Spectacles and Eyeglasses/Chapter 1

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4637086Spectacles and Eyeglasses — Chapter 1

I.	General Considerations, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		26
		The Material of Frames, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	26
		The Component Parts of Spectacles, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	27
		The Lenses: Their Material and Manufacture, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	28
		Eye Wires, Temples, and Bridges, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	38
		The Different Patterns of Spectacles, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	38
		The Varieties of Eyeglasses, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	46
		Spectacles for Cosmetic Effect, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	48

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I. General Considerations

By far the most generally useful method of placing glasses before the eyes is by spectacle frames, though the eyeglass, or pince-nez, has advantages in some cases, from the facility and quickness with which it may be placed in position or removed. The superiority of eyeglasses in appearance and becomingness is another point not unworthy of consideration, as the glasses will surely be more constantly worn if they are becoming than if they are not so. Moreover, the patient is justly entitled to the correction of his refractive error with as little injury to his appearance as possible. The disadvantages of eyeglasses are, that for constant wear they are seldom so comfortable as spectacles; that on some faces it is nearly impossible to keep them in place; while, where the contained glass is cylindrical or prismatic, the rotary motion which it is possible for the glass to take is a serious and sometimes fatal objection to their adoption.

Lorgnettes and single eyeglasses, or quizzing glasses, as they are called, are little more than playthings; though sometimes, as in aphakia, or high myopia, a strong convex or concave lens in one of these forms is of use when the spectacles constantly worn do not give the vision which may occasionally be required.

The Material of Spectacle Frames is usually gold, silver, or steel. Various alloys have also been employed, and sold as aluminium or nickel. So far as I have examined them, they consist principally of tin, and contain little or none of the metals whose names they borrow. Real nickel is too flexible a metal to be used with advantage for spectacle frames, while, so far, no means have been found of soldering aluminium firmly. Were this difficulty overcome, the lightness, stiffness, and freedom from rust of aluminium would make it an excellent material for cheap frames. Silver, like nickel, is too flexible, except for workmen's protective goggles, or some such purpose, where very heavy frames are allowable. Gold, of from 10 to 14 karat, is, by far, the best material for frames. Finer than this it is too flexible, while if less pure it may blacken the skin. In the end, such frames are cheaper than steel, as, owing to the liability of the latter metal to rust when in contact with the moist skin, the gold will outlast it many times over. In eyeglasses, however, the parts are heavier, and the metal is not in contact with the skin; so that there is not the same liability to rust. The gold frames furnished by opticians in this country usually have a stamp mark on the inner side of the right temple, near the hinge, which denote the fineness of the gold: thus 8 karat is marked + ; 10 karat, θ ; 12 karat, *; while 14 karat, or finer, is marked 14k, etc.

The Component Parts of Spectacles.—A pair of spectacles is made up of fifteen or seventeen pieces, whose positions are shown in Fig. 1. They are: two lenses, two eye wires, four end pieces, two screws, two pins, or dowels, two temples, and one bridge. Sometimes the rings upon the temples, through which the dowels pass, are formed as separate pieces. Fig. 2 shows the name and position of each part of an eyeglass. A glance at the more important of the many interesting processes required in making these different parts will contribute to an understanding of the subject.

The Lenses.—The word lens is the Latin name of the lentil, a small bean. The resemblance in shape caused the

Fig. 1.

Fig. 2.

name to be applied to the optical implement. Spectacle lenses are usually made of glass; sometimes of rock crystal (crystallized quartz). The latter substance has a slightly higher index of refraction, so that a lens of a given strength may be somewhat lighter when made of it than when made of glass. The notion is common that these “pebbles," as they are called, possess a peculiar virtue in strengthening the eyes or in some other direction. I suppose the idea is that, being the product of Nature's laboratory, they are necessarily superior. The advantage which they may have of being slightly lighter and harder than lenses of glass is more than counterbalanced by their higher cost, and by the fact that the index of refraction of rock crystal is not very constant.

