1893.] the Electric and Luminiferous Medium. 459 



merits examination ; we might conceive the opacity at the surface 

 to be so great that a sensible part of the light is lost before it has 

 penetrated more than a very small fraction of a wave-length. In the 

 extreme case of electric waves of finite length reflected from metals, 

 the absorption is complete in a very small fraction of the wave- 

 length, and the result is total reflexion, as from a vacuum ; on the 

 other hand, if the opacity is but slight, the phenomena ought to 

 agree approximately with those of transparent media. It seems 

 worth while to examine the consequences of assuming that the optical 

 phenomena of metallic reflexion are nearer the first of these limiting 

 cases than the second. It seems worth while also to compare the 

 facts for some medium not so opaque as metals with the formulas of 

 Cauchy and MacCullagh ; the examples of tourmaline crystal, and 

 some of the aniline dyes which exhibit selective absorption, suggest 

 themselves as affording crucial tests.* 



The considerations which have here been explained amount to an 

 attempt to extend the regions of contact between three ultimate 

 theories which have all been already widely developed, but in such a 

 way as not to have much connexion with one another. These 

 theories are Maxwell's theory of electric phenomena, including 

 Ampere's theory of magnetism and involving an electric theory of 

 light, Lord Kelvin's vortex-atom theory of matter, and the purely 

 dynamical theories of light and radiation that have been proposed by 

 Green, MacCullagh, and other authors. It is hoped that a sufficient 

 basis of connexion between them has been made out, to justify a re- 

 statement of the whole theory of the kind here attempted, notwith- 

 standing such errors or misconceptions on points of detail as will 

 unavoidably be involved in it. 



[While writing this summary it had escaped my memory that Lord 

 Kelvin has proposed a gyrostatic adynamic medium which forms an 

 exact representation of a rotationally elastic medium such as has 

 been here described. f If the spinning bodies are imbedded in the 

 aether so as to partake fully in its motion, the rotational forcive due 

 to them is proportional jointly to the angular momentum of a gyro- 

 stat and the angular velocity of the element of the medium, in accord- 

 ance with what is stated above. But if we consider the rotators to 

 be free gyrostats of the FoucauJt type, mounted on gymbals of which 

 the outer frame is carried by the medium, there will also come into play 

 a steady rotatory forcive, proportional jointly to the square of the an- 



* [An alternative view, in many respects preferable, is supplied by the assumption y 

 with Sir George Stokes, of the existence in metals of an adamantine property, such 

 as was discovered by Airy for the diamond. Cf. Sir GK GK Stokes, ' Proc. Roy. 

 Soc.,' February, 1883.] 



t Lord Kelvin (Sir W. Thomson), c Comptes Rendus,' Sept. 16, 1889; 'Col- 

 lected Papers,' vol. 3, 1890, p. 467. 



