292 Prof. Hertz on Rays of Electric Force. 



form. The height of the mirror thus became 2 metres, the 

 diameter of its aperture 1'2 metre, and its depth 0*7 metre. 

 The primary oscillation was adjusted in the centre of its focal 

 line. The wires by which the discharge was conducted were 

 allowed to traverse the mirror ; the inductorium and the accu- 

 mulators were placed behind the mirror, and produced no 

 disturbance. If, now, we examine the neighbourhood of the 

 vibration by means of our conductors, we find no action 

 behind the mirror and at one side in general ; but in the direc- 

 tion of the optic axis of the mirror the sparks are perceptible 

 to a distance of 5 or 6 metres. By placing a plane conducting 

 surface at right angles to the advancing wave the sparks were 

 perceptible near it to a much greater distance — as much as 9 

 or 10 metres. 



The waves reflected by the conducting surface strengthen 

 the direct waves at certain points. At other points, again, 

 the two waves weaken each other. With the straight con- 

 ductor we can recognize very distinct maxima and minima in 

 front of the plane conductor, and with the circular conductor 

 the interference phenomena characteristic of stationary waves, 

 which I have previously described. I was able to disinguish 

 four nodes which fell on the wall, at 33, at 65, and at 98 centim. 

 distance from it respectively. Hence we have 33 centim. 

 as a close approximation to the half wave-length of the waves 

 employed, and an oscillation-period of 1"1 thousand milhonths 

 of a second, assuming the velocity of light for the velocity of 

 radiation. In wires the oscillation had a wave-length of 

 29 centim. It seems, then, that also with these short waves 

 the velocity is somewhat less in wires than the velocity in 

 space ; but the ratio of the two velocities comes very near to 

 the theoretical value of 1, and does not deviate from it so 

 much as our experiments with longer waves made probable. 

 This remarkable phenomenon still requires explanation. Since 

 these phenomena are manifested especially in the neighbour- 

 hood of the optic axis of the mirror, we designate the result 

 as an electric ray issuing from the concave mirror. 



I now made a second concave mirror exactly similar to the 

 first, and placed the straight secondary conductor in it, so 

 that the two 50 centim. long wires fell in the focal line, but 

 the two wires leading to the spark-interval traversed the wall 

 of the mirror (being insulated from it) by the shortest route. 

 The spark-interval was thus close behind the mirror, and the 

 observer could adjust and observe it without interrupting the 

 course of the waves. I expected that on intercepting the ray 

 by this arrangement I should be able to recognize it at still 

 greater distances, and I found that I was not mistaken. In 