Of the different kinds of glass, that known as crown glass is preferred, on account of the superior brilliancy which it possesses. It differs from ordinary sheet glass only in the method of blowing. At one point in the process, the mass of glass on the blowpipe assumes the shape of a crown; hence the name. Although glass is theoretically a definite chemical compound, the different methods of handling make a considerable difference in the product of different makers. It consists, chemically, of silicic acid united with some two of the metallic bases: sodium, potassium, calcium, magnesium, aluminium, iron, and lead, but owing to impurities in the glassmaker's raw materials traces of several more of these bases are generally present. The bases calcium and sodium are those used for ordinary sheet and crown glass; iron, always present as an impurity, giving the product its greenish tinge. To lessen this tint, arsenic is employed as a bleaching agent. Peroxid of manganese is sometimes used for the same purpose, and it is a slight excess of this agent which gives to certain samples of glass their pinkish tint. The transparency of such glass is thought to be less durable than that having the greenish color.

The index of refraction of the varieties of glass used in spectacle making is as follows:—

Crown glass,....................................................................................................................................................................................................................................................1.50

Flint glass,....................................................................................................................................................................................................................................................1.57

Ordinary sheet glass,....................................................................................................................................................................................................................................................1.53

Rock crystal,....................................................................................................................................................................................................................................................1.56

The two broad, polished surfaces of a lens are called its refracting surfaces, since it is at these surfaces that the rays of light are refracted when the lens is in use. On the shape of these surfaces, and their position relative to each other, depend all the powers and properties of a lens. Each of these surfaces may be either plane, spherical, or cylindrical. A spherical surface is such a one as, continued in all directions, would form a sphere, and which is, therefore, a segment of a sphere. Similarly, a cylindrical surface is the segment of a cylinder. Spherical and cylindrical surfaces may be either convex or concave. A single surface of a lens may be, therefore, either

Plane,

Convex spherical,

Concave spherical,

Convex cylindrical,

Concave cylindrical.

Since every lens has two refracting surfaces, the list of lenses which it is possible for the lens maker to produce by combinations of these five primary surfaces is as follow:—

Prismatic.
Plano-convex spherical.
Plano-concave spherical.
Plano-convex cylindrical.
Plano-concave cylindrical.
Biconvex spherical.
Biconcave spherical.
Concavo-convex (two varieties):—
1. Radius of curvature of convex surface greater than that of concave. (Converging meniscus.)
2. Radius of curvature of convex surface less than that of concave. (Dispersing meniscus.)
Sphero-cylindrical (four varieties):—
1. Convex sphere combined with convex cylinder.
2. Convex„ sphere„ combined„ with„ concave cylinder.„
3. Concave sphere„ combined„ with„ concave„ cylinder.„
4. Concave„ sphere„ combined„ with„ convex cylinder.„
Biconvex cylindrical, axes coincident.
Biconvex„ cylindrical,„ axes„ crossed.
Biconcave cylindrical,„ axes„ coincident.
Biconcave„ cylindrical,„ axes„ crossed.
Concavo-convex cylindrical, axes coincident.
Concavo-convex„ cylindrical,„ axes„ crossed.

Sections of lenses are shown in Fig. 3, each section illustrating two or more lenses, accordingly as we regard the curved lines as sections of spheres, or cylinders, and the straight lines as planes, or as sections of cylinders in the direction of their axes.

Lastly, the prism may be introduced as an element into each of these lenses. Thus we have quite a long list of the possible forms of the lens, and that without considering the "toric" surface, which will be spoken of later. Of these lenses only the nine first mentioned, and the combination of some of them with the prism, are in practical use. The others, the bicylindrical lenses, besides being difficult of manufacture, have each its optical equivalent in some simpler form of lens, either plano-cylindrical or sphero-cylindrical. They are only mentioned now because their use has been advocated by a few writers in the past.

