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 will have lengthened wave-lengths. The total change in the position of the interference bands would be of the distance between the bands, a quantity easily measurable.
The conditions for producing interference of two pencils of light which had traversed paths at right angles to each other were realized in the following simple manner.
Light from a lamp a, fig. 1, passed through the plane parallel glass plate b, part going to the mirror c, and part being reflected to the mirror d. The mirrors c and d were of plane glass, and silvered on the front surface. From these the light was reflected to b, where the one was reflected and the other refracted, the two coinciding along be.
The distance bc being made equal to bd, and a plate of glass g being interposed in the path of the ray bc, to compensate for the thickness of the glass b, which is traversed by the ray bd, the two rays will have traveled over equal paths and are in condition to interfere.
The instrument is represented in plan by fig. 2, and in perspective by fig. 3. The same letters refer to the same parts in the two figures.
The source of light, a small lantern provided with a lens, the flame being in the focus, is represented at a. b and g are the two plane glasses, both being cut from the same piece; d and c are the silvered glass mirrors; m is a micrometer screw which moves the plate b in the direction be. The telescope e, for observing the interference bands, is provided with a micrometer eyepiece. w is a counterpoise.
In the experiments the arms, bd, bc, were covered by long paper boxes, not represented in the figures, to guard against changes in temperature. They were supported at the outer ends by the pins k, I, and at the other by the circular plate o. The adjustments were effected as follows:
The mirrors c and d were moved up as close as possible to the plate b, and by means of the screw m the distances between a point on the surface of b and the two mirrors were made approximately equal by a pair of compasses. The lamp being
