392 VISION it is perceived with imperfect outlines, since it is outside the line of distinct vision. But for the right eye it is to the right of this line, and for the left eye to its left; and the tvo images therefore no longer occupy the same spot, and the object accordingly appears double. On this account we never see all parts of a landscape, even directly in front, with distinctness at the same time. The fore ground, the middle ground, and the distance are each examined separately ; and when one is distinct, the others are always more or less confused. One great advantage of the simul- taneous action of the two eyes, as above de- scribed, is that we are thus enabled to judge in great measure of the distance of an object. As the angle of convergence of the two eyes varies according to the distance at which an object is placed, wo instinctively appreciate the amount of this convergence and conse- quently the distance of the object. A still greater advantage is, that by using two eyes we appreciate the qualities of solidity and pro- jection of foreign bodies. Vision with but one eye presents a simple expanse or variously shaded and colored picture in front of the visual organ. But since the two eyes are placed at a certain distance from each other, and their visual axes, as above mentioned, are convergent, it follows that for near objects it is not precisely the same figure which is per- ceived by each eye. If we look, for example, at a square box at the distance of a few feet, both eyes will see the front of the box equally well; but in addition the left eye will see a lit- tle of its left side in perspective, and the right eye will see a little of its right side. These two images, occupying the same spot at the con- vergence of the visual axes, are perceived as one ; and it is by this combination of two dif- ferent figures that we acquire the perception of the solidity of the object examined. We feel that it is not a fiat picture, because in that case the same image would be presented to each eye ; and however well it might be painted, such a picture could never deceive us, if looked at with both eyes simultaneous- ly. It is on this principle that the stereo- scope is constructed. (See STEREOSCOPE.) A change is also required in transferring the sight from near to remote objects, or rice versa, which is called the accommodation of the eye to distinct vision at different distances. The eye is a natural optical instrument, consisting essentially of a sensitive nervous expansion, the retina, intended to receive the impression produced by luminous rays, and of various re- fracting media, destined to converge these rays to a focus at the surface of the retina. The distinctness of vision depends upon the accu- racy with which the rays of light, diverging from all parts of a luminous object, are brought to a focus exactly at the level of the sensitive membrane destined to receive them. The most important and efficient of the refracting parts of the eye is the crystalline lens, a doubly con- vex transparent body, of considerable density, immediately behind the pupil and some dis- tance in front of the retina. As the origin:il di- vergence of the luminous rays passing through the pupil varies with the distance of the object from which they emanate, the crystalline lens, if both its refracting power and its position remained the same, could not bring the rays to a focus at the same point behind it, for lumi- nous objects at different distances. For ex- ample, suppose the lens to be so arranged that the rays from an object at the distance of 6 in. may be accurately focussed at the retina. If the object be now removed to a distance of 80 in., the divergence of its rays at their entrance through the pupil will be diminished ; but, the refractive power remaining unaltered, they will now be converged more rapidly than be- fore. They will accordingly meet and cross each other ; and when they reach the lens they will have already become partially dispersed, the effect of this being to produce partial in- distinctness of vision. This difficulty is met with in telescopes and spy glasses, and is ob- viated in them by shifting the relative distance of the object glass and the eye piece, when the instrument is turned from a near object to a remote one, or nice versa. In the eye the cor- rection might be accomplished either by mov- ing the lens backward and forward, so as to vary its distance in front of the retina, or by changing its refractive power, to correspond with the varying distances of visual objects. The experiments of Donders and others have shown conclusively that the latter method is that which is really employed by nature. If the eye be accommodated for vision of a dis- tant object, and a lighted candle be then held in front of it and a little on one side, an ob- server will perceive three reflected images of its flame in the eyeball ; namely, one from the anterior surface of the cornea, one from the anterior surface of the lens, and a third from the posterior surface of the lens. If the sight be now changed from the distant to a near object, the reflected image upon the cornea remains unaltered, but that from the anterior surface of the lens diminishes and moves slight- ly forward. The third reflection also dimin- ishes a little, but is not altered in position. These changes show that in shifting the ac- commodation of the eye from distant to near objects, the lens increases the curvature of its surfaces and of course its refractive power, the bulging taking place principally in front. When the sight is shifted from near to remote objects, of course these changes are reversed. Thus, when we look at a near object, the re- fractive power of the lens is increased, and the diverging rays are more powerfully refracted ; when we look at an object comparatively remote, the refractive action is diminished, and the slightly divergent rays are still brought to a focus exactly at the surface of the ret- ina. This alteration in the form of the lens is accomplished by means of the internal
