The coordinate-frame $(r, t)$ of a single observer does not cover the whole world. The range from $t = -\infty$ to $t = +\infty$ corresponds to values of~$\xi/z$ between~$±1$. The whole experience of any one observer of infinite longevity is comprised within a $90°$~lune. Changing the origin we can have another observer whose experience covers a different lune. The two observers cannot communicate the non-overlapping parts of their experience, since there are no light-tracks (generators) taking the necessary course. A further question has been raised, Is de~Sitter's world really empty? In \index{Horizon of world}% \index{Mass-horizon of world}% formula~\Eq{(70.1)} there is a singularity at $r = \Chg{\surd(3/\lambda)}{\sqrt{3/\lambda}}$ similar to the singularity at $r = 2m$ in the solution for a particle of matter. Must we not suppose that the former singularity also indicates matter---a ``mass-horizon'' or ring of peripheral matter necessary in order to distend the empty region within. If so, it would seem that de~Sitter's world cannot exist without large quantities of matter any more than Einstein's; he has merely swept the dust away into unobserved corners. A singularity of~$ds^{2}$ does not necessarily indicate material particles, for we can introduce or remove such singularities by making transformations of coordinates. It is impossible to know whether to blame the world-structure or the inappropriateness of the coordinate-system. In a finite region we avoid this difficulty by choosing a coordinate-system initially appropriate---how this \PageSep{166} is done is very little understood---and permitting only transformations which have no singularity in the region. But we can scarcely apply this to a consideration of the whole finite world since all the ordinary analytical transformations \index{Mass of the world, total}% \index{World!mass of}% (even a change of origin) introduce a singularity somewhere. If de~Sitter's form for an empty world is right it is impossible to find any coordinate-system which represents the whole of real space-time regularly. This is no doubt inconvenient for the mathematician, but I do not see that the objection has any other consequences. The whole of de~Sitter's world can be reached by a process of continuation; that is to say the finite experience of an observer~$A$ extends over a certain lune; he must then hand over the description to~$B$ whose experience is partly overlapping and partly new; and so on by overlapping lunes. The equation $G_{\mu\nu} = \lambda g_{\mu\nu}$ rests on the considerations of \SecRef{66}, and simply by continuation of this equation from point to point we arrive at de~Sitter's complete world without encountering any barrier or mass-horizon.