The Dispersion of Atomic Hydrogen: I-A Measure.

The apparatus used in the experiment which is the subject of this communication is related to that named after Puccianti.' It consists essentially of identical tubes about 85 cm. long with optically plane parallel ends placed in the arms of an interferometer and filled to the same pressure with hydrogen. A powerful discharge was passed through one of the tubes while "white-light" fringes from a high intensity carbon arc were being observed. The fringes were made horizontal and focussed on the narrow vertical slit of a spectroscope or a very fast spectrograph. The fiducial point was provided in the visual observations by the cross-hairs of the spectroscope and in the photographic by a very fine wire stretched horizontally across the slit of the spectrograph. The field presented was then a bright continuous spectrum traversed by a number of almost horizontal slightly curved and very black fringes and also by the very sharp horizontal shadow of the cross-hairs or the wire across the slit. The discharge is turned on and fixing attention to a part of the spectrum not disturbed by the Balmer lines which come out in great purity,2 a shift of the fringes is looked for. Otherwise a series of exposures of 1 to 4 seconds' duration is taken with the discharge alternately off and on and the position of the fringes measured with respect to the fiducial line. This procedure was repeated many times at pressures from 0 to 25 mm. with all varieties of conditions of discharge, purity of hydrogen, etc.