Page:Willich, A. F. M. - The Domestic Encyclopædia (Vol. 4, 1802).djvu/129

SPE or uniform manner, so is the ope- ration of medicines necessarily at- tended with different effefts, on various constitutions ; and even on the same patient, at different times. — See also Nostkum, and Quack-medicines. SPECTACLES, a well-known and useful optical contrivance, in order to aid the eyes of the aged, or young invalids of defedive vi- sion, by means of two appropriate lenses. Those used by short' sighted persons, are generally con- cave ; as the spectacles employed by the far-sighted, are convex. The choice of speiStacles being an objetSt of great importance, to all who are anxious to preserve their eyes, we shall seletl a few rules from Mr, Adams's valuable Essai/ on Vision; and the Editor's Lec- tures on Diet and Regimen. — By a careful attention to such direc- tions, the failure of sight may be retarded, and the eyes greatly re- lieved. Those, who stand- in need of speftacles, ought at lirst always to chuse such as represent objcfls, without enlarging or diminishing them ; and which, on being placed near the eye, exhibit printed cha- racters clearly and distinttly, with- out straining that organ. It will, therefore, in every instance, be ad- visable to consult the aitist of whom the glasses are purchased : for^, though every person must eventually determine what lenses affoid him the most accuratevision, the former will thus be enabled to accommodate them to the eye of ' the latter, with greater certainty and advantage. Besides, the fa- tigue of trying a variety of glasses, will thus be obviated 3 and the purchaser will procure a pair, best adapted to the structure of his SPE [to^ eyG5.-^Short-sighted persons ought always to make a very gradual al- lowance in changing their spe6ta- cles, so as to sele6t others which magnify a little more than the pre- ceding pair, though somewhat less distindly, without obscuring the obje6t. Thus, their sight will be imperceptibly improved ; and, after making use of less concave lenses, the defeft of vision may, in pro- cess 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 exaftly fit both eyes, or with a person whose or- gans of sight are both of a size and construdion perfectly equal, it ra- tionally follows, that such import- ant choice should b". separately made, with respcd to each of these useful organs, Spedacles are generally transpa- rent and colourless ; though some- times 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 objeS, 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 as- sume a reddish tin^e, after having tried the experiment for a few days. In all cases, h.owever, spec- tacles ought to be employed only in writing, reading, or similar oc- cupations that render this artificial aid necessary ; and during which, the eye is retained at an uniform distance. In December, 1793, a patent was