Abbe constant [V-value, ν-value]; g. Abbesche Zahl: constant that describes the ratio of the refractivity to the dispersion of an optical medium:

Vd = (nd − 1)/(nf −nc). n is the index of refraction. The indices indicate wavelengths. d: Helium line d (587.6 nm), f and c: hydrogen lines C and F (656.3nm and 486.1nm). Sometimes, slightly different wavelengths are used.