[ 48 ] 



VII. On Electric Radiation and its Concentration by Lenses. 

 By Prof. Oliver J. Lodge, D.Sc, F.R.S., and James 

 L. Howard, D.Sc* 



Introduction by Dr. Lodge. 



IN making exact optical experiments on electric radiation 

 it is necessary to be able to converge it and throw a beam 

 of it in any desired direction. To do this by means of mirrors 

 is possible, but not always very convenient. Prof. Fitzgerald 

 and Mr. Troutonf have related the difficulty they at first 

 found in making concave mirrors work ; and we experienced 

 the same difficulty, intensified probably in our case by the 

 fact that we tried to work with everything on an extra small 

 scale — half the linear dimensions of Hertz. 



It is much easier to work with a large oscillator than 

 a small one, because the same extraordinary suddenness 

 in starting the oscillations is not then essential ; only with 

 large waves, mirrors and everything have to be heroic to 

 match, and our laboratory was not big enough for optical 

 experiments on gigantic waves. Electrical experiments on 

 such waves I have made in large numbers, obtaining them 

 originally by means of discharging Leyden jars, but recently 

 sometimes by a gigantic Hertz oscillator consisting of a pair 

 of copper plates, each consisting of a couple of commercial 

 sheets soldered together and rimmed round with wire, con- 

 nected by a length of No. copper wire interrupted in the 

 middle by a couple of large knobs. The plates and con- 

 necting-rod are hung from a high gallery, so that everything 

 occupies one plane, their distance and dimensions being here 

 shown. 



Fig. 1. — Large Oscillator used for violent and distant effects. Scale J^. 



■o o 



Plates 120 centim. square. Knobs 3*2 centim. diameter. 



Each rod 230 centim. long and 8 millim. diameter. 

 Spark-gap about 1*5 centim. 



* Communicated by the Physical Society : read May 11, 1889. 

 f ' Nature/ vol. xxxix. p. 391. 



