It is prized for its exceptional dispersive power ( 0.051, B to G interval ) which exceeds that of diamond.
2.
Lead-free crystal has a similar refractive index to lead crystal, but it is lighter and it has less dispersive power.
3.
Newton failed to perceive the existence of media of different dispersive powers required by achromatism; consequently he constructed large reflectors instead of refractors.
4.
Glass with weaker dispersive power ( greater v ) is named " crown glass "; that with greater dispersive power, " flint glass ".
5.
Glass with weaker dispersive power ( greater v ) is named " crown glass "; that with greater dispersive power, " flint glass ".
6.
Consequently the powers of the two must be different ( in order that f be not zero ( equation 2 ) ), and the dispersive powers must also be different ( according to 4 ).
7.
For the construction of an achromatic collective lens ( f positive ) it follows, by means of equation ( 4 ), that a collective lens I . of crown glass and a dispersive lens II . of flint glass must be chosen; the latter, although the weaker, corrects the other chromatically by its greater dispersive power.
8.
Should the cemented system be positive, then the more powerful lens must be positive; and, according to ( 4 ), to the greater power belongs the weaker dispersive power ( greater v ), that is to say, crown glass; consequently the crown glass must have the greater refractive index for astigmatic and plane images.
9.
In all earlier kinds of glass, however, the dispersive power increased with the refractive index; that is, v decreased as n increased; but some of the Jena glasses by E . Abbe and O . Schott were crown glasses of high refractive index, and achromatic systems from such crown glasses, with flint glasses of lower refractive index, are called the " new achromats, " and were employed by P . Rudolph in the first " anastigmats " ( photographic objectives ).
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