Periscopic or meniscus lenses were introduced in the year 1770, and revived at a later period by Dr. Wollaston. Their properties were investigated by many scientific persons, and again candidly examined by Mr. W. Jones; yet, notwithstanding the oblivion to which they were consigned by the universal consent of practical opticians, they are still foisted on the public, under various disguises, and embellished with sundry new appellations. They are concave on one side, and convex on the other, the inner and outer curve differing, in order to produce a focus. When placed in the spectacle-frame, their convex surface is always from the eye; this necessarily exposes their centres to be rubbed and scratched. The halo, or aberration of light, is greater in lenses of this form than in any other. This can be easily proved by placing a double convex, and a periscopic lens, each of the same diameter and focus, say one-and-a-half inch diameter, and three or four inch focus, edgeways together. Let the image of a lighted candle be thrown against a white wall or paper, in a dark situation, through the two lenses, and it will be conspicuously seen that the periscopic lens exhibits this dazzling indistinctness, while in the double convex lens it is scarcely perceptible.
Parallel or flat surface lenses are produced from tools perfectly level, and without any curvature. For spectacle purposes their only use is as shades or shields for the eye, to protect it from the effects of bright and dazzling objects, dust, and wind. They are, however, indispensable in the construction of sextants, artificial horizons, and other mathematical instruments.
Spectacles for travelling by rail-road, on the old beaten path, or by vessels, are usually fitted with parallel glasses, unless the wearer requires focal power as well as a screen, in which case they are of the concave or convex figure, before explained. Almost every combination of light and shade has been used for this class of spectacles; violet, grey, blue, green, crape, wove wire, &c.; but some sensitive and tender eyes failed to receive the relief expected from any of these, and opticians have been repeatedly baffled in their attempts to produce a shade of glass congenial to the requirements of the eye under such circumstances. I have made extensive use of the new neutral tint, or twilight tinge glass, and find it most agreeable to the eye while employing it, and when removed, it leaves the vision undisturbed by the flickering and confused halo so much complained of after wearing other coloured glasses. The cause of this superiority is clearly seen when we remember that, after taking off a pair of green glass spectacles, every object appears of a red colour, while, upon the removal of blue colours, an orange or yellow mantle seems to rest on all which meets the view.
It is of consequence that all such glasses should be really parallel, otherwise a broken and disjointed appearance will be given to objects, the rays of light being abruptly dispersed instead of being transmitted through the transparent medium to the eye, in their natural direction. I have always discountenanced the use of wire, gauze, crape, and muslin substitutes for glass, because, in my opinion, it is a fallacy to assert that they are cooler and more agreeable to the eye. There is abundant space for the circulation of air in the region of the eye if the spectacle-frame adapts itself pleasantly to the wearer’s face; while the eye and common sense may answer together, that to look on things around us, a transparent medium is preferable to a hazy and indistinct one. We do not choose bars and gratings, or coarse curtains, in preference to glass, for the windows of apartments; but if the light is sometimes too intense, we place a shade to soften its dazzling effects. Such precisely is the reason why tinted glass spectacles, for defending the eyes from rain, dust, and wind, are recommended.
Lenses worked by machinery are produced in greater quantities, within a given time, than those worked by hand. They are passed through the different stages of grinding and polishing without having the keen eye of the workman carefully watching their progress, and adjusting the inequalities in their surfaces or edges, which will always appear more or less in the course of working.
The price at which competition demands those lenses shall be rendered, operates against the wearer of spectacles; for the producer cannot afford to throw aside such as are faulty, and the wholesale agent and retail dispenser cannot expect to have, at the low price charged, lenses which will bear a critical examination; and thus all which can possibly be used are thrust into frames of one kind or other, from the common iron or horn sold by the poor hawkers at sixpence, eightpence, and one shilling per pair, to the more expensive frames; while many faulty glasses, after being dubbed with some ear-tickling appellation, and imbibing extraordinary “light-modifying and refractive virtues,” by passing into the hands of the hawker of a higher class, are palmed upon the unfortunate spectacle-purchasers who are simple enough to give credence to the wondrous tale.
Women and children are chiefly employed to cut and edge those cheap glasses to the spectacle-frames; and who can expect they should do them better for the price? And if one glass should be unequally thick, like a wedge, while its companion in the same spectacles is miserably thin; or if the centres, instead of being equidistant from all parts of the rim are nipped into a corner; how can you feel surprised when you consider that for them to earn a living, it is necessary they should finish several dozen pairs per day, and therefore, expedition, rather than excellence, is the point at which they aim? In many departments, where machinery has supplanted manual labour, the work produced is of a superior character, and will bear more critical examination; but the contrary is the fact in the case of machine-worked optical glasses, and is more especially apparent in such as are intended for microscopic and achromatic purposes. It cannot be denied that, for all such uses, lenses worked by hand, with the ordinary care of a skilful workman, as much excel those produced by machinery, as the accurate and scientific touch of the artist eclipses the random splash of the plasterer.
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.