Spectacles, a well-known and useful optical contrivance, in order to aid the eyes of the aged, or young invalids of defective vision, by means of two appropriate lenses. Those used by short-sighted persons, are generally concave ; as the spectacles employed by the far-sighted, are convex.
The choice of spectacles being an object of great importance, to all who are anxious to preserve their eyes, we shall select a few rules from Mr. Adams's valuable Essay on Vision; and the Editor's Lectures on Diet and Regimen. - By a careful attention to such directions, the failure of sight may be retarded, and the eyes greatly relieved.
Those, who stand in need of spectacles, ought at first always to chuse such as represent objects, without enlarging or diminishing them ; and which, on being placed near the eye, exhibit printed characters clearly and distinctly, without straining that organ. It will, therefore, in every instance, be advisable to consult the artist of whom the glasses are purchased : for, though every person must eventually determine what lenses afford him the most accurate vision, the former will thus be enabled to accommodate them to the eye of the latter, with greater certainty and advantage. Besides, the fatigue of trying a variety of glasses, will thus be obviated ; and the purchaser will procure a pair, best adapted to the structure of his eyes. - Short-sighted persons ought always to make a very gradual allowance in changing their spectacles, so as to select others which magnify a little more than the preceding pair, though somewhat less distinctly, without obscuring the object. Thus, their sight will be imperceptibly improved; and, after making use of less concave lenses, the defect of vision may, in process of time, be entirely remedied. These transitions, however, ought not to be sudden ; lest the resources of art should be too early exhaust-ed. And, as it would be difficult to meet either with a pair of glasses in the shops, that exactly fit both eyes, or with a person whose organs of sight are both of a size and construction perfectly equal, it rationally follows, that such important choice should be separately made, with respect to each of these useful organs.
Spectacles are generally transparent and colourless ; though sometimes green lenses are preferred by those, whose eyes are unable to support a vivid light. Such colour is believed to be the most soothing to the human eye; though it tends, at first, in some degree to darken the object. Hence, this shade will prove beneficial only to persons who possess strong, but irritable eyes; yet even such individuals should not indulge in it, if light-coloured objects continue to assume a reddish tinge, after having tried the experiment for a few days. In all cases, however, spectacles ought to be employed only in writing, reading, or similar occupations that render this artificial aid necessary ; and during which, the eye is retained at an uniform distance.
In December, 1793, a patent was granted to Mr. John RICHARD-son; for his invention of a machine that may be applied to spectacles or glasses, and pebbles of every kind. As, however, no distinct idea can be formed of his contrivance, without a proper delinea-tion, we refer the reader to the 10th volume of the Repertory of Arts, etc. where it is described, and illustrated with an engraved figure. Hence, we shall only observe, that the chief object of the patentee's invention, appears to be that of preventing the necessity of employing two pair of spectacles, where these are occasionally required ; as two distinct sights may thus be obtained, for any purpose, according to the desire of the wearer.