The period at which the sight begins to fail does not at all depend on age, but varies in different persons according to the formation of the eyes, the treatment they have received, and the constitutional capability; therefore, the age of the person requiring spectacles gives but a vague general idea to the optician as to what is required, unless other particulars are stated; such as whether glasses have been used before; the distance at which writing and printing is seen pleasantly without assistance, the focus of those last used, or sending even but a broken piece of the same. “The proper selection of spectacles for imperfect vision is a point of much deeper importance than is generally believed. An _oculist_ who is only acquainted with the diseases of the human eye, without possessing any knowledge of it as an optical instrument, is often led professionally to recommend glasses when they ought not to be used, or to fix on focal lengths entirely unfit for the purpose to which they are applied, and the _mere vender_ of spectacles and lenses is still more frequently in the habit of proffering such counsel.”—BREWSTER. The near-sighted, or those who require concave spectacles should use those of the slightest power; No. 0, or No. 1, will generally be sufficient at first, but this, by the aid of the trial box, can readily be determined by the wearer himself. There is such an immense benefit experienced by the short-sighted from spectacles which suit their sight, that to argue for their adoption of them would be quite superfluous. Without spectacles they are excluded from observing beautiful landscapes, recognizing individuals, or viewing to advantage any of the crowd of interesting objects around them; but by adopting them they are placed on a par with the long-sighted in such circumstances, while the sharp and microscopic character of their sight _without_ spectacles, gives them many advantages over those possessing ordinary vision. The short, or near-sighted eyes, have the cornea, and often the crystalline lens, more convex or arched out than in long-sighted eyes. This formation causes the rays to converge to a focus before they reach the retina, but by the application of a concave lens the difficulty is corrected, and the rays are carried on to their proper point for giving a perfect representation on the retina. This character of sight is very frequent, and is more particularly remarkable among those whose mode of life restricts them to crowded cities, sedentary employments, and confined situations. Those whose infantine and youthful years have been passed in the country, or where the eyes have had a free range of view, not circumscribed by the walls of the nursery, or limited to the observation of objects near at hand, rarely require concave spectacles.[4]