Page:The wonders of optics (1869).djvu/40

 single. We have only to look at the same object first with the right eye, and we shall see it directly against some portion of the wall of the room in which we are sitting; then looking at it with the left eye, we shall see that it covers a different part of the wall. This experiment is easily tried, and is very convincing. Thus we see that an image is formed on both eyes, and we consequently see the object, whatever it may be, repeated twice. By degrees, however, the eyes gain the power of converging their axes on objects at different distances, so that they fall on similar portions of each retina, and so convey a single impression to the brain. Thus, for instance, if we look at a pencil held up at arm's length, and then, without changing the position of the eyeball, look at some distant object, we shall see it double. Let us, however, converge the eyes upon it, and the two images unite. Reverse the experiment by now looking at the pencil without converging the eyes upon it, and we shall see that object double in its turn. The same thing happens if we push aside one of the eyes with the finger while looking at any object. During severe illness it often happens that the patient from extreme weakness loses the power of convergence, and consequently sees every thing double, and we continually see children's faces wearing a most distressing appearance through having temporarily lost the power of moving the muscles of the eye. It is a common expression to use in speaking of drunken people, that they see double, but the saying, unlike many others, is no metaphor; when a man gets drunk he loses his power over the muscles of his eye, just as he does over those that sustain his body, and the instinctive closing of one eyelid, in order that he may see objects single, is an effort of his weakened judgment to set things right once more.

While on this subject we may mention the experiment