Difficulties in grinding, and the near equivalence of certain of the lenses mentioned among the first nine of our list, render the use of some of these lenses quite rare. It is, for instance, more difficult to grind a perfect planospherical lens than it is to grind a bispherical, and as in weak lenses, such as are used in spectacles, the action of every plano-spherical lens can be nearly exactly duplicated by some bispherical one, we seldom find plano-sperical lenses in use. Among sphero-cylindrical lenses also it is usual to consider certain combinations as equivalent to each other. For example, a convex spherical combined with a convex cylindrical, as equivalent to some stronger convex spherical combined with a concave cylinder. These lenses are only strictly equivalent, however, for a small area near their optical centers. When their influence on the field of vision is taken into account, they can no longer be considered identical, as we shall see in considering periscopic lenses.

Fig. 3.

In Fig. 3, the first three lenses shown act as convergers of rays, and are all considered as convex, or “plus” lenses, being designated by the sign +, or sometimes by cx. The remaining lenses in the figure act as dispersers of the rays and are known as “minus,” or concave lenses, and receive the sign —, or sometimes cc. For the terms, spherical lens, cylindrical lens, prismatic lens, sphero-cylindrical lens, etc., the words sphere, cylinder, prism, sphero-cylinder, etc., are frequently employed and are unobjectionable. Finally, the sign () is used for "combined with" in the formula of a combination lens, as + 4. sphere () + 2. cylinder.

TABLE I.
OLD SYSTEM					NEW SYSTEM
I.	II.	III.	IV.	V.	VI.	VII.	VIII.
No. of the Lens, Old System.	Focal Distance in English inches.	Focal Distance in Millimeters.	Equivalent in Dioptrics.	No. of the Lens, New System.	Focal Distance in Millimeters.	Focal Distance in English inches.	No. corresponding of the Old System.
72	67.9	1724	0.58	0.25	4000	157.48	166.94
60	56.6	1437	0.695	0.5	2000	78.74	83.46
48	45.3	1150	0.87	0.75	1333	52.5	55.63
42	39.6	1005	0.90	1	1000	39.37	41.73
36	34	863	1.16	1.25	800	31.5	33.39
30	28.3	718	1.39	1.5	666	26.22	27.79
24	22.6	574	1.74	1.75	571	22.48	23.83
20	18.8	477	2.09	2	500	19.69	20.87
18	17	431	2.31	2.25	444	17.48	18.53
16	15	381	2.6	2.5	400	15.75	16.69
15	14.1	358	2.79	3	333	13.17	13.9
14	13.2	335	2.98	3.5	286	11.26	11.94
13	12.2	312	3.20	4	250	9.84	10.43
12	11.2	287	3.48	4.5	222	8.74	9.26
11	10.3	261	3.82	5	200	7.87	8.35
10	9.4	239	4.18	5.5	182	7.16	7.6
9	8.5	216	4.63	6	166	6.54	6.93
8	7.5	190	5.25	7	143	5.63	5.97
7	6.6	167	5.96	8	125	4.92	5.22
6½	6.13	155	6.42	9	111	4.37	4.63
6	5.6	142	7	10	100	3.94	4.17
5½	5.2	132	7.57	11	91	3.58	3.8
5	4.7	110	8.4	12	83	3.27	3.46
4½	4.2	106	9.4	13	77	3.03	3.21
4	3.8	96	10.4	14	71	2.8	2.96
3½	3.3	84	11.9	15	67	2.64	2.8
3¼	3.1	79	12.7	16	62	2.44	2.59
3	2.8	71	14	17	59	2.32	2.46
2¾	2.6	66	15.1	18	55	2.17	2.29
2½	2.36	60	17.7	20	50	1.97	2.09
2¼	2.1	53	18.7
2	1.88	48	20.94

The new system of numbering lenses, the dioptric system, has so entirely fulfilled the requirements of the users of lenses, and has so simplified and facilitated our every-day work and calculations, that the old or inch system of numbering is rapidly becoming of historical interest only. As its use, however, still survives in certain quarters, and lenses are frequently met with which are marked by this system, a table showing the equivalence of the ordinary lenses of the test case in the two systems is shown on page 33. It is calculated for an index of refraction of 1.53.

