We are now to venture into regions where we shall no longer have clear lights to guide us. Tremendous as are the dimensions of the solar system, the widest sweep of the planetary orbits sinks into insignificance compared with the distances which separate from us even the nearest of the fixed stars. From beyond depths which the human mind is utterly unable to conceive, there come to us the rays of light which myriads of those orbs are pouring forth, and it is from the lessons taught us by these light-rays that we are to form our ideas concerning the nature of the orbs which emit them. Very carefully and cautiously must we proceed, if we would avoid being led into vain imaginings. It will but mislead us to pass a single step beyond the path which is dimly lighted for us, and yet that path is so narrow and so obstructed with difficulties, that we find ourselves continually tempted to leave it, and to venture forward on the alluring and easy paths which speculation opens out on every hand around us.

And yet we may well remain content to listen only to the teachings of known facts. Even so restraining ourselves, we have in reality a wide and noble domain to explore.