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viii unnoticed ones; but it would be a breach of scientific method to complicate its properties by any hypothesis, as distinct from logical development, beyond what is required for this purpose. It may therefore be held that, in so far as theories of the ultimate connexion of different physical agencies are allowed to be legitimate at all, they should develope along the lines of a purely electric aether until critics of such a simple scheme are able to point to a definite group of phenomena that require the assumption of a new set of properties and that moreover can be reduced to logical order thereby: a charge of incompleteness without indication of a better way, is not effective criticism in questions of this kind, because, owing to the imperfection of our perceptions and the limited range of our intellectual operations, finality can never be attained.

It appears to be not without value, as regards clearness and definiteness of view, that the conception of an elastic aethereal medium that had been originally evolved from consideration of purely optical phenomena, is capable of direct natural development so as to pass into line with the much wider and more recent electrodynaniic theory which was constructed by Maxwell on the basis of purely electrical phenomena, — in fact largely as the dynamical representation and development of Faraday's idea of a varying electrotonic state in space, determined by the changing lines of force.

In the following discussions some care has been taken to trace the connexion with the historical course of physical ideas. While there has been a steady gain in precision, the trend of fundamental physical speculation has always been much the same, and thus does not in its main features pass out of date: but owing to the rapid accumulation of new phenomena and the consequent necessity of formal treatises digesting the state of actual knowledge, there is less and less opportunity to become critically acquainted with the points of view of the original discoverers in physical science, and the general lines of thought in which the definite recorded advances are often only