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74 SECTION B. of occurring may be indefinitely diminished by making several independent observations with different kinds of light. Perhaps one of the mbstr'important applications of the method is the determination of the^wave length of standard lines, both relative and absolute. In the paper above referred to, it was stated that the maximum difference of path at which interference fringes are visible, had been increased to over two hundred thousand waves (Fizeau’s number is 50,000) by using light from highly rarefied sodium vapor in^an exhausted tube. Since then I have observed interference under similar conditions with thallium with a difference of over three hundred and seventy thousand waves, and with mercury, with a difference of more than five hundred and forty thousand waves. By repeated measurements of the diameters of the interference rings, the fractions of a wave can be found to within a fiftieth—which means that the number of waves in this fixed distance can be found to within less than one part in twenty-five million. Any two kinds of light of this degree of purity can be compared with this same precision. The accuracy of the measurement of absolute wavelengths will of course depend on the accuracy with which the fixed distance can be compared with the standard meter; and this may be estimated as one part in two million. The results of the remarkable work of Rowland do not claim a much greater degree of ao^uracy than one part in half a million for relative determinations; while the elaborate research of Bell on absolute wave-lengths claims but one in two hundred thousand. We have thus at any rate a very promising method of excelling by far the best results that can possibly be obtained by the most perfect gratings. It may possibly help to realize the very considerable superiority of this instrument over the grating—at any rate for the class of work in question — if I recall to your attention the fact that by its means it has been possible to show that the red line of hydrogen is a very close double. A short time ago the same was found true of the green thallium line. Both these lines are something like a fiftieth of the distance of the sodium lines, and like these are of unequal intensity. It is even possible to measure this very small interval easily to within a fourth of one per cent. Following are the numbers obtained for the distance from one maximum or minimum of distinctness to the next:— | ADDRESS BY ALBERT A. MICHELSON. 75
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 importance—namely; 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 unsymmetrical—causing 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 |