Page:Aether and Matter, 1900.djvu/223

 Michelson, above discussed, which has hitherto stood by itself as the only quantitative observational evidence that has a bearing on this question. It can be said on the other side that this view of aethereal action does not directly cover gravitational phenomena, unless the rather artificial pulsatory theory of gravity is allowed. But there is another aspect of the matter. The equations of the free aether, as revealed by MacCullagh's optical analysis, are linear equations: they in fact must be so if all kinds of radiations are to travel with the same speed in the celestial spaces. In Maxwell's hands, equivalent relations with the appropriate generalization were arrived at on the electric side, and formed a basis for the explanation of the whole plexus of electrodynamic and optical phenomena. Further theoretical discussion has in all directions tended to widen the scope and enhance the inherent simplicity of this scheme. The question arises whether there is anything to gainsay a view that this simple linear scheme is only the first approximation, a very close one however, to an analytical specification of the aether: just as the linear scheme of equations of the theory of propagation of sound covers the whole of the phenomena of acoustics, although in arriving at those equations from the dynamics of the atmosphere all terms involving the square of the ratio of the velocity of the actual aereal disturbance to the velocity of its propagation are neglected, for the reason that their consequences are outside the limits of observation in that domain. Why then should not relatively minute phenomena like gravitation be involved in similar non-linear terms, or terms involving differentials of higher orders, in the analytical specification of the free aether, which are as insignificant compared with the main fully ascertained linear terms as is the gravitation between two electric systems compared with their mutual electric forces? Against this there is a subjective reluctance to disturb the ideal simplicity of the aethereal scheme: but there is no help for that if its content is not sufficiently extensive for the facts. Of more weight is the circumstance that a train of radiation from a distant star would change its form as it advanced across