  In the space $\hh$ of quaternions, we investigate the natural, invariant
geometry of the open, unit disc $\Delta_{\hh}$ and of the open half-space
$\hh^{+}$. These two domains are diffeomorphic via a Cayley-type
transformation. We first study the geometrical structure of the groups of
M\"obius transformations of $\Delta_{\hh}$ and $\hh^{+}$ and identify original
ways of representing them in terms of two (isomorphic) groups of matrices with
quaternionic entries. We then define the cross-ratio of four quaternions, prove
that, when real, it is invariant under the action of the M\"obius
transformations, and use it to define the analogous of the Poincar\'e distances
on $\Delta_{\hh}$ and $\hh^{+}$. We easily deduce that there exists no isometry
between the quaternionic Poincar\'e distance of $\Delta_{\hh}$ and the
Kobayashi distance inherited by $\Delta_{\hh}$ as a domain of $\mathbb{C}^{2}$,
in accordance with a direct consequence of the classification of the non
compact, rank 1, symmetric spaces.
