Attempt to Separate the Isotopic Forms of Lead by Fractional Crystallization

must be high and M' low. If C' is at G' the whole of each component beam may be caught and passed through the respective shanks of the U-tube. The fringes are strong, easily found and large, so that the center of ellipses is not far outside of the field of the telescope. Finally if the connecting tube p is nearly horizontal when in place, the fringes are usually found at about the same position of the micrometer (at M') after the liquid is introduced into the U-tube. Experiments were also made with this apparatus. Displacement interferometers in which the rays do not retrace their respective paths have an important special property which I wish to accentuate in conclusion. If either opaque mirror is displaced on its micrometer normal to itself or if a plate compensator rotates in one beam on a vertical axis, the center of ellipses moves parallel to the length of the spectrum. If however the plate compensator rotates on a horizontal axis, the center of ellipses moves nearly transversely to the length of the spectrum. The phenomenon is quite sensitive. To this result I shall return in a succeeding note, in connection with the development of the Jamin design for displacement interferometry. The present note will be presented in more extended form in a report to the Carnegie Institution of Washington.