This, however, is more clearly seen in the case of vision. When we look through the window and see an object such as a house before us, we do not habitually localize the sensation in a certain part of the retina, but we refer the object to a particular position more or less distant in the world around us. This projection of the object outwards in a right line from the eyes is really a marvellous process, though the wonder of it is lost in its familiarity. It is the outcome of the experience of hundreds of generations. And the experience is not gained through vision alone, but through this in combination with other senses and activities. We see an object, but we have to go to it before we can touch it. It is not in contact with us, but distant from us. Its outness and distance is a matter of what is termed the geometry of the senses; and this geometry has been elaborated through many generations of organized beings, from data given by sight, touch, and the muscular sense. It is true that I can now estimate the distance of the house without going to it; but my eyes go to it, and I can feel them go. The panes of my window are separated by iron bars. As I look from them to the distant house and back to them again, I can feel my eyes going from one to the other. The lens of the eye is adjusted for near or far distance by the action of a ciliary muscle, through which its anterior surface can be flattened, returning again by its own elasticity to the more convex form when the muscle ceases to act. Each eye, moreover, is moved in its orbit by six eye-muscles, and in normal vision the two eyes act as one organ. For near distances they converge; for far distances there is less convergence. Through the muscular sense, which is here extraordinarily delicate, we can feel the amount of accommodation and convergence; and thus we can feel the eyes going to or coming from a near and a distant object. Of course, we are aided in judging or estimating distances by the apparent size of the object when the real size is known, by the clearness of its outlines in a slightly hazy atmosphere, and so forth. But apart from such judgments, it would probably be impossible to perceive that an object is near or distant in the absence of muscles of accommodation and convergence affording the data of the muscular sense. Not only the distance of two objects from the eye, but their distance apart, can be measured by the aid of the muscular sense as we move the eyes from one to the other. And in us this is so delicate that, according to Weber, a distinct muscular sensation is attached to a displacement of a sensitive point of the yellow spot through less than 1/6000 of an inch.