A crown glass prism of refracting angle `A= 6^@` is to be achromatised for red and blue light using a flint glass prism. Find the angle of flint glass prism (A') and also, the mean deviation from the following data : `mu_b=1.531` `mu_r=1.520` `mu'_b=1.684` `mu'_r=1.662`