flame without any magnetic field. The corresponding intensity curve A (Fig. 79) shows that the center of the line has the greatestFIG. 78 intensity and that the intensity falls off rapidly on either side, the width of the line corresponding to something like one-hundredth of the distance between the two sodium lines. When the field was created by simply closing the current through the magnet, the visibility curve assumed the form indicated in curve B. The corresponding distribution of light is shown in the second of the intensity curves, B (Fig. 79) and we see that the line shows simply a broadening, with a possible indication of doubling. TheFIG. 79 field was then increased considerably; curve C (Fig. 78) represents the visibility. The corresponding intensity curve shows clearly that the line is double. The other curves were obtained by increasing the field gradually, and it will be noted that the result is an increasing separation of the line and, at the same