest I have got at present are 125 

 per second, and for some way above this the sparks emit a musical 

 note: but no one has yet succeeded in directly making electric oscilla- 

 tions that are visible, — though indirectly every one does it when he 

 lights a candle. 



Here however is an electric oscillator which vibrates 300 million 

 times a second, and emits a'therial waves a yard long. The whole range 

 of vibrations between musical tones and some thousand million per 

 second, is now filled up. 



These electro-magnetic waves have long been known on the side of 

 theory, but interest in them has been immensely quickened by the dis- 

 covery of a receiver or detector for them. The great though simple 

 discovery b^' Hertz of an ''electric eye," as Sir W. Thomson calls it, 

 makes experiments on these waves for the first time easy or even jios- 

 sible. We have now a sort of artificial sense organ for their apprecia- 

 tion, — an electric arrangement which can virtually ''see" these inter- 

 mediate rates of vibration. 



The Hertz receiver is the simi)lest thing in the world; nothing but a 

 bit of wire or a pair of bits of wire adjusted so that when immersed in 

 strong electric radiation they give minute sparks across a microscopic 

 air gap. 



The receiver 1 have here is adapted for the yard-long waves emitted 

 from this small oscillator; but for the far longer waves emitted by a 

 discharging Leyden jar an excellent receiver is a. gilt wall-paper or 

 other interrupted metallic surface. The waves falling u])on the metallic 

 surface are reflected, and in the act of reflexion excite electric currents, 

 which cause sparks. Similarly, gigantic solar waves may produce 

 aurora' ; tuid minute waves from a candle do electrically disturb the 

 retina. 



The smaller waves are however far the most interesting and the 

 most tractable to ordinary optical experiments. From a small oscilhi- 

 tor, which may be a couple of small cylinders kept sparking into each 



