Determination of the angle of minimum deviation of a prism and the refractive index of the material of the prism using the spectrometer

If $D$ is the angle of minimum deviation, $A$ the prism angle and $n$ the refractive index of the material of the prism,

1. Illuminate the slit of the collimator of an adjusted spectrometer by sodium light.
2. As shown in Figure 20.2 place the prism on the prism table so that the light from the collimator would fall at a small angle of incidence and get refracted through the face of the prism.
3. Turn the telescope to the position $T_{1}$ so that the refracted rays could be observed.
4. Now rotate the prism table in the sense of increasing the angle of incidence $i$ gradually. Then as shown in Figure 20.3 it would be seen that the image of the slit would first move in one direction, stop at a certain position and then move back to the initial position.
5. Turn the telescope until the vertical cross wire in its field of vision coincides with the image of the slit at the position where it stops. At this position $T_{2}$ of the telescope, observe and note down the reading on the scale.
6. Then remove the prism from the table and turn the telescope to the position $T_{3}$ where it is in line with the collimator.
7. After coinciding the vertical cross wire in the field of view of the telescope with the image of the slit, note down the reading on the scale.
8. Use the value obtained for prism angle $A$ in experiment no. 19. Calculate the refractive index $n$ of the material of the prism as explained in the theory.