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XXXV. On Opacity, By Professor Oliver Lodge, D.Sc, 

 LL.D., PM.S., President of the Physical Society*. 



MY attention has recently been called to the subject of the 

 transmission of electromagnetic waves by conducting 

 dielectrics — in other words, to the opacity of imperfectly 

 conducting material to light. The question arose when an 

 attempt was being made to signal inductively through a 

 stratum of earth or sea, how far the intervening layers of 

 moderately conducting material were able to act as a screen ; 

 the question also arises in the transmission of Hertz waves 

 through partial conductors, and again in the transparency of 

 gold-leaf and other homogeneous substances to light. 



The earliest treatment of such subjects is due of course to 

 Clerk Maxwell thirty-four years ago, when, with unexampled 

 genius, he laid down the fundamental laws for the propagation 

 of electric waves in simple dielectrics, in crystalline media, and 

 in conducting media. He also realised there was some strong 

 aualogy between the transmission of such waves through space 

 and the transmission of pulses of current along a telegraph- 

 wire. But naturally at that early date not every detail of the 

 investigation was equally satisfactory and complete. 



Since that time, and using Maxwell as a basis, several 

 mathematicians have developed the theory further, and no 

 one with more comprehensive thoroughness than Mr. Oliver 

 Heaviside, who, as I have said before, has gone into these 

 matters with extraordinarily clear and far vision. I may 

 take the opportunity of calling or recalling to the notice of 

 the Society, as well as of myself, some of the simpler develop- 

 ments of Mr. Heaviside's theory and manner of unifying 

 phenomena and processes at first sight apparently different ; 

 but first I will deal with the better-known aspects of the 

 subject. 



Maxwell deals with the relation between conductivity and 

 opacity in his Art. 7b>8 and on practically to the end of that 

 famous chapter xx , (' Electromagnetic Theory of Light ' ) . He 

 discriminates, though not very explicitly or obtrusively, be- 

 tween the two extreme cases, (1) when inductive capacity or 

 electric inductivity is the dominant feature of the medium — 

 when, for instance, it is a slightly conducting dielectric, and 

 (2) the other extreme case, when conductivity is the pre- 

 dominant feature. 



* Communicated by the Physical Society of London, being the Presi- 

 dential Address for 1899. 



