
48 Hamilton’s Quaternion Algebra
q
− q
q
− q
q’
− q’
(a) (b)
FIGURE 4.1 The set of all quaternions that represent rotations corresponds to a h emisphere of
radius 1 in 4D such that all antipodal points q and −q on the boundary are pasted together. (a)
If a closed path that represents continuous variations of rotation reaches the boundary, it appears
on the opposite side. This loop cannot be continuously shrunk to a point. (b) If a closed loop
passes through the boundary twice, it can be continuously shrunk to a point: we first rotate the
diametrical segment connecting q
′
and −q
′
so that they coincide with q and −q and then shrink
the loop to q and −q, whic ...