1890.] on Electromagnetic Radiation. 79 



placed in the focal line of a cylindrical parabolic mirror, and its re- 

 ceiving wires were connected with the wires leading to the galvano- 

 meter by some very fine brass wire. With the large sized generator 

 and receiver, which were placed abont 3 metres apart, it was shown 

 that the sparking was stopped by placing a thin zinc sheet so as to 

 reflect the radiations from a point close behind the receiver. By 

 means of a long indiarubber tube hung from the ceiling it was shown 

 how, when waves are propagated to a point w^hence they are reflected, 

 the direct and reflected waves interfering produce a system of loops 

 and nodes, with a node at the reflecting point. It was explained that 

 these nodes, though places of zero displacement, were places of 

 maximum rotation, and that the axis of rotation was at right angles 

 to the direction of displacement. It was explained that an analogous 

 state of affairs existed in the electromagnetic vibrations. If the 

 electric force be taken as analogous to the displacement of the rope, 

 the magnetic may be taken as analogous to its rotation, and the two 

 are at right angles to one another. In the ether the electric node is 

 a magnetic loop, and vice versa. Though the two are separated in 

 loops and nodes, they exist simultaneously in a simple wave propaga- 

 tion, just as in a rope when propagating waves in one direction the 

 crest of maximum displacement is also that of maximum rotation. 

 It was explained that by placing the reflector at a quarter of a wave- 

 length from the receiver this would be at an electric loop, and have 

 its sparking increased. It may thus be shown that tliere are a series 

 of loo]3S and nodes produced by reflection of these electromagnetic 

 forces, like those produced in any other case of reflected wave- 

 propagation. This was Hertz's fundamental experiment, by which 

 he proved that electromagnetic actions are propagated in time, and 

 by some approximate calculations he verified Maxwell's theory that 

 the rate of propagation is the same as that of light. It follows that 

 the luminiferous ether is experimentally shown to be the medium to 

 which electric and magnetic actions are due, and that the electro- 

 magnetic waves we have been studying are really only very long 

 light waves. 



A rather interesting deduction from Maxwell's theory is that light 

 incident on any body that absorbs or reflects it should press upon it 

 and tend to move it away from the source of light. Illustrating this, 

 an experiment was shown with an alternating current passing through 

 an electro-magnet, in front of which a good conducting plate of silver 

 was suspended. When the alternating current was turned on the 

 silver was repelled. It was explained that as the silver could only 

 be afiected by what was going on in its own neighbourhood, and that 

 if sufiiciently powerful radiations from a distant source were fallino- 

 on the silver, it would be acted on by alternating magnetic forces 

 this experiment was in effect an experiment on the rej)ulsion of light, 

 which was too small to have been yet observed, even in the case of 

 concentrated sunshine. These slow vibrations are not stopped by a 

 sheet of zinc, though much reduced by a magnetic sheet like tin- 



