80
Light Waves and Their Uses
double. The distance between these small components and the larger ones is something like one-thousandth of the distance between sodium lines, corresponding to a separation of lines far beyond the possible limit of the most powerful spectroscope.
The curve of the green radiation of mercury is shown in Fig. 65. This curve is really so complicated that the character of the source is still a little in doubt. The machine has not quite enough elements to resolve it satisfactorily, having but eighty when it ought to have eight hundred. The curve
looks almost as though it were the exceptional result of this particular series of measurements, and we might imagine that another series of measurements would give quite a different curve. But I have actually made over one hundred such measurements, and each time obtained practically the same results, even to the minutest details of secondary waves. The nearest interpretation I can make as to the character of the spectral source is given at the left of this diagram. It will be noticed that the width of the whole structure is, roughly speaking, one-sixtieth of the distance between the sodium lines. The distance between the close components of the brighter line is of the order of one-thousandth of the distance between the sodium lines. The fringes in this case remain visible up to a difference of path of 400 millimeters, and they have actually been observed up to 480 millimeters, or nearly one-half meter’s difference in path — corresponding to something like 780,000 waves.
Interference Methods in Spectroscopy 81
In the curve of Fig. 66 we have quite a contrast to the preceding. Here we have a radiation almost ideally homo-geneous. Instead of having numerous maxima and minima like the curves we have been considering, this visibility curve diminishes very gradually according to a very simple mathematical law, which tells us that the source of light is a single line of extremely small breadth, the breadth being of the order of one eight-hundredth to one-thousandth of the
FIG. 66
distance between the sodium lines. It is impossible to indicate exactly the width of the line, because the distribution of intensity throughout it is not uniform. The important point to which I wish to call attention, however, is that this curve is of such a simple character that for a difference of path of over 200 millimeters, or 400,000 light waves, we can obtain interference fringes. This indicates that the waves from this source are almost perfectly homogeneous. It is therefore possible to use these light waves as a standard of length, as will be shown in a subsequent lecture. The curve corresponds to the red radiation from cadmium vapor in a vacuum tube. In using this red cadmium wave as a standard of length it is very important to have other radiations by which we can check our observations. The cadmium has two other lines, which serve as a control or check to the result obtained by the first.
Fig. 67 represents the green radiation of cadmium. This curve is not quite so simple as that of the red, but