398 Dr. Oliver Lodge on Opacity. 



tion does no serious harm. Articulation depends on the 

 features of the wave, and the preservation of the features 

 demands, by Fourier's analysis, the transmission of every 

 frequency at the same rate. 



But now suppose any cause diminishes one of the two 

 fields without diminishing the other : for instance, let the 

 electric field be weakened by leakage alone, or let the mag- 

 netic field be weakened by wire-resistance alone, then what 

 happens ? The preservation of E and the diminution of H, 

 to take the latter — the ordinary — case, may be regarded as 

 a superposition on the advancing wave of a gradually growing 

 reverse field of intensity SH ; and, by the relation E = /LtuH, 

 this reversed field, for whatever it is worth, must mean a 

 gradually growing wave travelling in the reverse direction. 



The ordinary wave is now no longer left alone and un- 

 injured, it has superposed upon itself a more or less strong 

 reflected wave, a reflected wave which constantly increases 

 in intensity as the distance along the cable, or the penetration 

 of the wave into a conducting medium, increases ; all the 

 elementary reflected waves get mixed up by re-reflexion in 

 the rear, constituting what Mr. Heaviside calls a diffusive 

 "tail" ; and this accumulation of reflected waves it is which 

 constitutes what is known as " distortion " in cables, and 

 what is known as "opacity" inside conducting dielectrics. 



There is another kind of opacity, a kind due to hetero- 

 geneousness, not connected with conductivity h\x' v due merely 

 to a change in the constants K and //,, — properly a kind of 

 translucency, a scattering but not a dissipation of energy, — 

 like the opacity of foam or ground glass. 



This kind of opacity is an affair of boundaries and not of 

 the medium itself, but after all, as we now see, it has features 

 by no means altogether dissimilar to the truer kind of opacity. 

 Conducting opacity is due to reflexion, translucent opacity is 

 due to reflexion, — to irregular reflexion as it is called, but of 

 course there is nothing irregular about the reflexion, it is 

 only the distribution of boundaries which is complicated, the 

 reflexion is as simple as ever ; — except, indeed, to some 

 extent when the size of the scattering particles has to be 

 taken into account and the blue of the sky emerges. But 

 my point is that this kind of opacity also is after all of the 

 reflexion kind, and the gradual destruction of the advancing 

 Avave — whether it be by dust in the air or, as Lord Bayleigh 

 now suggests, perhaps by the discrete molecules themselves, 

 by the same molecular property as causes refraction and dis- 

 persion — must result in a minute distortion and a mode of 

 wave propagation not wholly different from cable-signalling 



