SIGHT, in physiology, one of the five human senses, having for its organ the EYE (q. v.). In this organ, then, we have the retina, an expansion of the optic nerve, to receive and transmit to the brain the impressions of light; certain refract ing media for so disposing the rays of light traversing them as to throw a cor rect image of an external body on the retina ; and a ccntracting diaphragm, with central apertures for regulating the quan tity of light admitted into the eye. When the eye is directed to any object, an image of that object is depicted on the back of the eye by means of the rays of light en tering the pupil, and duly refracted by the different humors. The image, which is inverted, produces, somehow, an im pression on the retina with the assistance of the choroid coat, and this impression passes inward to the nervous centers, whence the optic nerve takes its rise. The subject of binocular vision, or that with two eyes we only perceive one object, was long a perplexing subject to philosophers, till it was satisfactorily explained by means of that now well-known instrument the STEREOSCOPE (q. v.).
The power of adapting the eye to vision at different distances has received the most varied explanations; but the opinion now generally entertained is that it de pends mainly on some alteration, either in position or form, which takes place in the crystalline lens. Some persons pos
sess this power of adaptation in a very slight degree, and thus labor under de fective vision, of which there are two kinds. Nearsightedness (myopia) is caused by anything—such as undue convexity of the cornea—which increases the refract ing power of the eye, and so causes the image of an object to be formed at a point anterior to the retina, and is rem edied by the use of concave glasses. Long-sightedness (hypermetropia), on the other hand, is owing to conditions the re verse of this, and is remedied by the use of convex glasses, which diminish the focal distance of an image formed in the eye. We judge of the motion of an object partly from the motion of its image over the surface of the retina, and partly from the motion of our eyes following it; and of the form of bodies, partly from the mere sensation, and partly from the as sociation of ideas.
In law, bills of exchange are frequently drawn payable "at sight" (i. e., on pre sentation), or a certain number of days "after sight." In the last case the time begins to run from the period of present ment and acceptance. "Sight draft" and "Sight bill" are bills payable at sight.