A crown glass prism of refracting angle $6°$ is to be used for deviation without dispersion with a flint glass of angle of prism $\mathrm{\alpha }$. Given: for crown glass ${\mathrm{\mu }}_{\mathrm{r}}=1.513$ and ${\mathrm{\mu }}_{\mathrm{v}}=1.523$ , for flint glass ${\mathrm{\mu }}_{\mathrm{r}}=1.645$ and ${\mathrm{\mu }}_{\mathrm{v}}=1.665$. Find $\mathrm{\alpha }$