SPECTACLES. The mention of magni fying-glasses by Roger Bacon, who died about 1292, justifies the supposition that something like what are now called spectacles were in use at least several years earlier. Extensively as these useful instruments are employed, there can be no doubt that, were the subject more generally understood, the amount of advantage obtained from them would he greatly aug mented. The eyes of an individual whose sight is much tried often receive the most serious injury from improper delay in the use of spectacles ; while the sight of many persons is prematurely worn out by the use of glasses of too high a power, or too short a focal length. The use of a single reading-glass in stead of spectacles is very injurious; since, by occasioning one eye to be more used than the other, the focallengths of the two are rendered unequal. The unsteadiness of the glass is also a disadvantage. Varieties in the con formation of the eyes, and in the manner and degree in which they are affected by use, render it impossible to lay down any rule for the focal length of convex glasses for persons of a given age.
It is very essential that the frame of the spectacles should fit comfortably to the head, and be of such a form as to bring the centre of each lens exactly opposite the centre of the eye it is intended to serve. The endless variations met with in the width between the eyes, thc total width of the head, and the form of the nose, render it frequently difficult to suit an individual out of even a very large stock Convex spectacles, being used for viewint near objects, may generally be placed down upon the wearer's nose than those use by short-sighted persons. The oval form i
usually preferred for the lenses, because i allows most room for the motion of the eye determine what combination of metals were most useful for the purpose, with respect to whiteness, porosity, and hardness, one of cop per and tin (the materials employed by New ton in the first reflecting telescope) united in the proportion of 126-1 parts of copper to 58.9 parts of tin, was found the best. It is stated that the compound is of admirable lustre and hardness, and has a specific gravity of 8.8, that of water being 1. Lord itosse has subsequently cast specula 6 feet in diameter, of the like composition, and each in one piece, with complete success. One of these is mounted at Parsonstown, in Ireland, in a telescope 56 feet long, with which important discoveries have been made in the Leavens. These specula were cast in moulds made by binding together layers of hoop-iron ; and both the grinding and the polishing were executed by machinery which was put in motion by a small steam engine. The speculum of the largest tele scope, when mounted, rests on a bed consisting of twenty-seven triangular pieces of iron, sup ported at their centres of gravity on the corners of nine other triangular pieces, which are also supported at their centres of gravity. By this contrivance any change in the form of the box or tube, by warping or otherwise, pro duces no effect on the speculum.