and the numerical calculation being performed, we find
Such is the difference of path which, under the present hypothesis, ought to exist between the two rays.
Strictly speaking, this number has reference to a vacuum, and ought to be divided by the index of refraction for air; but this index differs so little from unity, that, for the sake of simplicity, the correction, which would not alter the last figure by a unit, may be neglected.
The above quantity being divided by the length of an undulation, will give the displacement of the bands in terms of the breadth of one of them. In fact, for a difference of path amounting to
undulations, the system of bands suffer a displacement equal to the breadth of
bands.