Formula for dispersive power is (where symbols have their usual meanings)   
or
If the refractive indices of crown glass for red, yellow and violet colours are respectively  ${\mathrm{\mu }}_{\mathrm{r}}, {\mathrm{\mu }}_{\mathrm{y} }\text{ and }{\mathrm{\mu }}_{\mathrm{v}}$, then the dispersive power of this glass would be