SENSATIONS.
Vision. The mode in which the rays of light are affected, in passing through the various parts of the eye, is explained un der the article Orgies. See also VIsrox. We have only to add a few remarks on the physiology of the eye.
The quantity of light that can enter the Z z eye depends on the state of the pupil ; which is again influenced by the motions of the iris. When, after shutting the eye lids, they are suddenly opened in a strong a disagreeable impression takes place on the eye, and the iris dilates or becomes broader : hence the pupil is contracted, and the quantity of light ad mitted into the eye diminished. An op posite change takes place when we go from a strong into a weak light. These motions depend entirely on the mode in which light affects the retina ; for the iris is of itself insensible to luminous rays. The painful effect produced on the reti na by a strong light is obviated by the contracted state of the pupil ; while the opposite condition of that opening, in darkness, is designed to admit a suffici ent quantity of rays, to produce a proper impression on the retina.
The seeing of bodies erect, although their images are painted inverted on the retina, is thus explained. Au object is said to be inverted in respect to others are erect : now all objects what evetstre painted inverted on the retina, and all therefore correspond to each other in situation and connexion, just as if they had been represented in their natural position. All confusion is, there fore, guarded against in the mind ; to which the image itself on the retina is not communicated, but only an impression caused by its formation.
The motion of the iris contributes to distinctness of vision by regulating the quantity of light admitted into the eye : and there is another provision tending to the same effect; viz. the absorption of any superfluous rays, which may have en tered the eye by the black covering of the choroid coat. The utility of this dark pigment may be understood from observing the effects of its absence in the albino, where it causes a tenderness of the organ, and an impatience of the light.
Distinct vision also requires that the focus of the refracted rays should fall ex actly on the retina, without falling short of it in the vitreous humour, or being elongated beyond it. The former fault constitutes near-sightedness (myopia); where the cornea and lens are too con vex, and the sight of remote objects is imperfect. The latter defect is far-sight edness (presbyopia, as being common in old persons); where an opposite condi tion of the eye obtains, and near objects are seen imperfectly.
As the eye possesses a very considera ble range of power in seeing distinctly both near and distant objects, it must possess some powers of accommodation adapting it to these differences of dis tance. Various opinions have been held on this subject; but none are supported by sufficiently direct and convincing ar guments to command universal assent. The changes in the condition of the pupil have some effect: it contracts when we look at a near object, to exclude those rays which would be too divergent for the powers of the eye; and it dilates in the opposite case, to take in the divergent rays. Besides this, different physiologists have admitted a power of motion in the crystalline, by which its convexity may be altered; a movement of that body back wards and forwards, in the eye, by the ciliary processes, so as to place it at dif ferent distances from the retina; a com pression of the globe by the four recti muscles, and consequent elongation and shortening of the optical axis.
That the retina, in the very axis of the eye, is insensible, owing to the entrance of an artery at that part, is shewn by ex periments, in which objects vanish when their images are brought on that point.
Single vision, with two eyes, probably arises from habit; for children seem to see double ; and the same affection, after diseases, has been conquered by use.