centre at our earth, and with a radius extending all the way from the earth to this last star knowable by man. Every star that we can see, every star whose existence becomes disclosed to us on our photographs, lies inside this sphere; as to the orbs which may lie outside that sphere we can know nothing by direct observation. The imagination doubtless suggests with irresistible emphasis, that this outer region is also occupied by stars and nebulae, suns and worlds, in the same manner as the interior of that mighty sphere whose contents are more or less accessible to our scrutiny. It would do utter violence to our notions of the law of continuity to assume that all the existent matter in the universe happened to lie inside this sphere; we need only mention such a supposition to dismiss it as wholly indefensible. I do not now make any attempt to express the number of miles in the diameter of the sphere which limits the extent of space known directly to man. What that number may be is quite immaterial for our present purpose. But the point that I specially want to bring out is that the volume occupied by this stupendous globe, which includes within it all possible visible material, must be but a speck when compared with the space which contains it. Think of the water in the Atlantic Ocean, and think of the water in a single drop. As the drop is to the Atlantic Ocean so is the sphere which we have been trying to conceive to the boundless extent of space. As far as we know it would seem that there could be quite as many of such spheres in space as there are drops of water in the Atlantic Ocean. And, in all probability, these other spheres throughout space are tenanted by stars, systems, and galaxies just as grand in themselves, just as imposing in their colloca-
