As the circular summit of the _raised_ cone _appears_ to be _nearest_ the eye of the observer, the summit of the _hollow_ cone _farthest_ off, and the similar central circle in the flat drawing on each side, at an intermediate distance, the apparent distances from the eye of different and equal circles will represent the apparent distance of the moon in the _zenith_, or very high in the elliptical celestial vault,—the same distance when she is in the _horizon_, and the same when at an intermediate altitude. Being in reality of exactly the same size, and at the same distance from the eye, these circular summits, or sections of the cone, are precisely in the same circumstances as the moon in the three positions already mentioned. If we now contemplate them in the lenticular stereoscope, we shall see the circular summit of the _hollow_ cone the _largest_, like the _horizontal_ moon, because it _seems_ to be at the _greatest_ distance from the eye,—the circular summit of the _raised_ cone the _smallest_, because it appears at the _least_ distance, like the _zenith_ or culminating moon,—and the circular summits of the flat cones on each side, of an _intermediate_ size, like the moon at an _intermediate_ altitude, because their distance from the eye is intermediate. The same effect will be equally well seen by placing three small wafers of the same size and colour on the square summits of the drawings of the quadrangular pyramids, or more simply, by observing the larger size of the square summit of the hollow pyramid. This explanation of the cause of the increased size of the horizontal moon is rigorously correct. If any person should suspect that the circles which represent the moon are unequal in size, or are at different distances from the eye, they have only to cut the diagram into three parts, and make each drawing of the frustum of the cone occupy a different place in the binocular slide, and they will obtain the very same results. Hence we place beyond a doubt the incorrectness of Dr. Berkeley’s theory of the size of the horizontal moon,—a theory to which the stereoscope enables us to apply another test, for if we make one or more of these circles less bright than the rest, no change whatever will be produced in their apparent magnitude. CHAPTER XIV. APPLICATION OF THE STEREOSCOPE TO PURPOSES OF AMUSEMENT. Every experiment in science, and every instrument depending on scientific principles, when employed for the purpose of amusement, must necessarily be instructive. “Philosophy in sport” never fails to become “Science in earnest.” The toy which amuses the child will instruct the sage, and many an eminent discoverer and inventor can trace the pursuits which immortalize them to some experiment or instrument which amused them at school. The soap bubble, the kite, the balloon, the water wheel, the sun-dial, the burning-glass, the magnet, &c., have all been valuable incentives to the study of the sciences.