Page:MichelsonSodium1887.djvu/6

158 send out, according to our experiments, as many as 200,000 vibrations before it receives a new impulse, and begins to send out a new set of waves, with perhaps the original amplitude. These 200,000 waves make a length of four inches. Suppose we could see the disturbance in a straight line of ether particles made when the set of 200,000 waves was included between the distances 30 feet, and 30 feet 4 inches from the source. The waves at the head of this line would have an amplitude one per cent, less than they had when 30 feet from the source. But the waves at the rear end of this 4 inches might have an amplitude only one-half or one third of those at the head.

We can get interference only between waves of the same set; that is, waves sent out by a sodium particle, after receiving one impulse and before receiving another. And, further, we can observe interferences only while the amplitude of the waves of this set remains sufficient. So far we have been able to observe interferences up to 200,000 wave lengths, which enables us to count wave lengths up to 4 inches.

Prof. Michelson: An article received a week ago recorded experiments made in Germany. The intensity of the light was varied from 1 to 250. There was no variation perceived in wave length. If there was any variation, it must have been less than one part in 20 or 30 millions.

Mr. Eisenmann: Is it more easy to count long waves than short?

Prof. Michelson: The shorter they are, the more easily they can be used for our purpose; the error is less.

Prof. Morley: Practically we are limited by the fact that we must take light in which we can obtain a sufficient amount of monochromatic light. Sodium light is perhaps the most convenient.

I may add that the method promises to be fruitful. It will enable us to measure expansions with an accuracy and convenience not yet attained. For instance, we can compare the length of a bar before and after heating. We can compare bars at the freezing and boiling points by an immaterial scale, an immaterial standard of length which cannot alter. I will show you how we can measure the expansion of a bar.

We will surround the bar a with a tube enclosed in a jacket in which we can have either ice or steam, so that the bar can be heated or cooled to the desired temperature. Let b be a similar bar in a tube surrounded by ice, and so kept of constant length. The tubes c and d are exhausted of air. Between them at e we put the mirror of our refractometer. The air around it is so far from the hot or cold masses that it can be kept at a uniform temperature. Now our ray of light going from e to a and b is our immaterial scale, the two rays can be made exactly equal in length and are not affected by any amount of heat or cold.