The apparatus for observing the interference phenomena is
the same as that used in the experiments on the relative motion of the
earth and the luminiferous ether.
Light from the source at s (Fig. 1), a sodium flame, falls on the
plane parallel glass, a, and is divided, part going to the plane
mirror, c, and part to the plane mirror, b. These two pencils are
returned along _cae_ and _bae_, and the interference of the two is
observed in the telescope at e. If the distances, _ac_ and _ab_, are
made equal, the plane, c, made parallel with that of the image of b,
and the compensating glass, d, interposed, the interference is at once
seen. If the adjustment be exact, the whole field will be dark, since
one pencil experiences external reflection and the other internal.
If now b be moved parallel with itself a measured distance by means of
the micrometer screw, the number of alternations of light and darkness
is exactly twice the number of wave lengths in the measured distance.
Thus the determination consists absolutely of a measurement of a length
and the counting of a number.
The degree of accuracy depends on the number of wave lengths which it is
possible to count. Fizeau was unable to observe interference when the
difference of path amounted to 50,000 wave lengths. It seemed probable
that with a smaller density of sodium vapor this number might be
increased, and the experiment was tried with metallic sodium in an
exhausted tube provided with aluminum electrodes.
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