Fig. 4.Optician's Lathe for Grinding Spherical Lenses.

The simple apparatus used for grinding a single spherical lens is shown in Fig. 4. The disc of glass (a) of which a lens is to be made is fastened, by means of pitch, to a small, cubical block of iron (b) having a pit in the surface opposite that to which the glass is fastened. Into this pit fits a pin (c) upon a lever, which is in the hand of the workman. When the free surface of the glass is applied to the surface of the “tool” (d) to whose form it is to be ground, it, together with the block of iron, turns upon the pin. The joints at e and f allow of lateral and vertical movements of the lever, so that the workman is able to carry the glass freely over all portions of the tool.

The tool which gives the shape to the surface of the glass is made of steel; and for spherical glasses is in the form of a disc, with its surfaces looking upward and downward, and revolving about a vertical axis, like a potter's wheel. The upper surface of this disc is convex for grinding concave glasses, or concave for grinding convex glasses. Of course, each strength of lens requires a separate tool having the requisite convexity or concavity of surface. The abrading material placed upon the surface of the tool is wet powdered emery of successively finer and finer grades until the desired amount of glass has been ground away. When this process is complete, the surface of the glass has the desired spherical curvature, but it is rough: that is, it is “ground glass." To polish it, a piece of wet broadcloth or felt is smoothly applied to the surface of the tool upon which the glass was ground, conforming, of course, to that surface. The cloth, being sprinkled with wet “rouge” (a carefully calcined sulphate of iron), gives the glass held against it a beautiful polish without altering its spherical curvature. The same processes must now be gone over with the other surface of the lens, after which it is cleaned and cut to a shape suitable for its future mounting.

This is done by means of an implement called a lens-cutter, in which the lens rests on a leather cushion and is held firmly in position by a rubber-tipped arm, while a diamond-tipped glass-cutter, guided by a pattern, traces the oval or other desired outline upon the glass. The superfluous glass is removed piecemeal by means of pincers, and the lens passes to the next process, which is the smoothing and, if necessary, beveling of the edges. This is done by hand upon large Scotch grindstones. If the lens is to be mounted in a round eyewire, its edge must be grooved by means of a file, while a skeleton frame will require the drilling of the glass, which may be done by hand with a steel drill or by a special machine.

In grinding a cylindrical lens the surface of the tool is, of course, a portion of the surface of a cylinder, and the glass is ground by a to-and-fro motion. It is evident that the axis of the cylinder in the future spectacle need not be taken into account in grinding, but only in the process of cutting to shape for mounting. As a matter of fact, very few cylinders are, at present, ground in this country; the glass is brought from Europe with a cylinder already ground upon one side and glued to its block of iron for the grinding of a spherical or plane surface upon the other.

When the lenses are of high power it is of advantage that they be made in the form of a meniscus, giving what are known as periscopic glasses. For instance, if a + 4 diopter lens is required, the anterior surface is ground to a + 6. D. and the posterior surfaces to a — 2. D. It is just as advantageous to a cylindrical or sphero-cylindrical glass to be periscopic as it is to a spherical, but under present methods of grinding it is manifestly impossible to give them this form, as the cylinder is ground on one side, and the other ground to a plane or sphere, as the case may be. Glasses which overcome the difficulty have, however, been made, and were described by Dr. George C. Harlan at the meeting of the American Ophthalmological Society in 1885, and again in 1889. From the latter communication I quote the following description of the glass:—

"The lens to which I wish again to call the attention of the Society consists of crossed cylinders ground on one surface of the glass, the outer being left for any desired spherical curve. In this way a meniscus may be produced. Here, for instance, is a combination lens giving the effect of + 4. () + 2. Cyl. To produce this effect crossed cylinders of + 4. and + 6. are required, supposing the other surface of the glass to be left plain. If we wish to give the periscopic form to this glass, it can be done by making the cylinders 6 and 8, and grinding a — 2. sphere on the other side. If a simple cylinder is needed, the spherical curve must equal that of the weaker cylinder.

