Spectacles are devices for aiding sight, and consist of lenses of glass, rock crystal, or plastic, mechanically supported before the eyes. The term is more generally used for those glasses which are fixed in frames and fitted with supports for the ears. Spectacles without ear supports, which grip the nose are called pince-nez. The lenses may be surrounded by a rim for further support, or be rimless for the sake of appearance.
The invention of spectacles is probably due to the Chinese, and there are references by Latin writers to an eyeglass of beryl worn by Nero. Alessandro de Spina and Roger Bacon are both credited with the invention of spectacles in Europe, while Benjamin Franklin is credited with the invention of bifocals.
Eyeglass lenses are made from optical glass, rock crystal and plastic. The lenses are divided into two main classes, spherical and cylindrical, each sub-divided into concave and convex and their combinations. Concave lenses are used for the correction of myopia, convex lenses for hypermetropia. Cylindrical lenses are used to correct astigmatism. The surface of such lenses are sections of cylinders. Combinations of lenses are used where the sight of one eye is differently affected from the other. Prismatic glasses are used in cases of double vision, caused by weakness of the muscles of the eye-ball.
In compound spectacles the same pair of glasses may be used for long sight and for reading purposes. The bifocal lenses consist of a weak upper half and a stronger lower one, the latter for reading, the former for ordinary vision. In the torric lens one part of the glass is a spherical curve and the other cylindrical, and acts in a similar way to the bifocal. Research Spectacles
Abbreviations
SciTech
Research Spectacles