One of the grave difficulties we have in gaining a satisfactory comprehension of Einstein's conceptions, is that they do not readily relate themselves to our modes of geometrical thought. Within limits we may choose our own geometry, but it may be at the cost of unwieldy complication. If we think with Newton in terms of Euclidean geometry and consider the earth as revolving around the sun, the motions of our solar system can be stated in comparatively simple terms. If, on the other hand, we should persist in stating them, as Ptolemy would have done, from the earth as a relatively stationary center, our formulas will become complicated beyond ready comprehension. For this reason it is much simpler in applying the theory of relativity, and in considering and describing what actually happens in the physical universe, to use geometrical conceptions to which the actual conditions can be easily related. We find such an instrument in non-Euclidean geometry, wherein space will appear as though it were projected from a slightly concave mirror. It is in this sense that some speak of space as curved. The analogy is so suggestive it tempts one to linger over it. Unless there were material objects within the range of the mirror, its conformation would be immaterial; the thought of the space which the mirror, as it were, circumscribes, is dependent upon the presence of such material objects. The lines of light and of all other movement will not be quite "straight" from the view-point of Euclidean geometry. A line drawn in a universe of such a nature must inevitably return upon itself. Nothing therefore, can ever pass out of this unlimitedly great but yet finite cosmos. But even now, since our imaginary mirror is only very slightly concave, it follows that for limited regions like the earth or even the solar system, our conception of geometry may well be rectilinear and Euclidean. Newton's law of gravitation will be quite accurate with only a theoretical modification drawn from the theory of relativity.]82 The way in which a curvature of space might appear to us as a force is made plainer by an example. Suppose that in a certain room a marble dropped anywhere on the floor always rolled to the center of the room; suppose the same thing happened to a baseball, a billiard ball, and a tennis ball. These results could be explained in two ways; we might assume that a mysterious force of attraction existed at the center of the floor, which affected all kinds of balls alike; or we might assume that the floor was curved. We naturally prefer the latter explanation. But when we find that in the neighborhood of a large material body all other bodies move toward it in exactly the same manner, regardless of their nature or their condition, we are accustomed to postulate a mysterious attractive force (gravitation); Einstein, on the contrary, adopts the other alternative, that the space around the body is curved.]223