In visible region, the dispersive powers and the mean angular deviations for crown and flint glass prisms are${ }^{ }\omega ,{\omega }^{\text{'}}\text{ and d, }{d}^{\text{'}}$ respectively. The condition for getting deviation without dispersion when the two prisms are combined is 

A achromatic combination of two prisms can be made to get finite deviation without dispersion . 

$\mathrm{Dispersive} \mathrm{power}: \mathrm{\omega }=\frac{ \mathrm{angular} \mathrm{disperion} }{ \mathrm{mean} \mathrm{deviation} }$

so, Angular dispersion = Dispersive power x mean deviation $=\mathrm{\omega d}$

The condition for deviation without dispersion is that the net angular dispersion should be zero. $\Rightarrow \omega d+{\omega }^{\mathrm{\prime }}{d}^{\mathrm{\prime }}=0$.