Albert A. Michelson, "A Plea for Light Waves", Proceedings, AAAS, Section B, 37, 1888.

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TVe may confine our attention to the case of two parallel surfaces. Here it can readily be shown that the fringes are concentric circles, the common axis of the rings being the normal passing through the optical centre of the eye or telescope. Further they are most distinct when the eye or the telescope is focussed for parallel rays. In any other case we are troubled with the same perplexing changes of form and position of the fringes as already noted.

If now one of the mirrors have a motion normal to its surface the interference rings expand or contract; and by counting the fringes as they appear or disappear in the centre, we have a means of laying off any given distance in wave lengths.

Should this work of connecting the arbitrary standard of length — the yard or the metre — with the unalterable length of a light wave prove as feasible as it is hoped, a next step would be to furnish a standard of mass based upon the same unit. It may seem a little like exaggeration to say that the solution of this problem may admit of almost as high a degree of accuracy as the preceding.

Suppose a cube, ten centimetres on a side, with surfaces as nearly plane and .parallel as possible. Next suppose a testing instrument made of two parallel pieces of glass, whose inner surfaces are slightly farther apart than an edge of the cube.

The parallelism and the distance of these surfaces can be verified to a twentieth of a wave. Now apply this testing instrument to the three pairs of surfaces of the cube and determine their form, parallelism and distance to the same degree of accuracy. We have thus the means of measuring the volume of a cubic decimeter with an error less than one part in a million.

A very convenient and accurate method of making the determination of the weight of this volume of water at its maximum density has been suggested by Professor Morley, which consists in making the cube hollow, so that it will have almost exactly the same density as the water. On weighing the cube in water the excess of weight may be as small as required and may be most accurately measured by a very light and sensitive balance.

It does not seem extravagant to say that by some such plan as this we may obtain a standard kilogram which will be related to the standard of length with a degree of approximation far exceeding that of the present standard.

In the manufacture of plane surfaces, the only practicable method



of testing their accuracy is to place the surface close to a standard plane and examine the appearance of the Newton’s rings formed in the air film between\tluem. This process when executed with proper care is undoubtedly.'fche mo^ accurate, and, indeed, is the only one possible for producing a standard surface; but it is Attended with a number of inconveniences, among which may be mentioned the use of sodium light, the troublesome reflection from the first surface, the faintness of the fringes when the surface to be tested is metallic and the difficulty of getting rid of dust between the surfaces. All of these inconveniences are avoided by the use of the refractometer. For this purpose the apparatus is placed in a vertical plane, the lower mirror, which would then be horizontal, is replaced, first, by the test plane and then by the surface to be worked. The interference fringes in white light can then be conveniently studied while the surface is being corrected.

Another application of this apparatus, suggested by Professor Morle}', is the measurement of coefficients of expansion. For this purpose a bar is provided with silvered glass mirrors at each end (both facing the same way) and a second bar of the substance to be examined and of the same length is furnished in the same way. These are placed in the refractometer, so that the front mirrors, as well as the rear ones, give interference fringes in white light. The auxiliary bar is kept at zero. The bar to be examined is heated, and the fringes* which pass at the front surface are counted as the bar expands, the fixity of the rear mirror being controlled by the colored fringes at its surface. In this method the bars may be a meter in length, and, therefore, the accuracy of the determination would be proportionally greater than in the celebrated experiments of Fizeau, in which the length was limited to a few millimetres.

Evidently the same disposition will also serve Tor measurements of coefficients of elasticit}', with the evident advantage of studying the elastic properties of the substance in thick rods or bars instead of small wires. This method of investigation is not limited to the determination of changes in length, but is quite as applicable to changes in density and optical properties; particularly to the effect on the velocity of light in solids, liquids and gases due to alterations in temperature, pressure, or magnetic or electrical conditions.

It may be mentioned that a great deal of valuable work has already been accomplished in this direction. I need only cite the

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