50 



Dr. 0. Lodge on Electric Radiation. 



give waves from a foot to a yard in length ; and after reading 

 Hertz's experiment of the pitch prism*, I made preparations 

 for casting some great lenses that should give, I hoped, easy 

 concentration of such waves. 



Paraffin was a natural substance to use ; but it is rather 

 expensive, and has not a very high index. After consider- 

 ing many substances — beeswax, sulphur, &c. — I decided to 

 try resin, and laid in a stock of that substance. Meanwhile, 

 to gain experience in casting, and finding that a common 

 class of pitch could be obtained at an absurdly low price, 

 I procured several casks of the commonest pitch also. I 

 did not contemplate using this substance at first because 

 I feared it would be an imperfect insulator, and there seemed 

 no use in permitting any dissipation of energy whatever, so 

 long as one could get perfectly transparent substances. 



On casting a specimen of the pitch, however, it was found 

 so strongly insulating as nearly to fling off the leaves of a 

 gold-leaf electroscope it was brought near. It seems, there- 

 fore, an excellent cheap stuff for electrophorus and such like 

 use, wherever it is not expected to be strictly solid ; and it 

 can hardly help being transparent except to very little waves. 



Meanwhile we had calculated that to receive rays from one 

 point and convey them to another without aberration, a pair 

 Of piano-hyperbolic lenses were very suitable ; a parallel 

 beam being transmitted from one lens to the other. The 

 lenses would naturally be made cylindrical, instead of sphe- 

 rical, to suit the linear form of radiator. 



The optical calculation of a lens free from aberration for 



Fig. 2. 



one special point, S, from which it is to receive rays and emit 



* Wied. Ann. xxxvi. p. 769 (1889) ; translated in Phil. Mag. April 



1889. 



