The design of spectacles is to supply the loss of power which is experienced by the eyes at different periods of life, and arising from various causes. These productions of art are constructed with a close observance to, and act upon, the same principles as those by which the process of vision is regulated.
Spectacles ought not to do more than maintain or preserve to us the capability of seeing at the natural distance. This is, in fact, all they are intended to effect. When the chrystalline lens of the eye, losing its convexity, fails to converge the rays of light, and bring them to their natural focus on the retina, an artificial lens, of suitable convexity, supplies to it this capability, and compensates for its gradual diminution of capacity. Thus lenses for assisting the sight are fashioned upon the optical principles so apparent in the mechanism of the eye itself, which, it will be observed, is neither round nor flat, but of that nicely moulded convexity which is indispensable for the performance of its functions. If lenses were either spheres or planes they likewise would be ineffective for the purpose proposed.
There is not any material in existence beside pebble and glass, which is calculated for spectacle purposes. The pretended “improvements,” “pellucid lenses,” “refractive transparencies,” “patent amber,” “chrystal preservers,” &c., are new-fangled terms, coined to entrap the uninitiated.[2]
Convex lenses are produced by a series of operations through which a glass or pebble passes, as shaping out from the rough piece, affixing them to the block or frame on which they are worked, grinding their surfaces to the form and focus required, in brass or iron tools of the curvature or radius corresponding. Thus to produce a glass or pebble of any given focal length, we use a tool which is a portion of a sphere or globe of four, five, ten, or any other given radius, or half its diameter. For spectacles usually required, the focus of the pebble or glass, thus fashioned, varies from sixty inches down to five inches. The focus of a convex or magnifying glass is ascertained very readily, thus: hold the lens near to a white surface, as a sheet of writing paper, pinned to the side of the apartment opposite to the window; shift the lens gently backwards and forwards, until the objects before it, as the window frames, flower pots, or the Venetian blinds, are seen inverted upon the paper, clearly and distinctly measure the distance from the paper to the lens, and this distance is the focus: or, by placing a candle at the distance of twelve feet from the paper, and measuring, as before, when the reflected image of the flame is shown most accurately on the paper, we again have the focus denoted. The one method is as much practised as the other; though, in ordering a lens of a given focus, I would recommend a person always to state the method by which he has determined the focus; whether by the sun, or candlelight, which will prevent any mistake arising from the disparity between the two plans pursued.
Concave lenses are made upon the same principle, and pass through similar processes. They are designated by numbers, thus: a concave lens, worked in a tool of four-inch radius, is termed No. 12; five-inch, No. 10; ten-inch, No. 6. &c.
To determine the radius or focus of a concave lens, apply to its surface a convex lens of the same focus, and holding the two together as one glass, between the thumb and finger, at some distance before the eye, give the hand a gentle motion to and fro, and if the objects looked at through the lenses now in contact appear fixed, equal, and of their natural size, then is the focus of the one correspondent to that of the other; but if the convex lens is not of the same focal length as the concave to which it is thus applied, then all objects looked upon will appear to shake and have a tremulous motion.
Another method of ascertaining whether one concave lens is of the same number, and worked in tools of the same radius as another, is to hold one in each hand, placing their edges against each other, with their centres in a parallel line; now withdraw ten or twelve feet from the window, and observe the top and bottom bars of a square of glass: if they appear equal and uniform in all their parts, parallel to each other, and agreeing with the other squares in the window, then are they of the same radius; but if they do not match, the bars will appear disjointed, and higher or lower in one square than in the other. The trial box of sights will be found very useful in making these experiments.
Brazil pebbles, or crystallized quartz, are imported to this country in rough blocks; these are cut or slit, by the aid of pulverized diamond, into slabs or pieces, of the diameter required. Those pieces in which bubbles, waves, or blemishes appear, are thrown aside by the optician who is tenacious of his fair fame, as their imperfections become more apparent in every after-stage of their progress; and when polished, centred, and shaped for the spectacle-frame, they are really improper to be used at all; nevertheless, the needy, or dishonest, rather than lose a fraction of their gains, often persist in working up such imperfect material, and harping upon their being pebble—real pebble—palm them off upon the uninitiated as genuine articles. Pebbles have the following important advantages: they are of equal density, and exceedingly hard, firm, and clear; their surfaces are not liable to become misty or scratched (which circumstance alone often compels a change of glasses): they are of a pure, cool nature, and shew this contrast to glass (which is, on the contrary, produced by the action of artificial heat) in the touch of the finger or tongue to their surfaces.[3] They are, in consequence of these properties, calculated to suit the sight for a longer period than glass; but they need not be thrown aside, when, from the indications already referred to, we find an increase of magnifying power is required, as they can be re-worked readily enough to meet the requirement of the eyes, and at an expense scarcely more than a new pair of glasses, or about one-third of their original cost. The directions for ascertaining the focus of concave or convex pebbles, are the same as described for concave or convex glasses.
This consideration should weigh with those who are apt to be misled by the pretensions of the unprincipled; for pebbles have, in common with many other crystals, a double refracting property, which, if the pebble is cut carelessly, exhibits itself by painfully affecting the vision; two objects, instead of one, are seen, causing a confused and agitating sense of indistinctness, which, in proportion to the exertion of the eye to overcome it, is the more tiresome and distressing. Such faulty and blemished articles, technically called wasters, are refused by the optician of any real respectability and character, but are eagerly bought up by those venders, whose object is to purchase what costs them the least money, alike ignorant of, and indifferent to, any other consideration.
Pebbles, therefore, like all the precious stones, which, in fact, are crystals, cannot be properly cut or sliced except in a parallel direction to their natural faces.