The yellow fringes pass alternately from a condition of high contrast to one of almost complete disappearance. This latter condition occurs when one set of fringes is halfway between the other set. Move the mirror until the fringes almost most nearly disappear (maximum extinction coefficient). Record the micrometer reading D1. Turn the micrometer head to through ten more maximum extinctions. Record the micrometer reading, D2. Calculate the average distance d between conditions of disappearance: d = (D2-D1)/10.
The difference in wavelengths between the two extinction can be calculated by the formula:
Δλ = λ2/2/d.