On the other hand, while in the older crown and flint glasses the relation between refraction and dispersion had been practically fixed, dispersion and refraction increasing regularly with the density of the glass, in some of the new glasses introduced by Abbe and Schott this relation is altered and a relatively low refractive index is accompanied by a relatively high dispersion, while in others a high refractive index is associated with low dispersive power.
The dispersive powers of gases are, however, generally comparable with those of liquids and solids.
By altering the distance of the collective and dispersive members the magnification can be widely varied.
- The optical desideratum is uniformity of refractive index and dispersive power throughout the mass of the glass.
The symbol v represents the inverse of the dispersive power, its value being (nD-i)/(C-F).