Light Waves and Their Uses
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 was
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
l This prediction has since been amply confirmed by direct observation.
Interference Methods in Spectroscopy 79
glass in a Bunsen flame—the visibility curve due to its radiations diminishes so rapidly that it reaches zero when the difference of path is about forty millimeters; it is practically
impossible to go farther than this. It is seen that the curve is periodic, which would indicate that each one of the sodium lines is a double line. The intensity curve at the left represents one of the sodium lines only. The other, on the same scale, would be distant about half a meter. We can from this get some idea of the relative sensitiveness of this process of light-wave analysis, as compared with that of ordinary spectrum analysis. It will be observed that the intensity curve shows still another small component which corresponds to still another longer period, but the existence of these short companion lines is not absolutely certain.
Fig. 64 represents the curve of thallium. The oscillation shows that it is a double line, and not very close. The distance between the components is about one-sixtieth of the distance between the sodium lines. We have also a longer oscillation which shows that each one of the components is