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In order to find the new wave-front, draw the arc fg with b as a centre and ad as radius; the tangent to this arc from d will be the new wave-front, and the normal to the tangent from b will be the new direction. This will differ from the direction ba by θ, which it is required to find. From the equality of the triangles adb and edb it follows that θ = 2φ, ab = ac, or, neglecting terms of the order r Now let the light fall on a parallel mirror facing the first, we should then have and the total deviation would be where p is the angle of aberration, if only the orbital motion of the earth is considered. The maximum displacement obtained by revolving the whole apparatus through 90° would be With fifty such couples the displacement would be 0.2″. But astronomical observations in circumstances far less favourable than those in which these may be taken have been made to hundredths of a second ; so that this new method bids fair to be at least as sensitive as the former. The arrangement of apparatus might be as in fig. 2; s, in the focus of the lens a, is a slit. bb If it were possible to measure with sufficient accuracy the 460 Messrs. Michelson and Morley on the Relative Motion |