SPECTACLES are lenses so mounted in frames as to be conveniently held before the eyes to assist defective vision. The eyes of a person whose sight is much tried often receive injury from 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. Whether the glasses used be concave or convex, the lowest power tbat is available should be used. The use of a einglo reading-glass instead of spectacles is in jurious, since, by occasioning one eye to be more used than the other, the power and focal length of the two are rendered unequal. The unsteadiness of the glass is also a disadvantage. The defects of the single hand-glass are not removed by increasing its size so much that both eyes may see through it, because in that case the axis of each pencil of rays will be distorted by passing through the lens at points beside its centre. Hand-spectacles, which are made to fold up into nearly as small a space as a single glass, are better than reading glasses ; but, although steadied in some degree by resting upon the nose, they are not equal to spectacles well fitted to and supported by the head.
Varieties in the conformation 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 ; yet a general idea of the necessary power may be obtained from the following table, extracted from Dr. Kitchener's ' Economy of the Eyes In some cases it is advisable to use different spectacles for night and day, to suit different degrees of light; and it is, generally speaking, well to increase the power of the glasses as the sight becomes weaker from age. Even in this, however, caution must be exercised, lest, by over-atunulating the eye, its powers be too rapidly exhausted. It should always be borne in mind, both by young and old persons, and in changing spectacles as well as in first taking to them, that the legiti used in convex or magnifying spectacles of the periscopic construction ; and Fig. 2 shows that of a concave lens suitable for short-sighted persons. In the former case the curve of least radius is that of the anterior, and in tho latter, that of the posterior surface of the lens. Divided spectacles, each glass consisting of two half-lenses, are wino times used; the upper half of each glass being occupied by a concave lens, or one of very slight convexity, for seeing distant objects, while the lower half has a strong magnifier, for examining things near the eye. Such spectacles have an awkward appearance, on account of the
joint along tho middle, and require some practice to avoid inconve nience to the wearer ; but they have been used with advantage by artists and others requiring to look alternately at near and distant objects. Other plans have been tried for obtaining a similar advantage ; such as having a second pair of glasses hinged to the frame, and capable of either turning up out of the way, or being placed imme diately before the ordinary lenses, to modify their power ; or having two distinct pairs of spectacles, capable of being used either separately or together. Spectacles with glazed wings, or frames partly filled with crepe, are sometimes used by travellers ; the glasses, which may bo plain, unless otherwise required, being of such a form as to shield the eyes from dust.
In the Great Exhibition of 1851 the British exhibitors of spectacles wore contented with improvements in the method of mounting, but left out of sight the main object, the improvement of the glasses. Thus there wero fine and delicate frames, some weighing only 11 grains, and tho whole weight, including glasses, not amounting to more than two pennyweights. The steel frames of these spectacles resembled hairlines, and were imperceptible at a abort distance. There wero also ventilating eye-shades, designed to allow a current of air to circulate freely between the shade and the wearer : there were also spectacles for sketching, mounted without a rim to prevent obstruction of vision. Those, with every variety of gold, silver, and steel-mounted spectacles, wero to be found in abundance ; but it was left to the French exhibitors to present glasses to suit almost every peculiarity of vision. 31. Henri, uniting the skill of the optician with the knowledge of the physiologist, exhibited spectacles of an improved kind, the glass of which was so pure as to remove one source of uneasiness and fatigue incident upon the use of spectacles ; while tho foci were so arranged as to rectify certain defects and obliquity of vision. Some of theao spectacles were furnished with a moveable diaphragm, to be shifted right or left at will. These were designed for the cure of squinting, either converging or diverging. M. Pouillot exhibited metallic woven spectacles for the free admission of air to the eyes, while they serve as a screen against dust, insects, &c., and also subdue the light.