satisfied. This way of surmounting the discrepancies is therefore, on the very threshold of our present wider survey, illusory. Were it not so, it would only be necessary to proceed a step farther in order to encounter fresh difficulties. If the aether were carried on bodily by the Earth, we must assume that the aether very near a mass moving along the Earth's surface is at any rate partially carried along by that mass. This point has been tested directly with great precision by Lodge[1], who tried to detect whether the aether between two whirling steel discs partook to any extent in their motion; and the result has been decisively negative. The only possibility of escape from this result, that the aether is not carried on by the Earth's motion, would be in an assumption that the large mass of the Earth controls wholly the motion of the aether in its neighbourhood somehow as it does gravitation, so that the smaller mass of the rotating discs is inoperative in comparison. In any case the former difficulty remains decisive: we might indeed be tempted to replace the absolutely irrotational motion of the surrounding aether, involving surfaces of slip, by very slightly rotational motion such as would evade all tangential slip: but the law of astronomical aberration would thereby be upset, since the smaller the rotation thus imposed the greater the distance to which it must extend, while the resulting aberration is proportional to these quantities jointly[2]. A hypothesis that would allow the aether to be moved in any degree by material bodies passing across it thus has small chance of correspondence with the body of ascertained optical facts.
We are therefore thrown back on Fresnel's view that the aether is not itself set in motion by the movement of material systems across it, or, in terms of the simile of Young, that it passes through the interstices of material bodies like the wind through a grove of trees.
