But the discovery that the _dispersive_ powers of different glasses are not proportional to their reflective powers, supplied opticians with the means of remedying the defect.

Let us clearly understand what is the discovery referred to.

If with a glass prism of a certain form we produce a spectrum of the sun, this spectrum will be thrown a certain distance away from the point on which the sun's rays would fall if not interfered with.

This distance depends on the _refractive_ power of the glass.

The spectrum will have a certain length, depending on the _dispersive_ power of the glass.