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 curveFIG. 65 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.