page 313, Angel's book.

There are four versions of the shaded sphere on the website. 
The first, shadedSphere1, uses the true vertex normals and per-vertex shading. 
The second, shadedSphere2, uses the true normal and per-fragment shading. 
The third and fourth, shadedSphere3 and shadedSphere4, use the normal computed from the three vertices 
of each triangle and do per-vertex and per-fragment shading, respectively. 
The program shadedCube is a rotating cube with the normals computed from the vertices of each triangle.