The trial box, or frame of sights, ranging from the slightest focal power down to the deepest, is a very useful apparatus, both for the optician and his patient. It consists of eight or nine spectacle-fronts, clamped together at one end by a rivet, on which they are yet free to move. The focus of each pair is stamped on the front. They are enclosed by a pair of lids, which forms a handle, to be held by the hand, while one front after another is placed before the eyes, to determine what focus yields the desired assistance. This being ascertained, and having noted down the power we find to suit most naturally, as explained when treating of convex lenses, page 12, we have only to determine what style of mounting we prefer, and whether we will have glasses or pebbles, for the optician to suit us at once with lenses adapted to the sight, and mountings fitting pleasantly to the head. If an interview with the optician is impracticable, it will be ensuring greater accuracy, to mention the exact distance, measured as a straight line from the centre of the pupil of one eye to that of the other; and if the nose, on the bridge between the eyes, is flat or prominent, broad or narrow, since spectacles, suiting the sight most accurately, are frequently an annoyance, rather than an assistance, to the wearer, from these particulars being unheeded. Two trial boxes comprise the range of convex sight, one for young persons, and those who require spectacles for the first, second, or third time; the other for those eyes which have been long accustomed to spectacles, or from some constitutional or sudden cause, need those of strong magnifying power. The third box contains concave lenses, ranging from No. 1 to 12, which are the sights chiefly wanted, although, in extraordinary cases, No. 13, 14, and even 20, have been required. Although, throughout this treatise, I have endeavoured to convince my readers that they may and ought mainly to determine for themselves, yet I would recommend a visit to the optician, where it is practicable, in preference to a description of the case; for the same reason that the best written statement of a patient’s malady, sent to a physician, does not afford him that familiar acquaintance with the case which an interview of a few minutes would supply. FOOTNOTES: [2] The manufacture of glass was known very early, but glass perfectly transparent and colourless was reckoned so valuable, that Nero is said to have given a sum equal to £25 for two moderate sized colourless drinking glasses.—_Starke’s Oriental Letters._ Glass for optical uses is heavy, homogeneous, free from streaks and veins. More expensive chemical substances are employed in its manufacture than are used in making common glass.