VISION, the act of seeing, that faculty of the mind by means of which, through its appropriate material organ, the eye, we perceive the visible appearances of the external world. Vision is mainly concerned with the color, form, distance, and tridimensional extension of objects. It is caused by impact of ether waves on the retina of the eye, but if these waves be longer or shorter than a certain limit, there is no visual impression produced by them. The apparent color of an ob. ject depends partly on the wave length or wave lengths of the incident light waves, single or mixed, and partly on the state of the eye itself, as in color blindness, or after taking santonine, which makes external objects look yellow, or in jaun dice. The apparent brightness of an ob ject depends on the amplitude of the light waves which pass from it to the eye; and the smallest perceptible difference of brightness always bears a nearly con stant ratio to the full intensity of the bright objects (Fechner's psychophysi cal law). As between different colors the eye perceives them with different inten sities, even when the physical intensity is the same; thus yellow appears brighter in a bright light than an equally intense red; and as light fades away the differ ent colors fade away unequally, so that the ratio in Fechner's law is different for each color; red and yellow disap pear first, blue last; and thus in a dim light the blue is the brightest.
The leading problems in the theory of vision are, however, those which deal with the nature of our perception of dis tance and of three-dimensional extension. According to Bishop Berkeley, whose views ("On the Theory of Vision") have met with the widest acceptance, we do not by means of sight perceive either that external objects are outside our selves or their distance from us; but this knowledge is derived from touch and from our experience of motion from place to place; and as our experience is in general uniform, we come to associate the visible with the tangible so readily that we fancy we directly see visible ob jects. As regards the distance of any given point, Berkeley maintains that this cannot be seen, "for distance being a line directed endwise to the eye, it projects only one point in the fund of the eye; which point remains invariably the same, whether the distance be longer or short er." This may be true, and yet the eye may be obliged to put forth perceptibly different efforts in order to discriminate points situated at different distances. Rays proceeding from the distant point form a cone, whose base is the pupil of the eye; and in order to make this di vergent cone converge on a point in the retina a distinct effort of focussing or accommodation is necessary for each dis tance, That the distance may be judged by means of the necessary effort of ac commodation may be seen by taking a small thin-edged lens, holding it at arm's length, and viewing distant objects through it. A small inverted image of
the distant objects is seen; but on try ing to ascertain at what distance this image is situated, the necessary accom modation teaches us that it is situated beween the eye and the lens. The effort of accommodation appropriate to each distance is the same whether the rays have actually come from the apparent distant point or not, so long as they ap proach at a certain angle of divergence; hence rays from clouds reflected in still, turbid water, and continuing their diver gence after reflection, approach the eye, and are dealt with by it, as if they had proceeded from a great depth below the surface of the water. Similarly the ap parent depth of objects under water is diminished because the amount of diver gence of the rays is altered by refrac tion; and the apparent distance of an object is increased by repeated reflection because after such repeated reflection the rays originally diverging from the object reach the eye diverging as if they had come from a more distant point, the virtual apex of the incomplete cone of ultimately reflected rays.
The axis of the double cone of rays, first divergent as it approaches the eye, and then convergent upon the retina within the eye, fixes the direction of the apparent position of the point (which may or may not be the real position, ac cording to circumstances), and the strain to which the eye is subjected in accom modation measures the apparent distance in that direction.
On ordinary optical principles a point above the direct line of vision comes to a focus at a point of the retina below its center, and vice versa. If the retina could be looked at by another person it would be found that an image of the object is formed on the retina, and that this image is inverted. It has been much questioned how this inverted image can produce the sensation of direct vision. We may observe in the first place that the question is somewhat nugatory, since the individual never becomes directly aware of the inversion or, it may be, even of the existence of the physical image in his own retina; and secondly, that the individual has come strongly to associate, by experience, the top of an object with the act of looking up in or der to see it, and vice versa. Any in crease in the magnitude of the retinal image is generally associated with ap proach of the object, and in the excep tional cases in which this result can be brought about by means of lenses, even where the real distance is increased, the object seems to approach; this seeming to approach being the result of an un conscious process of reasoning. The mind, on the basis of tactile experience, interprets any given object as being of a known or ascertained size; if it comes to look larger, it is inferred that it has come nearer.