Electrical Methods.
1. The nature of the connexion between ether and gross matter is one of the most striking physical problems which now appear ripe for solution, and as a preliminary to the undertaking of fresh experiments I propose to review the subject in order to realize our present position in connexion with it.
The subject may be attacked either optically or electrically. It first prominently presented itself historically in connexion with the earth's motion through space and the finite velocity of light; and it will be convenient to consider the matter first on this side, and to realize precisely what ought to be expected to happen on any simple hypothesis concerning the ether; working it out, however, inmost cases with accuracy,[1] and by no means ignoring as negligible small quantities of the second order (hundred-millionths), which Michelson has practically shown to be nowadays within the limits of highly-refined observation.
Necessary Hypotheses or Postulates.
2. There are one or two hypotheses regarding the ether so elementally obvious that they may be regarded as almost axiomatic, such as the following:—
(i.) In interstellar space the ether is free; that is, its properties in no way depend on the existence or motion of gross matter. It may, therefore, be called at rest. Whether it is in absolute rest or not appears to be a question which can hardly be put into an intelligible shape. If it be moving relatively to itself, we have in those regions no obvious means of ascertaining the fact. But just as it is natural to assume that its properties in free space are uniform, so it is natural to assume that its motion there, whatever it is, is perfectly uniform, and it may be defined as absolute rest. When I speak of the ether anywhere as "stationary," I mean stationary with respect
- ↑ Whenever equations are approximate only, the symbol is used instead of the symbol
