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.
All material media, however, ' are more or less dispersive.
The dispersive powers of gases are, however, generally comparable with those of liquids and solids.
The symbol v represents the inverse of the dispersive power, its value being (nD-i)/(C-F).
The spectrum will be straight if the twoprismsaresimilar in dispersive property, but if one of them is con structed of a material which possesses any peculiarity in this respect it will be revealed by the curvature of the spectrum.
