Align an autocollimator such as the Model D-638, Coordinate Autocollimator to a flat mirror like the Model D-571, Adjustable Reference Mirror.

Insert the Wedge to be measured between the autocollimator and the mirror.  As light goes through the wedge it is deviated once and as the light returns from the reflective mirror and passes through the wedge again it is deviated once more.

Since the autocollimator is calibrated to indicate half the angle observed, the measurement provided by the autocollimator is equal to the deviation angle of the wedge.

D = (n-1) A for small angles

In principle, if one knows the wavelength of the light projected by the autocollimator and the refractive index of the material at that wavelength, it is possible to calculate the angle A of the wedge.

On the other hand, if one measures the wedge angle A using the above autocollimation technique, then it is possible to deduce the refractive index of the wedge material for the specific wavelength used by the autocollimator.