“I learn from a recent publication by Dr. George J. Bull, of Paris, entitled 'Lunettes et Pince-nez,' that glasses similar to these have been made with more or less success before, but have never come into general use. Dr. Bull describes them under the name of 'verres toriques' . The tore (Latin torus) is the surface engendered by a circle which turns about an axis situated on the plane of the circle. A familiar example of the torus is the circular convex molding at the base of an architectural column. A glass ground upon a wheel having this form will present two cylindrical curves at right angles to each other, one depending on the radius of the wheel, and the other on the radius of the convexity of its rim. It would seem that 'toric lenses' is the proper designation of these glasses." Fig. 5, A, represents a concave toric lens. In the same figure, B is a concave cylindrical lens, introduced for the sake of comparison.

Those who have used these glasses consider them much more satisfactory than glasses made by the common method, and they should be borne in mind when prescribing for high astigmatism in patients who use their eyes a great deal for work requiring accuracy.

Fig. 5.

Eye Wires, Temples, and Bridges.—Eye wires are made by wrapping the untempered wire, in the form of a spiral, closely about a flattened metal cylinder. Being tempered while in this position, the loops of the spiral will retain the shape given them. A single cut down the side of the cylinder converts each loop into a separate oval ring. End pieces and straight temples are stamped from sheets of metal, and afterward formed and tempered. Hook temples of steel are turned from wire upon a lathe. Bridges are usually made of oval or half-oval wire, and are simply pressed to the desired shape by a forming machine.

Of the Different Patterns of Spectacles.—In the common and strongest form of spectacle, the edge of the glass is beveled so as to enter a groove in the wire which surrounds it. In a second form, in which the edge of the glass is grooved for the reception of a fine, round wire, the object sought, of rendering the rim of the spectacles less conspicuous, is generally defeated by the fact that the glass must be made thicker than it otherwise need be, in order to give room for the groove on its edge. In concave glasses this is not the case, since the edge of the glass is here the thickest part, and such glasses may sometimes be mounted in this way with advantage. In a third form, called “frameless” spectacles (Fig. 6), the wire encircling the glass is

Fig. 6.

dispensed with altogether, small holes being drilled through the glass near its edge for the accommodation of screws which fasten the bridge and temples in place. The advantages of this form are its beauty and inconspicuousness. It should never be prescribed for children, as it is quite liable to break at the point where the glass has been drilled. The edges of these glasses should not be polished, but should be given a dull finish, otherwise they reflect the light disagreeably.

Sides, or temples, have been variously constructed. Those having sliding and turn-pin joints are examples of antiquated forms. Those now used are the “hook," or “riding-bow,” and the plain, “straight” temple. The former are to be preferred in all cases where the glasses are to be worn constantly or nearly so, and the latter for those who wear glasses for near work only, and require to remove them frequently from the eyes. Hook temples are made in three lengths, designated as short, medium, and long. These are sufficient for all cases.

