Vision

I

Introduction

Vision, physiological power of sight. Vision is the faculty by which, through the material organ, the eye, the visible external world is perceived. Many simple organisms have light receptors and can thus react to motion and shadows, but true vision involves the formation of images in the brain. Eyes of different organisms provide images of varying clarity: This article deals with vision in humans and other animals with eyes of comparable complexity.

II

Basic Principles

Vision is mainly concerned with the colour, form, distance, and three-dimensional extension of objects. It begins with the impact of light waves on the retina of the eye, but if these waves are longer or shorter than a certain limit, they produce no visual impression. The apparent colour of an object depends partly on the wavelength or wavelengths of the incident light waves, single or mixed, and partly on the state of the eye itself, as in colour blindness. The apparent brightness of an object depends on the amplitude of the light waves that pass from it to the eye, and the smallest perceptible difference of brightness always bears a nearly constant ratio to the full intensity of the bright object.

On ordinary optical principles, a point above the direct line of vision comes to a focus at a point of the retina below its centre, and vice versa. If a retina were observed by another person, the observer would see that the image of an object is formed on the retina, and that this image is inverted. Any increase in the magnitude of the retinal image is generally associated with approach of the object; if this same result is brought about by means of lenses, however—even where the real distance is increased—the object still seems to approach. This seeming approach is the result of an unconscious process of reasoning. The mind interprets any given object as being of a known or ascertained size.

III

Stereoscopic Vision

Humans and other animals that are able to focus both eyes on a single object are capable of stereoscopic vision, which is fundamental to depth perception. The principle of stereoscopic vision can be described in terms of the vision process involved in the use of a stereoscope, which presents an image from two slightly different angles so that the eyes can merge them into a single image in three dimensions.

The figure below shows L and R representing the two eyes and SS a line (the “horopter”) drawn through the point A where the optic axes LA and RA intersect, and parallel to a line joining the two eyes L and R. The point A is seen in corresponding points of the two eyes, axially situated. Two points r and l, however, may be so placed—either in the plane of the horopter or outside it—that the two eyes together perceive the points r and l as one point, B (in Fig. 1 point B is nearer to the eye, and in Fig. 2 farther from the eye than the horopter SS itself.) Suppose now, in Fig. 1, a diagram is made representing l and A, and another representing r and A. Then suppose the former is laid before the left eye and the latter before the right eye. The two optic axes are thus made to converge, so that the image of A is formed in corresponding points in the two eyes. As a result, the points l and r will appear to blend into one, situated either nearer the eye than A or further from it. This explains the action of the stereoscope and also the “pseudoscopic” effect produced when the pictures are reversed. See also Optics.

A type of stereoscopic picture called an autostereogram, not requiring a special viewing instrument, has been made possible by the advent of computer graphics. Two computer-generated pictures, each representing the view seen by one eye, are overlaid, as in an anaglyph. Viewed in the ordinary way the combined image looks like an abstract pattern, or else like some scene unrelated to the hidden stereoscopic view. The latter is revealed when the viewer's eyes are focused on some point that is nearer or farther away than the autostereogram.

IV

Sight Defects

The most common disturbance to vision is caused by crystals or other small opaque bodies in the humours of the eye, which are usually only a passing inconvenience. Much more serious are opacities called cataracts, which develop in the lens of the eye as a result of mechanical injury, advancing age, or dietary deficiencies. Opacity of the cornea also causes obscured vision, but this condition may be repaired through transplantation of a section of clear cornea from another person.