Interference Methods in Spectroscopy 77
ordinary spectroscope bed, and the light from only one of the lines in the spectrum thus formed is allowed to pass through the slit d into the interferometer.
As explained above, the light divides at the plate e, part going to the mirror/, which is movable, and part passing through to the mirror g. The first ray returns on the path fell. The second returns to e, is reflected, and passes into the telescope h. If the two paths are exactly equal, we have interference phenomena in white light; but for monochromatic light the difference of path (from the point e to the mirror /, and from the same point to the mirror g) may be very considerable. Indeed, in some cases interference can be obtained when the difference in the two paths amounts to over half a million waves.
It is rather important to note that the surface of the mirror g must be so set by means of the adjusting screws at its back that its image in the mirror e shall be parallel with the surface of the movable mirror /. When this is the case the fringes, instead of being straight lines, as in the case of the fringes in white light, are concentric circles very similar in appearance to Newton’s rings. Having thus adjusted the interferometer so that the fringes are circles, the difference in path is increased by turning the micrometer screw a definite amount, say half a millimeter at a time. At every half millimeter an observation is taken of the visibility, and then these readings are plotted on co-ordinate paper as ordinates, the corresponding difference of path serving as abscissae. The ends of these ordinates trace out the visibility curve. This curve is then set on the harmonic analyzer, as described above, and the machine turns out the curve corresponding to the distribution of the light in the line examined.
In this way the radiations of many substances were analyzed, and in almost every case it was found that the
7S
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
FIG. 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
l This prediction has since been amply confirmed by direct observation.