Securing a proper fit in the bridge, upon which so much of the comfort and efficiency of spectacles depends, was a difficult matter until the ingenuity of Dr. Charles Hermon Thomas, of this city, suggested what is known as the "saddle bridge,” which solved the problem. (See Fig. 7.) This bridge may be varied to suit every possible case, and is always to be preferred. The “K” bridge, formed of wires in the shape of the letter K, is allowable in some cases. The nearly similar "X” bridge allows the glasses to teeter, or see-saw across the nose, with the motions of the head. It is, however, the best form of bridge for reversible glasses; that is, glasses for persons having sight in one eye only, who may have their distant glass set in one side of a frame and their near glass in the other. By using this bridge and straight temples, or hook temples without a shoulder at the hinge, the spectacles may be turned over so as to bring either lens before the wearer's seeing eye. The old-fashioned bridge, called the “curl," is unobjectionable for cases in which the bridge of the nose is prominent, or for the spectacles of old people, who like to slip their glasses down toward the end of the nose. None of the forms mentioned, however, have any advantage over the saddle bridge for any case. A small piece of cork is sometimes attached to the under side of the bridge where it comes in contact with the skin. It is unnecessary if the frames fit the face of the wearer properly.

Fig. 7."X," "K," "Curl," and "Saddle" Bridges.

If it be desirable to remove all pressure from the bridge of the nose and to transfer it to the sides, it is best done by soldering a pair of guards, similar to those used on eyeglasses, to the spectacle bridge.

The earliest spectacles appear to have had round eyes. Various other shapes are occasionally seen, as octagon, oblong, etc. The oval has about displaced these antiquated forms, and is made in sizes known to the trade in America as follows:—

TABLE II.—SIZES OF EYES.
No. 00.	119/32 by 11/4 in.	or	41 by 32 mm.	large coq. size.
No.„ 0.	117/32 by 113/16 in.	or	39 by 30 mm.	coq. size.
No.„ 1.	17/16 by 11/8 in.	or	37 by 28 mm.	standard large eye.
No.„ 2.	113/32 by 31/32 in.	or	36 by 25 mm.	standard E. G. size.
No.„ 3.	13/8 by 11/32 in.	or	35 by 26 mm.	standard interchange.
No.„ 4.	111/32 by 31/32 in.	or	34 by 25 mm.	standard small eye.
No.„ 5.	11/4 by 29/32 in.	or	32 by 23 mm.	children's size.

Where glasses are used for near work only, the eyes are sometimes made of semi-oval shape, allowing the line of sight to pass over their upper, straight edge when the wearer views a distant object. These are known as "half," "pulpit," or "clerical" eyes, and are very convenient, especially to public speakers, as their name implies. They do not seem to me as well known or as generally used as they should be.

When glasses of different focusing power are required for distant and near vision, the trouble incident to frequent changing is obviated by "bifocal" glasses. That is, the lower part of the spectacle eye, which is used for near work, is made to differ in focusing power from the upper part, which is used for distant vision. Such bifocal glasses are also called Franklin glasses, from the philosopher who, as we have seen, invented them.

The object sought may be attained in various ways. In the early Franklin glasses each eye contained two half-oval pieces, with their straight edges in apposition (A, Fig. 9). This has been improved upon by making the line of junction a curved one, giving somewhat greater latitude of distant vision and rendering the glass more secure in its frame. A form of bifocal glasses which were never used to any great extent are called "ground" bifocals. They are very handsome, containing only one piece of glass in each

Fig. 8.Ovals Showing the .

eye, the upper and lower parts of which are ground of different strengths (C, Fig. 9). The mechanical impossibility of centering both spherical surfaces upon the same piece of glass, however, introduces a prismatic effect which destroys their usefulness. The latest and best variety of these glasses are called “cemented bifocals” (D, Fig. 9). They have been in occasional use in France for over twenty years, though their general manufacture by our opticians is due

Fig. 9.

to the efforts of Dr. Geo. M. Gould. To the back of the distance glass is cemented, by means of Canada balsam, a small lens whose strength added to that of the distance glass equals the strength required for near work. The upper edge of the supplemental lens should be ground as thin as possible, in order to render it inconspicuous. This nearly does away with the objectionable line of junction, the spectacles are strong, light, and handsome, and may even be made “ frameless," like those represented in Fig. 6, if the patient so desire. For cylindrical lenses this form is also cheaper, since only the distance glass need have the cylinder ground upon it, the supplemental segment being a simple sphere.

