Spectacles

Spectacles are lenses or other refracting objects used for aiding the sight when the eyes are defective. Spectacles of convex lenses are supposed to have been invented about the end of the 13th century, and are used by long-sighted people. These produce a virtual image of the object further away than the object from the eye, and hence at a more convement distance for a long-sighted person. Concave lenses were used soon after the others in spectacles for short-sighted people, an image being produced nearer than the object to the person. These are the commonest forms of spectacles; but other kinds are used in certain cases - e.g. prisms are employed in some cases of squinting, and cylindrical lenses are used to remedy astigmatism (q.v.). It was formerly the rule to number lenses according to their focal lengths given in inches, but the system was not convenient, especially as the inch is not a universal unit. A more scientific system is one in which 1/focal-length is taken as the number; this has been named a "dioptric," when the metre is the unit of length. The number of dioptrics, therefore, varies directly with the refractive power or strength of the lens.