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 the terrestrial motion to the opposite direction. This was afterwards confirmed by Lord Rayleigh, who found that the alteration, if it existed, could not amount to (1/100,000)th part.

In terrestrial methods of determining the velocity of light the ray is made to retrace its path, so that any velocity which the earth might possess with respect to the luminiferous medium would affect the time of the double passage only by an amount proportional to the square of the constant of aberration. In 1881, however, A. A. Michelson remarked that the effect, though of the second order, should be manifested by a measurable difference between the times for rays describing equal paths parallel and perpendicular respectively to the direction of the earth's motion. Ho produced interference-fringes between two pencils of light which had traversed paths perpendicular to each other; but when the apparatus was rotated through a right angle, so that the difference would be reversed, the expected displacement of the fringes could not be perceived. This result was regarded by Michelson himself as a vindication of Stokes's theory, in which the aether in the neighbourhood of the earth is supposed to be set in motion. Lorentz, however, showed that the quantity to be measured had only half the value supposed by Michelson, and suggested that the negative result of the experiment might be explained by that combination of Fresnel's and Stokes's theories which was developed in his own memoir ; since, if the velocity of the aether near the earth were (say) half the earth's velocity, the displacement of Michelson's fringes would be insensible.