A. Michelson and E. Morley. On the Relative Motion of the Earth and the Luminiferous Æther. // Phil. Mag. S. 5. Vol. 24. No. 151. Dec. 1887.

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velocity of light without returning the ray to its starting point, the problem of measuring the first power of the relative velocity of the earth with respect to the aether would be solved. This may not be as hopeless as might appear at first sight, since the difficulties are entirely mechanical and may possibly be surmounted in the course of time.

For example, suppose m and m/ (fig. 3) two mirrors revolving with equal velocity in opposite directions. It is evident that light from s will form a stationary image at s/ and similarly light from s/ will form a stationary image at s. If now the velocity of the mirrors be increased sufficiently, their phases still being exactly the same, both images will be deflected from s and s/ in inverse proportion to the velocities of light in the two directions; or, if the two deflections are made equal, and the difference of phase of the mirrors be simultaneously measured, this will evidently be proportional to the difference of velocity in the two directions. The only real difficulty lies in this measurement. The following is perhaps a possible solution.

gg/ (fig. 4) are two gratings on which sunlight is concentrated. These are placed so that after falling on the revolving mirrors m and m/, the light forms images of the gratings at s and two very sensitive selenium cells in circuit with a battery and telephone. If everything be symmetrical, the sound in the telephone will be a maximum. If now one of the slits s be displaced through half the distance between the image of the grating bars, there will be silence. Suppose now that the two deflections having been made exactly equal, the slit is adjusted for silence. Then if the experiment be repeated when the earths rotation has turned the whole apparatus through 180, and the deflections are again made equal, there will no longer be silence, and the angular distance through which s must be moved to restore silence will measure the required difference in phase.

There remain three other methods, all astronomical, for attacking the problem of the motion of the solar system through space.

1. The telescopic observation of the proper motions of the stars. This has given us a highly probably determination of the direction of this motion, but only a guess as to its amount.

2. The spectroscopic observation of the motion of stars in the line of sight. This could furnish data for the relative motions only, though it seems likely that by the immense improvements in the photography of stellar spectra, the information thus obtained will be far more accurate than any other.

3. Finally there remains the determination of the velocity of light by observations of the eclipses of Jupiter's satellites. If the improved photometric methods practised at the Harvard observatory make it possible to observe these with sufficient accuracy, the difference in the results found for the velocity of light when Jupiter is nearest to and farthest from the line of motion will give, not merely the motion of the solar system with reference to the stars, but with reference to the luminiferous æther itself.

LIX. On a Method of making the Wave-length of Sodium

Light the actual and practical Standard of Length. By Albert A. Michelson and Edward W. Morley*.

THE first actual attempt to make the wave-length of sodium light a standard of length was made by Peirce. This method involves two distinct measurements : first, that of the angular displacement of the image of a slit by a diffraction-grating, and second, that of the distance between the lines of the grating. Both of these are subject to errors due to changes of temperature and to instrumental errors. The results of this work have not as yet been published ; but it is not probable that the degree of accuracy attained is much greater than one part in fifty or a hundred thousand. More recently, Mr. Bell, of the Johns Hopkins University, using Rowland's gratings, has made a determination of the length of the wave of sodium light which is claimed to be accurate to one two hundred thousandth part. If this claim is justified, it is probably very near the limit of accuracy of which the method admits. A short time before this, another method was proposed by Macé de Lepinay . This consists in the calculation of the number of wave-lengths between two surfaces of a cube of quartz. Besides the spectroscopic observations of Talbots fringes, the method involves the measurement of the index of refraction and of the density of quartz, and it is not surprising that the degree of accuracy attained was only one in fifty thousand.

Several years ago, a method suggested itself which seemed likely to furnish results much more accurate than either of the foregoing, and some preliminary experiments made in

* Communicated by the Authors.

Nature, xx. p. 99 (1879) ; Amer. Journ. Sci. [3], p. 51 (1879).

On the Absolute Wave-lengths of Light, Amer. Journ. Sci. [3], xxxiii. p. 167 (1887) ; Phil. Mag. [5], xxiii. p. 365.

Comptes Rendus, cii. p. 1153 (1886) ; Journ. de Phys. [2], v. p. 411 (1886).

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