placed transverse to the direction of the Earth's motion; and that the observations might even be carried out by a microscope, reversible by hand, sighted on a bright point attached to its frame. It was pointed out by Fresnel that Wilson had shown that on the corpuscular theory no effect was to be expected: and he added a demonstration that the same was the case on his own view of the undulatory theory, thus predicting on a consensus of all points of view a negative result.
9. The theory was next taken up, from the undulatory standpoint, by Cauchy, who preferred the first of Fresnel's alternatives, that the Earth in its orbital motion pushes the aether in front of it so that the portions near the surface travel along with the Earth, as he was unwilling to admit that the heavenly bodies could move through the aether without disturbing it at all. He pointed out that astronomical aberration was then to be explained, not probably by any effect of changed aethereal elasticity or inertia, but merely by a kinematic slewing round of the advancing wave-fronts (or rather absence thereof) owing to the translatory motion of the medium in which the waves are propagated. The disturbance of the aether itself owing to the motion of the Earth he was prepared to regard as the source of the electric and cognate phenomena associated with that body. This mere preference of Cauchy's did nothing towards removing Fresnel's difficulty as to how such a motion of the aether is to be imagined as exactly adjusted so as to involve the correct amount of aberration in accordance with Bradley's law: but the view subsequently became a real theory in the hands of Sir George Stokes. That physicist had just had cause, in his hydrodynamic researches, to analyze the differential change of form, arising from the state of motion in a fluid medium around any given point, into pure strain made up of three superposed elongations, combined with pure rotation: and it became clear that if the latter component is absent in the aether, so that the motion of the aether is differentially irrotational, the advancing wave-fronts will not be slewed round at all, and therefore the waves will travel through space in straight lines as if the aether were at rest. When these rectilinear waves get into the region of aether imme-
