"If... an apparatus is so constructed as to permit two pencils of light, which have travelled over paths at right angles to each other, to interfere, the pencil which has traveled in the direction of the earth's motion, will in reality travel 4/100 of a wave-length farther than it would have done, were the earth at rest. The other pencil being at right angles to the motion would not be affected. If, now, the apparatus be revolved through 90° so that the second pencil is brought into the direction of the earth's motion, its path would have lengthened 4/100 wave-lengths. The total change in position of the interference bands would be 8/100 of the distance between the bands, a quantity easily measurable."