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

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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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