But there are defects of vision dependent upon the structure of the eye itself, as an optical instrument; and under this head we may first notice _myopia_, or _near-sightedness_. _Myopia_ results from an over-refractive condition of the eye, the rays of light passing from any object being brought to a focus in the vitreous chamber of that organ, before reaching or impinging on the retina. In this case, either the cornea or the crystalline lens is too convex, or both are in this condition, or the humours of the eye generally are too dense or too abundant, and the pupil is large. Persons thus affected see all objects indistinctly which are viewed at the ordinary distance of distinct vision, but objects held close to the eye are seen with microscopic accuracy. The distance from the eye, when in a normal state, at which objects are surveyed with the utmost distinctness, is from fifteen to twenty inches. An eye which cannot discern objects with ease and accuracy beyond ten inches, may be considered as myopic; but not unfrequently persons affected with myopia cannot regard an object with distinctness which is beyond four, three, two, or even one inch distant from the eye. To myopic persons, objects brought within their sphere of vision appear magnified; a small type is read more easily than a large type; and, indeed, such eyes see better through a pin-hole in a card than when not thus assisted; the reason is, that the pupil, from its dilatation, admits too many rays of light, but by diminishing the aperture through which the rays of light are admitted, all but those which are the most direct are excluded, and the images on the retina will be, therefore, the more defined. On the same principle, myopic persons, when endeavouring to see distant objects, half close the eyelids, (hence the term myopia, from μύω, I shut, and ὤψ, an eye.) Myopic persons generally see better in the dusk, or at twilight, than others; and their sight, though short, is strong and good, often lasting. It has been asserted, that as the eye flattens in age, the vision of myopic persons will improve as they advance in life, and the theory, it must be acknowledged, seems feasible, but experience proves that this is not the case; indeed, it would appear that the myopia rather increases than diminishes. Concave glasses greatly assist the myopic eye; they cause a divergence of the rays of light before entering the pupil, and thus counteract the over-refractive condition of that organ. The degree of concavity best suited to particular cases can only be determined by trials.