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These aids to failing sight were first used about the latter end of the thirteenth century, and their invention is ascribed to Roger Bacon. Sir David Brewster says:" Persons who ]llwe enjoyed distinct and comfortable vision in early life, it is remarked, are the mostlikely to appreciate the benefit to be derived from glasses. Between the ages of thirty and forty, they begin to experience a change in sight. During the progress of this alteration, much inconvenience is experienced, as no spectacles seem to be serviceablc in giving correct vision. Happily, however, two or three months end.s this difficulty, and as soon as the alteration is complete, distinct and comfortable vision is at once .obtained by the use of well selected glasses of a convex figure. During this transition state it is important that the eyes should be subjected to no severe strain, and great regard should be paid to the general health. The material of spectacle lenses should be glass, of a very low dispersive power or better still, of rock crystal. They should be as thin as practicable. To correct a common error in the manufacture of lenses, by which the distance between the centres of the lenses is equal to the distance between the pupils of the eyes, the following is given : 'Draw on paper an isosceles triangle, the two sides of which are equal to the distance of each pupil from the point to be seen distinctly; while the third side or base is equal to. the distance between the pupils when the eyes view that point. Then set off on each side of the triangle, from each end of the base, the distance of the center of lenses or their frames from the pupil, and the distance of these points will be the diitance of the centers of the lenses required.' The long-sighted persons will generally, for ten or twelve years, require glasses only for reading or work done by hands; but as life advances other spectacles will be needed for objects at greater distances, and it will be of great advantage to have two or three pairs of different local distances. It is a very incorrect notion that it is prudent to avoid the use of artificial helps'to the eyes as long as possible. The human eye is too delicate a structure to bear continued strain without injury, and the true rule is to commence the use of glasscs as soon as we can see better with tllan without them."
