hertz's researches on electrical waves. 



195 



Ordiuary masonry walls were found to be transparent to radiation of 

 this wave-length (that is, of about 10 meters), and some visitors to the 

 opening meeting of the Dublin University Experimental Society, last 

 November, were much astonished by seeing the spaiking of the resonating 

 circle out in the College Park, while the vibrator was in the laboratory. 



Attempts were tirst made last December to obtain retlection from the 

 surface of a non-conductor, with the hope of deciding by direct experi- 

 ment whether the magnetic or electric disturbance was in the plane of 

 polarization; that is, to find out whether the " axis of the vibrator" 

 should be at right angles to the plane of reflection or in it, when at the 

 polarizing angle, for obtaining a reflected radiation. It is to be ob- 

 served that in these radiations the electric vibration is parallel to the 

 "axis of the vibrator" while the magnetic is perpendicular to it, and 

 that they are consequently polarized in the same sense as light is said 

 to be polarized. 



Two large glass doors were taken down and used for this puri)ose, 

 but without success; and until lately, when reflection from a wall was 

 tried, the experiment seemed unlikely to be successful. 



In working with the glass plate, the resonator circle was first placed 

 so that the " vibrator" had no effect on it. Then the glass plate was 

 carried into position for reflection, but without result, though even the 

 reflection from the attendants moving it was amply sufdcient to be 

 easily detected. 



To obviate a difficulty arising from the fact that the wave was diverg- 

 ent, we decided to try Hertz's cylindrical parabolic mirrors, for concen- 

 trating the radiation. Two of these were made with sheets of zinc 

 nailed to wooden frames, cut to the parabolic shape required. 



In the " focal line" (which was made 12.5 centimeters from the vertex) 

 of one of these, a "vibrator" was placed, consisting of ^ ^ 

 two brass cylinders in line, each about 12 centimeters 

 long and 3 centimeters in diameter, rounded at the spark- 

 ing ends. 



In order that the " resonator" wire may lie in the " focal 

 line" of the receiving mirror, it has to be straight; this 

 necessitates having two of them. They each consist of 

 a thick wire 50 centimeters long, lying in the " focal line," 

 and of a thin wire, 15 centimeters long, attached to one 

 end at right angles, and which ])asses out to the back of 

 the mirror through a hole in the zinc, where the sparking 

 can be viewed, without obstructing the radiation in front. 

 The total length of each "resonator" is about two wave- 

 lengths, the wave-length being about 33 centimeters, so 

 that it may be that there are two vibrating segments in 

 each of these " resonators." 



With this apparatus it is possible to deal with definite 

 angles of incidence. No eftVct was obtained with glass plates using 



Fig. 3.— Plan. 



