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was sent through each notch to the mirror, which reflected back to the same notch, after having rotating one notch over, in front of the lens of a powerful telescope. In short, one pulse of the light signal, traveling over 17.2 km, should be caught within 1/720th of the wheel's rotation. Done at night, the first image would be of a light flashing in the distance. When the wheel's speed is adjusted so that when the light stops flashing and becomes steady, the tachometer is read, and the rest is simple arithmetic.

For the record, Jean-Bernard-Leon Foucault, Fizeau's assistant, repeated the experiment the next year, only using rotating mirrors.

By the time Michelson was studying optics in Europe (1880-82), the speed of light across the Earth's surface was known to such considerable degree of accuracy that it was considered "constant" everywhere, no matter what.

But it remained to be proved how it propagated through the ether.

The theory was that light propagates as rarefactions of the ether much the way sound travels through air, or waves travel on the ocean. If the ether exists, it will be distorted as a drag wake around the Earth as the Earth speeds around the Sun at such a considerable speed (30 kps) that the distortion of the ether should be measurable.

In 1881, Michelson used his first interferometer to look for the effect. He split one beam of light (he called them "pencils of light") through a half-silvered mirror (in the center) into two perpendicular beams sent to two mirrors on each arm of the device, and back to another half-silvered mirror in the center, given focus through a microscope. The instrument was simple at first; two spindly brass arms on a tripod light enough to set on the kitchen table without scratching the surface.

The difference between the light beam traveling ACROSS (transversely) the direction of the Earth's orbit, to the light beam traveling PARALLEL (longitudinally) to the direction of orbit, should throw the two recombined light beams out of phase to produce a measurable interference fringe in the magnitude of:

2(v^2 + c^2) : ((c + v) + (c - v))^2

The amount of fringing predicted would be from the same mechanism that spoils conversation in the back of a speeding pickup truck.

When the device was rotated 90 degrees, the fringe shift should have been still more dramatic, albeit still on a microscopic level.

Instead, Michelson saw NOTHING. At least, nothing beyond the margin of instrument error.

Thinking the error might be caused by the shortness

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