Admitting that, let us now suppose that at some point in the universe where there is weight—at the moon’s surface, for instance—there is a remarkable gun which can fire a ball that has and retains (along its whole path) the velocity of light. The trajectory of this ball will be very extensive, on account of its great velocity, yet curved toward the surface of the moon on account of its weight. As we may make our choice in the field of hypotheses, there is nothing to prevent us from supposing that the ball is of such a nature as to disclose its path by a faint luminous trail. There were projectiles of this character during the Great War. As the ball advances, it also falls every second toward the moon’s surface, to the same extent as any other projectile would which was fired at any velocity whatever, or had no velocity. All objects near the surface of the ground (in a vacuum) fall at the same vertical velocity, and this is independent of their motion in the horizontal direction. That is, in fact, the reason why the paths of projectiles are the more curved the less initial speed they have. Seen from the windows of Jules Verne’s projectile (which is itself falling toward the moon), the trajectory of the ball will seem to the passengers to be a straight line, because it falls with the same velocity as they. Now let us suppose that a luminous ray, from the flame of the gun, starts at the same time and in the same direction as the ball. This luminous ray will obviously be rectilinear for the passengers in the projectile, because light travels in a straight line when there is no weight. Consequently, since it has the same form, direction, and velocity as the luminous ball, the passengers will see the ray of light coincide in its whole course with the trajectory of the ball. It further follows that the “Interval” (both in time and space) of the luminous ray and of the ball is, and remains, zero. Now this “Interval” must remain the same, whatever be the velocity of the observer. Hence, if Jules Verne’s projectile ceases to fall, and is stopped at the moon’s surface, its passengers will continue to see the luminous ray coincide at every point with the trajectory of the ball. This trajectory is, as they now notice, curved on account of weight. Therefore, the luminous ray is similarly curved in its path on account of weight. This shows that light does not travel in a straight line, but falls, under the influence of gravitation, like all other objects. The reason why this was never known before, and it was always thought that light travels in a straight line, is that on account of the enormous velocity of light its trajectory is only very slightly curved by weight.