As a rule both eyes are moved at the same time in the same direction, so as to regard the same object. When one muscle becomes paralysed so that the eyeball cannot be turned in that direction, the two eyes no longer act to gether, when the person seeks to look that way. The sound eye is turned far enough, the other fails to go round. Squinting is produced, and the particular object looked at is seen double. As soon as the eyes are turned in other direc tions, they again act together, the squint disap pears and the vision is single. (Refer to Dou ble Vision, p. 458.) The Information gained by the Eyes.— It may be well to state here briefly the sub stance of the foregoing paragraphs. The eye is to be regarded as the peculiar form of ending of the optic nerve, designed to be affected only by light, and so excited by light as to send on to the brain an impression which there gives rise to a sensation of light. It is supplied with a series of structures that act as convex lenses, which so focus rays of light, passing through them from external objects, as to form small images of these objects on the retina, the ner vous coat that lines the inner surface of the back chamber of the eyeball. Now the first thing to notice is, that it is this image on the retina that produces the sensation of seeing something, and yet we are not conscious of the image on the retina but only of the outward thing from which the rays of light proceed. This is difficult to understand. It is doubtless the result of education. We learn that the things we see are the result of impressions reaching us from the outside, and we refer the object from which the impressions reach us out wards in the direction of the straight lines in which the rays of light fall upon the eye. Thus, it is related of a patient, who was blind from his birth owing to cataract, that when sight was restored by an operation, performed by the English surgeon Cheselden, he thought all ob jects he saw touched his eyes. His other senses corrected his mistake. He found when he put his hand up that the objects did not touch his eyes, that he had to walk towards them in order to touch them, &c. Thus he trained his eyes by means of other senses, and in other ways, to appreciate the distance from him of the objects he saw. Again, the brain of the man suffering from delirium tremens is dis turbed and excited by what he has drunk. The seeing centre in the brain is aroused by the stimulant and perverted by it, and he be comes conscious of images so produced, and be lieves them to represent actual existences. The creatures that leer at him, and crawl over him, and dance before him, are the creations of his excited brain, but his judgment is also per verted, and he is unable to perceive that they have no real objects corresponding to them in the external world. Again, when pressure is exerted on the eyeball, or when a sudden blow is received on the eyes, the nervous apparatus of vision is excited and colours or bright sparks (called phosphenes) are seen, which only ex perience teaches to be due to internal disturb awes. The production of what are called Pur kinge's figures is another example of the same thing. If a person goes into a dark room with a lighted candle, and, facing a blank plain coloured wall, holds the candle to the side of the head, moving it up and down, the appear ance of branching lines will be seen on the wall. These are shadows of the blood-vessels of the retina (p. 449), The sensitive portion of the retina (the layer of rods and cones) is be hind the blood-vessel layer, and thus the lines of vessels intercept the light passing in at the ex treme side of the eye, the shadows produced appearing to the person to be something out side. Then it is well known that minute floating bodies in the humours of the eye pro duce shadows which to the person seem to float across his vision in space. These are called muscse volitantes. It is then only by a process of education, in which the various senses take part, that a person learns to judge of the actual existence of an outward object corresponding to his sensation. It may be remarked that a simi lar explanation applies to hallucinations. This is the term given to things a person seems to see for which there is nothing externally to account. There are undoubted cases on record where an individual has seen a person or thing in the immediate neighbourhood, and by going up to the place has assured himself that nothing but simply space existed there. Sir David Brewster gives a case, in his Natural Magic, of a lady who on entering the drawing-room saw her husband standing on the hearth-rug with his back to the fire. She addressed him and sat down in a chair within two feet of the figure. After she had again spoken, the figure moved off to the window and then disappeared. Frequently afterwards she had similar experi ences, seeing other persons and things, in the presence, on one occasion, of her husband, who assured her that the cat she saw sitting on the rug at his feet had no actual existence. She
herself had the courage more than once to con vince herself that the appearance was a decep tion by sitting down on the chair on which she saw someone sitting, when the appearances ished. In these cases changes were excited in the nervous apparatus of vision not due to any outward existence, though as a rule only duced by such, and the lady was consequently for the time deceived, until she had corrected her sensations by other means.
Another thing to be noticed is that the image on the retina is upside down, and yet we see things in their upright position. When we di rect our eyes towards a particular object rays of light pass into the eye not only from that object but from other parts in its immediate surround ing, and we become conscious not only of the particular object we are looking at but of a region round about it. This region is called the visual field or field of vision. Now rays of light coming from the left of the field of vision fall on the right side of the retina, rays from the upper part of the field fall on the lower part of the retina, rays from the lower part of the field on the upper part of the retina, and so on. We refer the image on the lower part of the retina in the direction from which the rays come, that is, towards the upper part of the field of vision. Moreover we interpret by means of touch, for, to reach with the hand the part of the object whose image is on the lower part of the retina, we must raise the hand, and to touch the part of the object whose image falls on the right side of the retina we must pass the hand to the left side, and so on. Thus though the image is upside down on the retina, we see the object upright.
The estimate of size given by the eyes de pends on the angle formed by the rays of light before crossing in the eye. This is explained by Fig. 182. From the object PAH, rays Pp, xh pass to the eye. At o they form an angle r on. This is the visual angle, the angle under which PAn is seen. PAn forms an image ph on the retina, and its apparent size is dependent upon the angle at o. But the lines and r"c n" are seen under the same visual angle, and will, therefore, have the same apparent size, and form images of the same size on the retina. To this impression, however, there remains to be. added the idea of distance. We know that as objects pass farther and farther away from us they appear smaller and smaller, and as they ap proach they become larger. If, therefore, an object at a great distance off appears as large as an object very near to us, we judge the far-off object to be much more extensive than the near one.
Our appreciation of distance is guided to a large extent by the clearness with which the object looked at is perceived and its details made out If the atmosphere be very clear, mountains at a distance appear nearer than they do when the atmosphere is hazy. An ar tist gives an impression of distance to the objects in the background of his picture by the want of distinctness of their outline and detail. It is very difficult, however, to judge absolutely of distance. Between us and a distant object a great many other objects intervene, whose dis tance from us we can more readily estimate. We thus guide ourselves in forming au idea of the distance of the far-off object by the others which are between, and which afford us some , thing to measure by. Thus everyone knows the errors easily made by sailors at sea in judg ing of the distance between their ship and another, because of the absence of anything be ' tween to aid the vision.
For various reasons, therefore, judgments formed by vision of the real size and distance of things are not too reliable. A good illustra tion is given in Fig. 183. The distance between A and a seems greater than the distance between a and c, and yet it is the same, the apparent in creased space between A ands being due to the markings between. For the same reason, of two squares absolutely identical in size, one marked with alternately clear and dark cross bands, and the other with alternately clear and dark upright markings, the former will appear broader and the latter higher than the other. Thus a short stout person whose dress is cross striped or made with flounces appears stouter than she really is, and a tall woman whose dress has upright markings or folds that run up and down exaggerates her length. Conse quently a stout person who wants to increase her apparent height and diminish her apparent stoutness should wear dresses striped or folded up and down, and a tall person who wishes to diminish her apparent tallness and to appear stouter should wear cross-marked or folded dresses.