I had recourse to Newton's rings. These being produced, in yellow light, for instance, if one passes from one dark ring to the following, the variation of optical retardation in air is one wave-length of yellow light. If, now, with the same apparatus and the same incidence, rings are produced by means of "N" rays, and the number of these rings comprised between two dark rings in yellow light is counted, we shall obtain the number of times which the wave-length of "N" rays is contained in the wave-length of yellow light. This method, applied to rays of index 1.04, gave the values 0.0085 instead of 0.0081 found by the gratings; and for the index 1.85, the value 0.017 instead of 0.0176. Though the ring method is inferior to the grating method, on account of the uncertainty attending the exact position of the dark rings in the experiment, an uncertainty which is due to the necessity of rendering these rings very wide, the concordance of the numbers obtained by the two methods constitutes a valuable control.
In the tables given above I have retained all the decimals occurring in the calculation of the
