Page:Michelson1904.djvu/1



the Physikalische Zeitschrift a method is proposed by W. Wien for deciding the important question of the trainement of the æther by the earth in its motion through space, by measuring the velocity of light in one direction—that is, without reflecting it back from the distant station.

The essentials in the proposed method are two Foucault mirrors, or two Fizeau wheels (one at each station) revolving at the same speed. The control for this synchronism is to be furnished by the "Konstanz des hindurchgehendes Lichtes oder in bekannten stroboskopischen Methoden."

The flaw in the proposed method—as was pointed out by Simon Newcomb as long ago as 1880—lies in the fact that the effect which it is proposed to measure is exactly the same as the effect on the light which is to furnish the test of synchronism.

In November 1887 I proposed a method differing in no essential respect from the foregoing, except that the control of synchronism was to be furnished by electrical methods. This was before the celebrated work of Hertz showed that electrical impulses differed in no essential from light. This identity constitutes the same objection to this plan.

The possibility of a mechanical method of control was pointed out in a paper on the "Velocity of Light," in the Philosophical Magazine, March 1902. This is based on some experiments made in 1899, which showed that the vibrations of a timing-fork could be transmitted over a mile of piano-wire with but little diminution of amplitude.

Suppose it were possible to transmit two pencils of light in opposite directions around the earth parallel to the equator, returning the pencils to the starting-point. If the rotation of the earth does not entrain the æther, it is clear that one of the two pencils will be accelerated and the other retarded (relatively to the observing apparatus) by a quantity proportional to the velocity of the earth's surface, and to the length of the parallel of latitude at the place; so that a