Conference on the Michelson-Morley experiment held at the Mount Wilson observatory Pasadena, California February 4 and 5, 1927

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S attached to the slab, and passed through a small hole in the screen Z. The pencils of light are carefully limited by screens and by focusing in order to prevent stray light from reaching the eye and thereby reducing its sensitiveness. Adjustments are made so that broad fringes are formed at the surface of Mx and M2, on which the telescope is focused. Final adjustments are made by rotating the compensating plate C by means of a fine differential screw, and by placing small weights near the corner of the slab; under proper condi

tions a 5-g weight deflects the heavy slab just perceptibly. The adjusting screws are manipulated by means of spindles passing through short flexible tubes in such a way as to be freely rotatable but air tight. After the mirrors are given preliminary alignment, the cover is carefully lowered into place, sealed to the slab, and then filled with helium.

Schematically, the arrangement of the interferometer is shown in Figure 9. A beam of practically plane-parallel, homogeneous light, plane-polarized so that its electric vector lies in the plane of the paper, moves to the right and falls on the mirror Ms at the polarizing angle for the given wave-length. At the upper face the beam


intensity, one passing to the mirror Mx and the other to M2. From there they are reflected back to M3, where they recombine and pass to the eye through a telescope focused on Mx and M2• Two purposes are accomplished by the use of plane-polarized light : first, the noninterfering rays indicated by the dotted lines, which would be produced with natural light, are completely eliminated; and, second, the recombining beams can be adjusted to perfect equality of intensity by varying the relative reflecting powers of Mz and M2. Be

cause there are two more glass-air interfaces to be traversed by the upper beam than by the lower, it is impossible to equalize both components of natural light in this way.

The high sensibility necessary because of the short paths is secured chiefly by the simple device of raising one-half of the surface of mirror M2 a small fraction of a wave-length above the other, the dividing line between the two levels being straight and as sharp as possible. The mirror used was made by covering part of a plane plate with a flat sharp-edged microscope cover-glass and applying the extra thickness by cathode deposition of platinum, thereafter giving the whole plate a fully reflecting coat. I ran across the suggestion of using such a divided mirror in interferometry some years ago, but am unaware to whom the credit for it belongs.

is split by a thin platinum film into two parts of nearly equal

Fig. 9

Fig. 10