line was not produced by homogeneous vibrations, but was double, treble, or even more complex. The distances between the components of these compound lines are so small that it is practically impossible, except in a few cases, to observe them in the ordinary spectroscope.

The following diagrams (Figs. 62–8) present a number of these visibility curves. Thus Fig. 62 represents that obtained from the red radiation of hydrogen. The curve to the right represents the visibility curve, while on the left the corresponding distribution of the light is drawn. Beginning at a difference of path zero, the visibility wasFIG. 62 100, and at one millimeter it was somewhat less, and so on, until at about seventeen millimeters we find a minimum. As the difference in path increases, we find that there is a maximum at twenty-three millimeters. After that the curve slopes down, and at about thirty-five millimeters it disappears entirely. Since the curve is periodic, we may be pretty sure that this red line of hydrogen is a double line. This fact, I believe, has never yet been observed, though the distance between the two components is not beyond the range of a good spectroscope, being about one-fortieth or one-fiftieth of the distance between sodium lines.^[1]

Fig. 63 represents the curve which was obtained from sodium vapor in a vacuum tube. When we burn sodium at atmospheric pressure—as, for example, when we place sodium