which enables us to see the hidden object without the aid of photography by allowing the rays to fall on a plate coated with one of these phosphorescent substances.  Already the new method has been applied by doctors in examining malformations and diseases of the bones or internal organs, and in localising and extracting bullets, needles, or other foreign matters in the body.  There is little doubt that it will be very useful as an adjunct to hospitals, especially in warfare, and, if the apparatus can be reduced in size, it will be employed by ordinary practitioners.  It has also been used to photograph the skeleton of a mummy, and to detect true from artificial gems.  However, one cannot now easily predict its future value, and applications will be found out one after another as time goes on.

CHAPTER X.

The wireless telegraph.

Magnetic waves generated in the ether (see pp. 53-95) by an electric current flowing in a conductor are not the only waves which can be set up in it by aid of electricity.  A merely stationary or “static” charge of electricity on a body, say a brass ball, can also disturb the ether; and if the strength of the charge is varied, ether oscillations or waves are excited.  A simple way of producing these “electric waves” in the ether is to vary the strength of charge by drawing sparks from the charged body.  Of course this can be done according to the Morse code; and as the waves after travelling through the ether with the speed of light are capable of influencing conductors at a distance, it is easy to see that signals can be sent in this way.  The first to do so in a practical manner was Signer Marconi, a young Italian hitherto unknown to fame.  In carrying out his invention, Marconi made use of facts well known to theoretical electricians, one of whom, Dr, Oliver J. Lodge, had even sent signals with them in 1894; but it often happens in science as in literature that the recognised professors, the men who seem to have everything in their favour—­knowledge, even talent—­the men whom most people would expect to give us an original discovery or invention, are beaten by an outsider whom nobody heard of, who had neither learning, leisure, nor apparatus, but what he could pick up for himself.

Marconi produces his waves in the ether by electric sparks passing between four brass balls, a device of Professor Righi, following the classical experiments of Heinrich Hertz.  The balls are electrified by connecting them to the well-known instrument called an induction coil, sometimes used by physicians to administer gentle shocks to invalids; and as the working of the coil is started and stopped by an ordinary telegraph key for interrupting the electric current, the sparking can be controlled according to the Morse code.  In our diagram, which explains the apparatus, the four balls are seen at D, the inner and larger pair being partly immersed in vaseline oil, the outer