60i REPORT— 1895. 



say a very rough idea, because the actual state of motion in a ring vortex or a 

 Hill's vortex is not quite so simple as the analogy might lead one to think. 



Now a compound vortex atom of this kind is just what we want to produce 

 rotation of the plane of polarisation of light. The light passing through such a 

 vortex has the direction of vibration twisted in the wave front. In ordinary 

 matter no such rotation is produced, because the various atoms are inditferently 

 directed, and they neutralise each other's eii'ects. Let, however, a magnetic field be 

 produced, and they will range themselves so that, on the average, the primary ' 

 circulations through the apertures will point in the direction of the field. Conse- 

 quently the average direction of the secondary spin will be in planes perpendicular 

 to this, and will rotate the plane of polarisation of any light whose wave front 

 passes them. The rotation is produced onlj- on the light which is transmitted thmigh 

 the vortex. The rotation observed is a resultant etl'ect. In fact it is clear that in 

 the case of refraction the optical media belong to the type in which every portion 

 transmits the light, and not to the type in which refraction is produced by opaque 

 bodies embedded in the ether. Tlie atoms are only opaque if they contain vacuous 

 cores. The question of the grip of the particles on the ether does not enter, but 

 dilference of quality — showing itself in refraction and dispersion — is due to difference 

 in average rotational quasi-elasticity produced by the atomic circulations, and 

 possibly absorption is due to precessional and nutational motion set up by the 

 secondary spins. These, however, are perhaps rather vague speculations. 



Instead of attempting to invent ethers, to deduce their properties from tbeir 

 specifications, and then seeing whether tbey fit in with experience, we maj' begin 

 half way. "\Ve may assume difierent forms for the function giving the energy of 

 the medium when disturbed, apply general dynamical methods, and distinguish, 

 between tho.se which are capable of explaining the phenomena we are investigating 

 and those which are not. Invention is then called upon to devise a medium for 

 which the desired energy-function is appropriate. This was the method applied by 

 MacCullagh to the luminiferous ether. He obtained an algebraical form of the 

 energy function which completely satisfied the conditions for a luminiferous ether; 

 its essentiiil property being that the energy depended only on the rotational dis- 

 placements of its small parts. He was unable, however, to picture a stable material 

 medium which would possess this property. We recognise now that such a 

 medium is possible if the rotational rigidity is produced by intrinsic motions in the 

 small parts of the medium of a gyrostatic nature. In a most masterly manner 

 Larmor- has recently investigated by general dynamical methods the possibility of 

 explaining electric and magnetic phenomena by means of the same energy function. 

 Electric lines of force are rotational filaments in the ether,^ similar in fact to those 

 I suggested at Bath, whilst a magnetic field consists of a fiow of the ether. The 

 same difficulty in accounting for electro-dynamic induction arises, but the general 

 form of the equations for the electro-dynamic and magnetic fields are the same as 

 those generally received. 



Towards the end of this ])aper he is led to postulate a theory of electrons whose 

 convection through the ether constitutes an electric current. Two rotating round 

 each other are supposed to produce the same efiect as a vortex ring. The mass of 

 ordinary matter is attributed to the electric inertia of these electrons. The electrori 

 itself is a centre or nucleus of rotational strain. If I express a doubt as to the 

 possibility of the existence of these nuclei as specified, I do so with great diffidence.* 



' ' Primary ' refers to the motion as usually understood ; ' secondary,' to the super- 

 posed, as explained above. 



- 'A Dynamical Theory of the Electric and Luminiferous Medium, 'P/((7. Trans. ,\9<%i. 



^ The necessity that the filaments shall be in pairs docs not seem to lie recognised. 

 This is, however, essential. Moreover, if the complementary circulations of the filaments 

 between (say) a plate condenser be placed otlierwhere than in the same region, the 

 filaments between the plates must rotate as a whole ; that is, an electric tield would 

 always be combined with a magnetic one. 



* It would appear that the same results would flow if two particles oppositely 

 electrified — i.e. joined by two complementar)' filaments, as already described — were to 

 rotate round each other. 



