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.