Pulfrich refractometer is used to measure the refractive index of solids and liquid. It consists of right angled prism A having its two faces perfectly plane. One of the face is horizontal and the other is vertical as shown in Fig. The solid B whose refractive index is to be determined is taken having two faces cut perpendicular to one another. Light is incident in a direction parallel to the horizontal surface so that the light entering the prism A is at critical angle C . Finally, it emerges from the prism at an angle $i$ . Let the refractive index of the solid be $\mathrm{\mu }$ and that of the prism A be ${\mathrm{\mu }}_0$(which is known). Here ${\mathrm{\mu }}_0>\mathrm{\mu }$ and by measuring $i$, $\mathrm{\mu }$ can be determined.