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ADDRESS BY ALBERT A. MICHELSON.
One unit means a distance of 24.6 mm. which gives for the average distance 25.2 mm. and for the ratio of the wave-lengths of the two lines 1.0000212.
Closely connected with the preceding investigations is the study of the effect of the temperature, thickness, and density of the source on the composition of the radiations, as shown by the symmetrical or unsymmetrical broadening of the spectral lines and the consequent shifting of their mean position. This question lias quite recently been taken up by II. Ebert and the results he has already obtained are very promising. The principal effects noted are: first, the shortening of the difference of path at which interference can be observed ; secondly, the shifting of the fringes as the mean wave length changes. Ebert has shown that the interference method is far more delicate than the spectroscopic ; and by its means he has established two conclusions which, if verified, are of the greatest importanceЧnamely; first, that the chief factor in the broadening of the spectral lines is the increase in density of the radiating body; secondly, that the broadening, in all the cases examined is unsymmetricalЧcausing a displacement of the line toward the red end of the spectrum. The importance of these conclusions, in their relation to the proper motions of the heavenly bodies and their physical condition, can hardly be overestimated. The value of results of this kind would, however, be much enhanced ifit were possible to find a quantitative relation between the density of the radiating substance and the nature of its radiations. In the case of hydrogen enclosed in a vacuum tube this could readily be accomplished. It may, however, be objected that it would be difficult in this case to separate the effects of increased density from those due to the consequent increase in the temperature of the spark. The problem of the temperature of the electric discharge in rarefied gases is one which has not yet been solved. In fact it may seriously be questioned whether in this case temperature has anything to do with the accompanying phenomena of light; and it
76 SECTION B.
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 Хstudy 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:Ч
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