The refractivity of helium is 0.1238 (air =1).
Boron dissolves in molten aluminium, and on cooling, transparent, almost colourless crystals are obtained, possessing a lustre, hardness and refractivity near that of the diamond.
He introduced the idea of comparing the refractivity of equimolecular quantities of different substances by multiplying the function (n-1)/d by the molecular weight (M) of the substance, and investigated the relations of chemical grouping to refractivity.
In air and other gases, at ordinary pressures, the dispersion is very small, because the refractivity is small.
Glasses can now be made differing considerably both in refractivity and dispersive power.
