The near-sighted, or those who require concave spectacles should use those of the slightest power; No. 0, or No. 1, will generally be sufficient at first, but this, by the aid of the trial box, can readily be determined by the wearer himself. There is such an immense benefit experienced by the short-sighted from spectacles which suit their sight, that to argue for their adoption of them would be quite superfluous. Without spectacles they are excluded from observing beautiful landscapes, recognizing individuals, or viewing to advantage any of the crowd of interesting objects around them; but by adopting them they are placed on a par with the long-sighted in such circumstances, while the sharp and microscopic character of their sight without spectacles, gives them many advantages over those possessing ordinary vision.
The short, or near-sighted eyes, have the cornea, and often the crystalline lens, more convex or arched out than in long-sighted eyes. This formation causes the rays to converge to a focus before they reach the retina, but by the application of a concave lens the difficulty is corrected, and the rays are carried on to their proper point for giving a perfect representation on the retina.
This character of sight is very frequent, and is more particularly remarkable among those whose mode of life restricts them to crowded cities, sedentary employments, and confined situations. Those whose infantine and youthful years have been passed in the country, or where the eyes have had a free range of view, not circumscribed by the walls of the nursery, or limited to the observation of objects near at hand, rarely require concave spectacles.[4]
The spectacle-frames next demand our attention, as our utmost care in judiciously selecting lenses of the proper focus for our sight will be neutralized if the frame or mounting in which they are placed does not apply comfortably to the head, leaving the lenses they carry fair and parallel before the eyes. If the front of a pair of spectacles is too short for the wearer’s face, he will look upon the edge of the lens, and a portion of the exterior rim of the frame; if they are too long his eye will meet the opposite edge and inner curve of the rim.
Spectacle-frames are fashioned to suit the variety of formation in different individuals, and therefore such should be applied as adapt themselves pleasantly to the temples, across the forehead and before the eyes. The pupil of the eye should, when looking at an object directly before it, see through the centre of the spectacle-eye, so that if the whole of the glass, except the small centre, were painted black, the rays of light would pass through it without distortion of the pupil. The bridge or nose-part of the spectacle-frame should be arched out, made like the crank of a lathe, or perfectly parallel with the knuckles of the sides, according to the formation of the wearer’s nose, otherwise the spectacles sit awkwardly on the face too far from or too near to the eye-lashes, with their centres too high or too low for the natural exercise of sight. Trifling as these particulars may seem, they cannot be neglected with impunity. Where these defects are unattended to, the eye is tortured by straining to overcome the difficulty, and striving to accommodate itself to them, instead of their being shaped to meet its requirements. Some old-fashioned spectacles have short side-pieces, which press most unpleasantly on one point of the temples, and when closed scratch and rub the lenses. Others have very cumbrous and heavy sides, with double lift or common hinge-joints. These also scratch the lenses when folded up, and can only be worn with any degree of comfort when the head is uncovered. Decidedly the best constructed spectacle-frames are those which, without being clumsy, have enough strength to admit of the sides being extended the full width of the face without bending the front, with single sides rather longer than their front for ladies’ use, and with turnpin sides (an extending joint, turning on a pivot,) for gentlemen.
The material of which they are composed should be either gold, silver, or enamelled blue steel. Tortoiseshell, also, when well made, is very light and pleasant to wear, particularly for ladies. There is no advantage in large size, or round-eye spectacles, to compensate for their clumsy appearance and great weight; we get a sufficient expanse of observation with the oval-shape glasses without harassing the eye with an excess of light, which the large glasses admit.
Solid blue steel mountings are a decided improvement, and are invaluable for persons who constantly require spectacles. They are wrought from a plate of steel, and shaped as light and uniform as any other town-made elastic blue-steel spectacles, with the advantage of being more durable, and eventually less expensive; for as they have no soldering in their composition, it is scarcely possible to break them, and therefore they rarely want repairing. Blue steel spectacles, in consequence of their being so much in request, are coarsely imitated, and vast numbers are sold by the pretended cheap shops, at apparently low prices which the wearer will find to be immensely beyond their real value. There are thirteen different qualities of the spectacles termed blue steel. The chief part are country made, and roughly put together, some being all iron, others having iron fronts and steel sides; others again bear a tolerably close resemblance to the best town-made articles, and unless the two are compared together, are likely to deceive a casual observer.[5]
The fine elastic blue steel, of the best quality, are exceedingly light, highly polished, and beautifully enamelled. They will retain their appearance for two or three years, when carefully used; and they can be restored to their original appearance at any time for a few shillings’ expense.
The anti-pressure spectacles are made of gold, silver, or the elastic steel. They are not adapted for ladies’ use, but for gentlemen have been much in demand, as they obviate the necessity of opening and closing a second joint or lengthening piece, and do not press or annoy the nose or temples, but by an equal tenacity preserve their proper position before the eyes.
Single eye-glasses, though very convenient for occasional use, are not to be compared with spectacles for the agreeable and natural assistance they give to vision. The eye should not be dependent on them for continuous assistance. Those who persist in using them should do so as little as possible; but the wiser course is to discard them entirely, and adopt the double eye-glasses where it would be tedious to apply spectacles of the usual kind.