THE THEORY OF RELATIVITY

So far everything is quite satisfactory. But now, in the sixth, and last, section of Lorentz's Essay the difficulties begin.* In this section Lorentz investigates three problems, of which two concern the rotation of the plane of polarization and Fizeau's polarization experiments. But without dwelling on these, we shall pass straight on to the third one, namely to the famous interference experiment of Michelson and Morley. This second order or /^-experiment, originally suggested by Maxwell,! was performed by Michelson in 1881, and six years later repeated on a larger scale and with a higher degree of exactness by Michelson and Morley. \ A beam of luminous rays coming from the source s, after having been made parallel in the usual way, is divided by the semi-transparent

B

->-v

c

FIG. 7.

plane mirror (half-silvered plate) al>, which is inclined at an angle of 45° to sOA, into a transmitted beam OA, and a reflected one OB. After having been reflected by the mirrors placed at A and B (at right angles to OA, OB, which directions are perpendicular to each other), the two beams of light return to the central mirror; here a part of the first beam is reflected along OC and a part of the second

* As is explicitly stated in the title : ' Abschnitt VI.—Versuche, deren Ergeb-nisse sich nicht ohne Weiteres erklh'ren lassen.'

t See Note at the end of chapter.

\ A. A. Michelson, ' The relative motion of the earth and the luminiferous ether,' A met'. Journ. of Science, 3rd Ser. Vol. XXII., 1881. A. A. Michelson and E. W. Morley, Sill. Jottrn., 2nd Ser. Vol. XXXI., 1886; Awer. Journ. of Science, 3rd Ser. Vol. XXXIV., 1887; Phil. Mag., jth Ser. Vol. XXIV., 1887. What is given above is but the usual rough scheme ; details of the actual arrangement will he found in the original papers quoted and, to a certain extent, also in Michelson's popular book on Light Waves and their Uses, where a diagram of the actual apparatus is given (Fig. 108),