Note 24 — Chapter VIII · ACCESSORIES
Reading edition
If A be the brightness of one object and B that of the other, α the reading of the index when one image disappears and β the reading when the two images are equal then A/B = tan²(α-β). There are four positions of the Nicol, 90° apart, for which equality can be established, and usually all are read and the mean taken. (H. A. II, 1.)