“The subject is of universal importance, since every man, woman, and
child in the empire will probably require optical assistance.”—ABBOTT.
The eyes, when in a sound and healthy state, instinctively adjust
themselves at a distance of twelve inches from a book or paper, when
they are observing the same. This distance is found to be most natural
and agreeable; for when we extend it to sixteen, twenty, or thirty
inches, the chrystalline lens is stimulated to keep a distinct and
clear perception, until, as the distance increases, the object becomes
less and less perceptible. When we are compelled to extend this natural
distance, experience difficulty in reading small characters, or find
it necessary to get more light on what we are observing, we may safely
conclude that artificial assistance is needed, and that, judiciously
applied, the tendency to decay will be mildly arrested.
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.
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.
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.
[3] Pebbles, again, are proved by grinding the edges briskly on a
moderately smooth file, or porous stone; they will resist the action
of these, and emit sparkles of light as the velocity of the friction
is increased, while glasses, on the contrary, yield, and are ground
without difficulty.