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

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that the actual limits observed are of the same order of magnitude as those given by the kinetic theory.

There is still a third cause which might limit this distance, but which would not have any effect in broadening the lines ; namely, the diminution in the amplitude of the vibrations after collision. There must be such a diminution and it would evidently be the more marked the more rapidly the energy was transferred to the ether, that is, the brighter the light. If the effects due to this cause alone could be separated from the others it would be possible to measure the diminution in amplitude and therefore the rate of transference of the energy. Thus it maybe shown that a vibrating sodium atom gives up to the surrounding ether less than six millionths of its energy at every oscillation.

Returning to the first and chief cause of broadening, it may be remarked that the universal opinion of scientific men seems to be that during collisions between the molecules the vibrations are entirely “irregular;” and the longer the collisions last in proportion to the time between collisions, the more intense will be the light due to these “ irregular” vibrations, and consequently the broader the lines and the more impure the light.

The following consideration would seem to show that this explanation will not hold. t

If, in the refractometer, so frequently referred to, white light be used, all phenomena of interference are lost to sight when the difference of path exceeds a few wave lengths, for the well-known reason that the fringes due to the infinite number of different kinds of light are superposed, thus producing a uniform illumination. If now this light be analyzed by a spectroscope, the spectrum will be traversed by well-marked interference fringes which are the finer and closer, the greater the difference of path of the interfering pencils. Now, I have observed such interference fringes in the white light from the incandescent carbons of an arc light when this difference amounted to thirty thousand waves. And it may be added that this limit was reached by the closeness of the lines rather than by their indistinctness.

It seems to me that the only conclusion which can be drawn from this experiment is that in the light from an incandescent solid the vibrations must be isochronous for at least thirty thousand waves. The same observation applies also to the so-called “irregular” vibrations of the broadened sodium lines, for the same limit (about thirty thousand waves was also observed in this case). The



inference seems irresistible that the broadening is not caused by “ irregular” vibrations, but by the addition of vibrations whose intensity is greater the dearer their period is to that of the normal vibrations and which may ok almost if not quite as regular as the normal vibrations themselves.

If these conclusions be granted we must profoundly modify our conception of radiation in solids and liquids, or at least that part of it which supposes that such radiation produces a continuous spectrum because the molecules have no “ free path,” and, therefore, no proper periodic vibrations.

What, then, is the nature of the effect produced b}' the collision of molecules? If it be to produce or reinforce vibrations differing from the normal type, it must be granted that these new vibrations are isochronous. If so, they must be due either to a change in the form, or in the mass of the molecule itself produced by collision, such changes tending to revert back to the type when the frequency of the collisions is not too great. The only alternative is to suppose that the molecules differ among themselves, either in form or weight. In this case, the molecules agreeingmost nearly with the type and hence having a proper period differing but little from the normal would be more easily set in vibration than the others, or their vibrations once started would outlast the others. Accordingly, when a gas is very much rarefied, the collisions are few, hence only the typical vibrations persist; but when the collisions are frequent the other vibrations are also sustained.

• I fear I have wandered so far from the subject of this address, if such a name be at all appropriate, ever to return ; and, though many other interesting and important applications of light-waves clamor for recognition, I fear they would be wearisome even to enumerate.

I fear also that it may with some justice be said that I have made a plea for my own instruments and theories, rather than “a plea for light waves;” and still more that I have presented many crude and half digested ideas, when it would have been more to the purpose to present facts and results of diligent study and careful experiment.

In extenuation and in conclusion I can only hope that if I have created the slightest interest in the matters here presented for your consideration, if there be any chance that even a few of the seeds may germinate, grow, blossom and bring forth fruit worthy of acceptance, my purpose will be accomplished.

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