SPECTACLES, the name given to lenses of any required form which are supported in front of the eyes to assist vision. They may be required to correct errors of refraction, or to cor rect imbalance of the ocular muscles, or to cut off injurious or unpleasant light or glare. The lenses for the first two purposes are usually made of crown glass, but in the early days, when it was not easy to obtain glass of good quality, free from striae (i.e., streaks of glass of a different refractive index, due to im perfect mixing of the constituents), they were also made of quartz, or rock crystal as it was called. Some years ago quartz lenses were revived by the opticians, and sold at fancy prices, on the erroneous plea that they were cooler to the eyes.
Professor M. von Rohr has discussed the history of Spectacles in several papers. (See Thomas Young Oration, Trans. Optical Soc. 1923-4 No. 2 and also in 1926-7 No. 3.) Briefly, it appears that Roger Bacon explained how to magnify writing by placing a segment of a sphere of glass on the book with its plane side down in his Opus Majus 1266. A portrait of Cardinal Ugone in a fresco in a church at Treviso painted in 1352, shows two mounted lenses with their handles riveted together and fixed in front of his eyes, so evidently a form of spectacles was known by this time. Up to the invention of printing in the middle of the 15th century, very few other than the monks could read, but the advent of the printed book greatly stimulated the demand for spectacles, and the trade in them grew rapidly during the loth century, especially in North Italy and South Germany where there were glass workers, and by about 1600 opticians were to be found in most of the towns of any size on the continent. The early mounts were made of horn and leather, and by about this time metal began to be used. The nomenclature of the lenses varied ; in Venice it is possible that the numbering represented the power of the lens, with a unit of about o.6 diopters. Else where they were either numbered to indicate the age for which they were supposed to be used, or according to the radius of the tool in which they were ground. In Germany spectacles were made for the very low price of 21- to 4 farthings, and were ex tensively exported. We have no information of the beginning
of the optical trade in England, but in 1629 a charter was granted by Charles I. to the Spectacle Makers' Guild. The invention of the method of grinding a number of lenses together on one large block, which Mr. Court has shown was made by Marshall between 1690 and 1693 (Trans. Opt. Soc. 1926-7, p. 122), gave the London opticians a very great advantage over their foreign rivals, as it enabled them to produce lenses of a better quality; and during the 18th century they acquired a great reputation, and were able to charge as much as sixteen guineas for the best double-jointed standard gold spectacles (gold case included) (Trans. Opt. Soc. 1923-4, p. 64). They intro duced the fashionable monocle about this time.
Bifocals, that is, lenses of which one portion has a higher power than the remainder, appear to have been first made for Benjamin Franklin about 1760.
Astigmatism, that is, a variation the refractive power of the eye in different meridians, was first recognised by Thomas Young, who found it present in his own eyes. He found he could correct it by looking obliquely through his spectacles, so it must only have been slight. Sir George B. Airy in 1825, who had one eye so badly astigmatic as to be unusable, showed that the astig matism could be corrected by a spherocylindrical lens, and had such a lens made for himself by Fuller, an Ipswich optician, in 1827. Cylindrical lenses, however, were seldom prescribed or supplied until towards the end of the 19th century.
In the last few years great improvements in the construction of bifocals have been made. The early bifocals were made by cutting away a portion of the weaker lens, which corrected the wearer's error for distant vision, and replacing it by a part of a stronger lens, adapted for his reading distance. The two partial lenses were mounted together in the frame, so that when he looked up he looked through the weaker lens, and when he looked down, he saw through the stronger one. The first improvement was to cement an extra lens over the lower part of the weak lens, so as to increase its power by the required amount over this area.