Page:MichelsonSodium1887.djvu/4

156 these is that, as the difference in path increases, the interference becomes less distinct, and sometimes finally disappears, reappears and becomes most distinct again when the distance is an exact multiple of both wave lengths.

Thus there is an alternation of clear interference fringes with uniform illumination or less clear interference fringes, and if the length to be measured, the centimeter for instance, is such that the interference does not fail exactly in the middle of the series but say a tenth of the distance to one side, there will be an error of one-twentieth of a wave length, which is of the same order as the error of observation.

Among other substances tried in these preliminary experiments were thallium, lithium and hydrogen, all of which gave interference up to 50 or 100 thousand wave lengths, and could therefore all be used as checks on the determinations with sodium.

It may be noted that in the case of the red hydrogen light, the interference phenomena disappeared at about 15,000 wave lengths and again at about 45,000 wave lengths. So that the red hydrogen line must be composed of two lines about one-sixtieth as distant as the sodium lines.

DISCUSSION.

Mr. C. G. Force: Is the change from the black spot to the white instantaneous?

Prof. Morley: No, it is gradual.

Mr. Force: Then the coincidence of the two planes is a matter of judgment.

Prof. Morley: It is; but the limits of possible error of judgment are very narrow. A motion of one plane by the hundred thousandth of an inch produces all the gradations from blackness to maximum light and blackness again. It is hardly possible to err in judgment by one tenth of this quantity, or the millionth of an inch. But if so much dependence on the judgment of the observer be deprecated, we can use micrometric measurement, and then we can trust the mean of repeated measurements even to the hundredth of a wave length, or the fifty millionth of an inch. In some observations made by use of interference phenomena (though for a very different purpose), we made the mean of our measurements 42 per cent, of a wave length, and found that the true value was 43, so that our measurements were within one per cent, of a wave length.

Prof. A. Michelson: Permit me to state that Professor Morley has given me more than my due in attributing so large a share of this work to me. Without his assistance, our present results would never have been attained.

You will observe that if we are to make our measurements, say to within one part in 10 or 20 millions, it would be an absurdity to allow the surfaces to be in error by more than the same quantity. That would mean that the surfaces must not be anywhere different from a plane more than one five hundred thousandth of an inch. So far such surfaces have not been made, no mechanician had been found who could do that work; but there is now one who will be enabled to do it, and he is Professor Morley.

Mr. Whitelaw: Do these fringes enable you to see the summit or depression of the waves of light?