The eye – is the most expressive human part and therefore you should be more careful when drawing it. It’s essential to draw it in the correct position and having the exact sphere shape. The white section of the eyeball is not pure white but changes its color due to the shadow. Be careful that the eyeballs and the pupils have the same axes.
“Human anatomy” by Gottfried Bammes
“Portraits.Faces.and.Figures” by Giovanni Civardi