* This is intelligible without calculation--but only for the
two-dimensional case--if we revert once more to the case of the disc
on the surface of the sphere.

In this way, by using as stepping-stones the practice in thinking
and visualisation which Euclidean geometry gives us, we have acquired
a mental picture of spherical geometry. We may without difficulty
impart more depth and vigour to these ideas by carrying out special
imaginary constructions. Nor would it be difficult to represent the
case of what is called elliptical geometry in an analogous manner.
My only aim to-day has been to show that the human faculty of
visualisation is by no means bound to capitulate to non-Euclidean
geometry.