Page:MichelsonSodium1887.djvu/2

154 some preliminary experiments made in June last have confirmed the anticipation.

The apparatus for observing the interference phenomena is the same as that used in our experiments "on the relative motion of the earth and the ether."



Light from the source at s, a sodium flame, falls on the plane glass a, and is divided, part going to the plane mirror c, the rest to plane mirror b. These two pencils are returned along c a e and b a e, and the interference of the two pencils is observed in the telescope at e.

The distances a c and a b being made equal by moving the mirror b, and the plane of c being made parallel with the image of b in a, and the compensating glass d interposed, the interference is at once visible.

If the adjustment is exact, the whole field will be dark (since one pencil experiences internal reflection at a and the other external).

If now b be moved parallel with itself a measured distance by 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 length and the counting of a number.

The degree of accuracy attainable depends on the number of wave lengths which it is possible thus to count. Fizeau was able 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 sodium in an exhausted tube provided with aluminium electrodes, and it was found possible to increase this number to over 200,000. But it is very easy to estimate tenths or even twentieths of a wave length, which means that it is possible to find the number of waves in a given fixed