It is necessary, however, to distinguish between these effects and true double vision, as well as a certain defect which exists in the eyes of many people, consisting in the apparent multiplication of distant objects by the same eye. In these cases, there is a superposition of images upon the retina, each having its proper bounds. With the majority of individuals afflicted in this way, it only happens when they look at a very distant object, the moon or stars for instance. There are many, however, who suffer from it in the case of everything they look at, whether far or near. Stephenson, who was affected with it, made it the subject of many interesting experiments. When he looked at a clear mark on a white ground, and gradually walked away from it, not only did the image become indistinct, but it seemed to unfold itself into several, independently of many others much more indistinct, more especially two situated on each side, whose distance increased the farther he walked away. As these latter images became more and more separated, they also became more confused. The image seen by the right eye was a little higher than that seen by the left. Griffin states, that after having used the telescope for any length of time, the eye that he kept shut always saw objects triple and double for some hours afterwards. These phenomena are possibly connected in some way with the disposition of the plates and fibres of which the crystalline lens of the eye is composed. Semi-vision, or _hemiopia_ as it is called, is much more rare and more difficult to explain than the phenomena of double vision; and consists in the power of being able to see only the right or left half of the object looked at, the separation being vertical when the eyes of the observer are in the same horizontal line. Thus, in looking at the word NEWTON, the person so afflicted would only see either the letters NEW or TON according to which half of the eyes were defective. Wollaston was afflicted with hemiopia on two different occasions; the first time after violent exercise, during two or three hours, when he could see distinctly only the left-hand halves of the objects he was looking at. Both eyes were similarly affected, and the phenomenon only lasted about a quarter of an hour. Twenty years afterwards he suffered again from the same accident, but on this occasion in the contrary manner; that is to say, he only saw the right halves of the objects he was looking at—to use his own words, he could only see the right half of every friend he met. At certain distances from the eye, one of two persons would become invisible, and by simply changing his own position or that of the persons he was near, he could make one or other of them, or indeed both, disappear at will. It must be acknowledged that similar tricks of Dame Nature, due to an unconscious insensibility of the eye, are most singular, and at first sight appear to have a supernatural origin.