LECTURE VI
ANALYSIS OP THE ACTION OF MAGNETISM ON LIGHT WAVES BY THE INTERFEROMETER AND THE ECHELON
A little over a year ago the scientific world was startled by the announcement that Professor Zeeman had discovered a new effect of magnetism on light. The experiment that he tried may be briefly described in the following way: If we place a sodium flame in front of the slit of a spectroscope, we get in the field of view a bright double line. If tho flame is placed between the poles of a powerful electro-magnet, it is found that the lines are very much broadened; at least this was the way in which the announcement of the discovery was first made. It may be mentioned that a somewhat similar observation was made by M. Fievez a long time before. He found that the sodium lines in the spectrum were modified by the magnetic field, but not quite in the way that Zeeman announced; instead of the lines being broadened, he thought that each separate sodium line was doubled or quadrupled. It seems that, long before this, the experiment had actually been tried by Faraday, who, guided by theoretical reasons, conjectured that there should be some effect produced by a powerful magnetic field upon radiations.
The only reason why Faraday did not succeed in observing what Fievez and Zeeman observed afterward was that the spectroscopic means at his disposal at the time were far from being sufficiently powerful. The effect is very small at best. The distance between the sodium lines being taken as a kind of unit for reference, the separate sodium lines, as was shown in a preceding lecture, have a width of about one-hundredth of the distance between the two. The broadening, or
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Light Waves and Their Uses
doublings or other modification which is produced in the spectrum by the magnetic field, is of the order of one-fortieth, or perhaps one-thirtieth, of the distance between the sodium lines. Hence, in order to see this effect at all, the highest spectroscopic power at our disposal must be employed. Subsequent investigation has shown, indeed, that still other modifications ensue, which are very much smaller even than this, and which cover a space of perhaps only one-hundredth to one liundred-and-fiftieth of the distance between the sodium lines. They are, therefore, beyond reach of the most powerful spectroscope.
It occurred to me at once to try this exj)eriment by the interference method, which is particularly adapted to the examination of just such cases as this, in which the* effect to be observed is beyond the range of the spectroscopic method. The investigation was repeated in very much the same way as described by Zeeman, namely: A little blow-pipe flame was placed between the poles of a powerful electro-magnet; a piece of glass was placed in the flame to color it with sodium light. The light, instead of passing into the spectroscope, was sent into an interferometer and analyzed by the method described in Lecture IV. The visibility curves which were thus obtained showed that, instead of a broadening, as was first announced by Zeeman, each of the sodium lines appeared to be double. The visibility curves which were observed are showTn in Fig. 78, and in Fig. 70, the curves which give the corresponding distribution of the light in the source. In the former figure the vertical distances of the different curves represent the clearness of the fringes, and the horizontal distances the differences in path. In curve A, as the difference in the paths increases, the fringes become less and less distinct, until at forty millimeters the fringes have almost entirely disappeared. This curve represents the visibility of the sodium