shows part of the same close-packed
arrangement of spheres; hollows lie between the spheres
and we can build a second layer of spheres upon the first
by placing spheres in these hollows. However, if we arrange
the spheres in the second layer so that close-packing is again
achieved, it is possible to occupy only every other hollow.
This is shown on going from Figure 5.2a to 5.2b.
Now consider the hollows that are visible in layer