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

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appears to me much more reasonable to suppose that the vibratory motion of the molecules is not produced by collisions at all but rather by the sudden releJteeTof tension in the surrounding ether.

Whether true or not, the results obtained and interpreted by this hypothesis would be of great interest. The method could be applied directly to any substance, mercury for instance, for which the relation between the temperature and the pressure is known. For substances for which this relation has not been established, as sodium, thallium, etc., the density may be found by heating the stib--stance in a tube closed with plane parallel glass ends and measuring its index of refraction. The density will be very approximately .proportional to the excess of this index over unity.

Aside from its application to this problem, it seems highly probable that this method of measuring the density and pressure of vapors may be made to yield excellent results in cases where these are far too small to be measured directly.

It may not be entirely out of place in this connection to present a few.observations concerning the causes of the broadening of the spectral lines. It seems to me that by a thorough and systematic •study ami discussion of this phenomenon we have a possible means of materially increasing our knowledge of a subject, of which we are ,at,present in almost total ignorance : namely, the real action of the forces and motions of vibrating atoms and of the ether which transmits these vibrations in the form of light.

The possible causes of the broadening of spectral lines are as ifollows:—

First, the addition of vibrations of periods differing from the normal period, due to the influence of neighboring molecules ; secondly, the effect on the wave length due to the velocity of the molecules.

It is evident on considering the second cause, that it could not possibly account for more than a small fraction of the effects observed. For example, to effect a change in wave-length corresponding to the difference between the two sodium lines, would require velocities of the order of three hundred thousand meters per second, over a hundred times as great as the velocities given by the kinetic theoiy. But it is also clear that when a gas is so rarefied that the first cause cannot possibly produce any perceptible effect, the second cause would be quite sufficient to limit .the fineness of the lines, and to impose a definite limit to the difference of path at which interference is visible; and it is worthy of note



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

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