JEtlier and Electrons, 703 



involves infinite velocity for those points o£ the medium 

 which at any given instant lie in the surface of discontinuity, 

 and is in fact characterized by an infinitely rapid shearing 

 motion of the medium just at the surface in question. It is 

 seen to be a direct kinematical consequence of the assumed 

 mobility that, as the surface of discontinuity is transferred to 

 new positions, there is a continual shearing asunder and re- 

 uniting of the medium in violation of the postulated " locked " 

 condition. Under special assumptions we may evade the 

 difficulty of impulsive displacements, if we suppose the motion 

 of the locked strain-figure to be such that the surface of dis- 

 continuity is moving purely along itself, but even in this 

 restricted case the locked condition would be violated. 



These are broadly the considerations advanced in support 

 of the contention that locked strain-figures are not in general 

 mobile ; and that the only self-sustaining distributions of 

 strain which are freely mobile are such as have been desig- 

 nated free strain-figures (see § 10). 



14. We must, however, examine Professor Larmor's setherial 

 strain-figure, or model electron, more closely before attempting 

 to decide as to its mobility. The aether is supposed to have 

 quasi-rigidity in virtue of which each element experiences a 

 restoring couple proportional to its small absolute angular 

 displacement. " Let, then, in the rotationally elastic medium 

 a narrow tubular channel be formed, say for simplicity a 

 straight channel AB of uniform section : suppose the walls 

 of this channel to be grasped, and rotated round the axis of 

 the tube . . . . : this rotation will be distributed through the 

 medium, and as the result there will be lines of rotational 

 displacement all starting from A and terminating at B : and 

 so long as the walls of the channel are held in this position 

 by extraneous force, A will be a positive electron in the 

 medium, and B will be the complementary negative one. 

 They will both disappear together when the walls of the 

 channel are released. But now suppose that before this 

 release the channel is filled up (except small vacuous nuclei 

 at A and B which will assume the spherical form) with a 

 plug of aether so as to be continuous with the surrounding 

 medium ; the effort of release in this surrounding medium 

 will rotate the plug slightly until it attains the state of equi- 

 librium in which the rotational elasticity of the new part 

 of the medium formed by the plug provides a balancing 

 torque, and the conditions all round A or B will finally be 

 symmetrical " *. 



A difficulty arises when we attempt to apply to electrons 

 * < ^Ether and Matter/ p. 326. 



