the moon is seen with all its natural roundness, and every mountain projects as in a model, placed onlj a few inches from the eye. But how is it that a stereoscopic picture of the moon can be obtained? This, at first sight, appears impossible, as she always turns the same side to us. When a stereoscopic portrait is taken, two views of the party must be obtained, and this may be done in two ways. When one picture is taken, the camera is moved a little to one side and a second taken, the party sitting immovable all the time; or the camera may be fixed, and the party may turn his body a little round for the second picture. It is in this latter way, a stereoscopic picture of the moon is obtained. The camera, of course, cannot be moved sufficiently aside to take a picture from a different point of view, and it is therefore stationary. The moon, however, effects the object required by turning her face a very little round, so that a somewhat different perspective is obtained. This small movement is called her libration, and, though small, is quite sufficient to give the required stereoscopic effect. The moon always presents the same face to us, as she rotates on her axis in the same time that she revolves round the earth; but these two motions are not perfectly coincident during a revolution, and we are, therefore, permitted to see, a small way, round her globe. It is from this slight oscillation that the stereoscope gives us so perfect a representation of the lunar surface. If the student's first acquaintance with
