30 Mr. 0. Heaviside. On the Transformation of [Jan. 18, 



moisture, or condensed gases, or to combinations of these causes. 

 And it affords an explanation of the details of reflection, which is 

 rigid, and at least as good as the representation given by the em- 

 pirical formulae of Cauchy, even as modified by Qnincke. 



VI. "On the Transformation of Optical Wave-Surfaces by 

 Homogeneous Strain." By OLIVER HEAVISIDE, F.R.S. 

 Received December 20, 1893. 



Simplex Eolotropy. 



1. All explanations of double refraction (proximate, not ultimate) 

 rest upon the hypothesis that the medium in which it occurs is so 

 structured as to impart eolotropy to one of the two properties, 

 associated with potential and kinetic energy, with which the ether is 

 endowed in order to account for the transmission of waves through 

 it in the simplest manner. It may be elastic eolotropy, or it may be 

 something equivalent to eolotropy as regards the density. In Max- 

 well's electromagnetic theory the two properties are those connecting 

 the electric force with the displacement, and the magnetic force with 

 the induction, say the permittivity and the inductivity, or c and fi. 

 These are, in the simplest case, constants corresponding to isotropy. 

 The existence of eolotropy as regards either of them will cause double 

 refraction. Then either c or ft is a symmetrical linear operator, or 

 dyadic, as Willard Gibbs calls it. In either case the optical wave- 

 surface is of the Fresnel type. In either case the fluxes displace- 

 ment and induction are perpendicular to one another and in a 

 wave-front, whilst the electric and magnetic forces are also per- 

 pendicular to one another. But it is the magnetic force that is 

 in the wave-front, coincident with the induction, in case of 

 magnetic isotropy and electric eolotropy, the electric force being 

 then out of the wave-front, though in the plane of the normal and 

 the displacement. And in the other extreme case of electric 

 isotropy and magnetic eolotropy, the electric force is in the wave- 

 front, coincident with the displacement, whilst the magnetic force 

 is out of the wave-front, though in the plane of the normal and 

 the induction. Now, as a matter of fact, crystals may be strongly 

 eolotropic electrically, whilst their magnetic eolotropy, if existent, is 

 insignificant. This, of course, justifies Maxwell's ascription of double 

 refraction to electric eolotropy. 



Properties connected tvith Duplex Eolotropy. 



2. "When duplex eolotropy, electric and magnetic, is admitted, we 

 obtain a more general kind of wave-surface, including the former two 



