One of the most obvious difficulties in the way of the ether-theory is that the planets, and even the atoms, move through space as though it were absolutely empty. According to modern ideas, however, the atom may be only an aggre gate of smaller "electrons," each of which may transpire to be nothing but a state of strain in the ether; and if this proves to be the case, we are certainly not in position at present to say that the ether would oppose in the slightest de gree the transmission of such a state of strain through its own substance. The difficulty with the theory of aberration is more formidable. If a shower of rain is falling vertically, the drops will appear to an observer to descend ver tically so long as he remains stationary:. If he moves forward, however, the drops will strike him in the face and will therefore appear, to him, to come from some point slightly- in ad vance of the zenith, rather than from the zenith itself. A similar phenomenon is observed in connection with light and is known as aberra tion. Every star is seen in its true position when the earth is moving directly toward it; but three months later, when the earth is mov ing at right angles to this directioni the observ er's telescope will have to be inclined slightly toward the direction in which the earth is mov ing, in order that the light from the star may come down through the instrument centrally. The maximum displacement that a star can have, from this cause, is known by observation to be about 20.47 seconds of arc on the heavens. If the ether were motionless, the analogy with the raindrops would be perfect, and the °con stant of aberration,° whose value has just been given, could be calculated from the lcnown ve locity of light, and the known velocity of the earth's orbital motion. It is found, however, that the theory of aberration is exceedingly complicated when the possibility of currents in the ether is admitted, and hence physicists have been much concerned to know whether or not the earth drags the adjacent ether along with it, in its motion around the sun. As long ago as 1859 Fireau showed, by a justly celebrated ex periment, that the ether is apparently dragged along by a current of water flowing through a tube; and Michelson and Morley have since shown, by an even more ingenious experiment, that there is evidence that the ether in the immediate vicinity of the earth participates in the earth's motion to such an extent that any difference that may exist does not amount to the twentieth part of the whole motion. Lodge, on the other hand. found no evidence of any "ether drag° in the space between two rapidly whirled steel plates that were separated by an interval of one inch. (Consult Preston, (Theory of
Light)). The whole subject of the "drag° of the ether is still unsettled; but the observed value of the constant of aberration appears to require that the ether is not disturbed by the motion of the earth through it.
As indicative of confused condition of thought in regard to the ether, even among the most illustrious scientific minds, the following quota,tions are appended. Sir William Thomp son (Lord Kelvin) says: "The luminiferous ether is an elastic solid, for which the nearest analogy I can give you is this jelly which you see.° Fitzgerald remarks upon this, "I cannot conclude without protesting strongly against Sir William Thompson's speaking of the ether as like a jelly.° Alfred Sang remarks, °Some of the most eminent physicists have adopted the view that the universal medium must be solid. We are asked to conceive our planet moying at the rate of 18 miles per second through it, and, what is still more incredible, that this takes place without any friction." Sir William Ram sey says, °It is almost universally held that all phenomena are 'mechanical,' that is, they are the result of matter in motion, and can be pic tured to the mind in a concrete form ; that some kind of 'machine' can be imagined which, if it existed, would reproduce the phenomena in question.' And, further, "It has not yet been found possible to think out a structure and mode of motion of the ether which will explain or make it possible to realize as a land of machine, all the phenomena in which the ether appears to play a part" J. Clerk Maxwell offers the hypothesis that the constitution of ether is made up of elastic centres or vortices in close proximity, but goes on to say, "No the ory of the constitution of the ether has yet been invented which will account for such a system of molecular vortices being maintained for an indefinite time." It has more recently been postulated of the ether (Erwin 1916), that it is structureless, in compressible, motionless, but capable of taking on motion, non-elastic, and capable of indefinite subdivision, and that the subdivided parts can be moved over each other without friction. On the other hand such eminent physicists as Ein stein, Ritz and Poincare deny the necessity for any such suppositional substance as the ether. It is a fact that scientific inquiry is attacking this and similar problems along quite a differ ent line, upon the hypothesis that matter has no existence otherwise than as a mode of ma Lion Consult Erwin, M., 'The Universe and the Atom' (New York 1916) ; Larmor, J., (yEther and Matter' (Cambridge, Mass. 1900) ; Pres ton, S. J., 'Physics of the Ether' (London 1875).