Theoretical Optics since 1840. 541 



left it, is not mechanical. Electric displacement and mag- 

 netic force are vector quantities which accompany each other 

 in a changing electric field. They satisfy certain equations ; 

 and it follows from these, and the result is verified by experi- 

 ment, that they are propagated according to the same laws as 

 light. It is reasonable to suppose that the periodic dis- 

 turbance which constitutes light is very intimately connected 

 with one or other of these ; the supposition that it is identical 

 with Maxwell's electric displacement leads to consequences 

 consistent with fact, and, indeed, in the able hands of those 

 who have developed the theory has been the fruitful means 

 of correlating many varied phenomena ; but it does not tell us 

 what electric displacement is, or how it is related to the 

 movements of the aether ; neither does it enlighten us as to 

 the structure and mechanical properties of the aether, beyond 

 the simple fact that in the aether transverse waves only are 

 propagated, no forces can be called into play which tend to 

 set up a pressural wave. Maxwell himself attempted to 

 formulate a mechanical model of the aether, and to some 

 extent succeeded. Lord Kelvin, so fertile in his thoughts, 

 has made various suggestions, we will return to one later. To- 

 day the electron theory of electricity, thanks mainly to the 

 brilliant work of Stokes'' Cambridge colleague, J. J. Thomson, 

 holds the field ; but the relation of the electron to the aether 

 and the mechanism by which electrons produce aether waves 

 have yet to be discovered. 



Larmor's suggestion that the flow of aether constitutes mag- 

 netic force, while a twist in an aether endowed with rotational 

 elasticity produces electric displacement, forms perhaps the 

 most consistent picture of the process which we possess. 



Lord Kelvin, indeed, in 1888 suggested a structure for 

 the aether which allows of a homogeneous mechanical account 

 of optical phenomena being given. 



On this view the resistance to compression of the aether is 

 negative, if free it would collapse, but the necessary stability 

 is given by the supposition that it is fixed at the boundaries; 

 it is a structure like a collection of soap-films stretched across 

 a wire framework ; if the connexion be broken the whole 

 collapses, so long as it remains the system can propagate 

 transverse waves. With such an aether there is no difficulty 

 in giving a consistent account of Optics, but it is difficult to 

 imagine that the aether has such properties. I believe, how- 

 ever, that Lord Kelvin now thinks that a slight modification 

 of his original hypothesis will lead to the same result so tar 

 as optics are concerned, but will enable him to get over the 

 difficulty of postulating fixed boundaries. 



Phil. Mag. S. 6. Vol 5. No. 29. May 1903. 2 