All bifocals have the inconvenience that in walking, the floor just in front of the patient's feet is not seen clearly because viewed through the near glass. Spectacles which revolve on the long axis of the “eye," bringing the distance glass to the lower portion of the frame, have been contrived to overcome this difficulty, but they are cumbersome and, moreover, it requires more effort to effect the revolution of the glass than it does to bend the neck sufficiently to bring the upper segment into the line of vision when the ordinary bifocals are worn. Some persons declare that they cannot become accustomed to bifocals however well adjusted. Parallel, horizontal lines, as those of a staircase, are particularly confusing, it being possible to see each line doubled if the junction of the two segments of the glass is placed just opposite the pupil. Such persons may prefer "extra front” (Fig. 10); that is, a second pair of spectacles whose temples are replaced by short hooks, by means of which they are hung in front of the frame already upon the face. This is a rather clumsy device; less so, however, when the eyes of the extra front are made half oval instead of oval.

Fig. 10.

Eyeglasses.—Most that has been said of the varieties of spectacle frames applies as well to eyeglass frames. As was mentioned, the chief objection to the latter is that they allow a displacement of the axis of a cylindrical or base of a prismatic lens. This is a necessary concomitant of their being joined by a spring instead of a rigid bridge. More or less ingenious frames have been made in which the glasses are rigidly joined, and the spring placed in some other position (Fig. 11); their weight and cumbersome appearance have, however, so far prevented their becoming popular. Moreover, the addition of extra parts, which must have working room, and the necessarily small amount of metal which these parts contain render it extremely difficult to construct a really rigid eyeglass frame. So far, I have seen none which I consider superior or even equal to a carefully fitted frame made in the usual style.

Another difficulty in the past arose from the fact that it was impossible to always place the glass at the proper distance from the eye, since the frame must be placed at that point where it obtains the best grip upon the sides of the nose. This has been in great measure overcome by what is called the “offset guard," in which the nose-pieces are placed back of the plane of the glasses, instead of in the same plane, as formerly. This part is varied in its shape so as to adapt it to variously proportioned faces. By the use of some one of these variations we are always enabled to place the lenses in proper position without removing the guards from that position on the nose where they obtain the best bearing. When the glasses contain no prism or cylinder, or only weak cylinders, a well-adjusted eyeglass with “offset guards” is fairly satisfactory. When made frameless, as in Fig. 12, it is the most modern, and certainly the most handsome, mounting we have to offer our patients.

Fig. 11.Forms of Rigid Frame or "Bar Spring" Eyeglasses.

Fig. 12.

Fig. 13.Various Patterns of the Offset Guard.

"A." For shallow bridge, prominent eyes, flat forehead.

"B." For shallow bridge, prominent eyes and forehead.

"C." The guard used for the average case.

"D." Deep-set eyes, prominent nose and forehead.

"E." Same as "C," but for lowering glasses (for reading).

"F." Same as "B," but for lowering glasses (for reading).

"G." Same as "C," but somewhat smaller and neater, although having less bearing surface.

Spectacles for Cosmetic Effect.—Something may legitimately be done, at times, in the way of improving the appearance of a patient by the application of glasses. The blind whose eyes are not only sightless, but unsightly, very commonly hide them behind colored glasses. Neatly fitting spectacles with large eyes of ground glass render the appearance of such persons less lugubrious. When one eye is useless for vision, and at the same time small, and the orbit undeveloped, a gratifying improvement in the appearance of the patient may be attained by placing before the shrunken eye a convex glass of sufficient strength to magnify it to the size of its fellow. The condition known as epicanthus can generally be removed by wearing eyeglasses whose nose pieces draw just enough on the inner canthi to smooth out the offending fold of skin. As the subjects of epicanthus are generally flat-nosed, it may be necessary to furnish the eyeglasses with a pair of hook temples to keep them in place. Since operations for this disfigurement are so unsatisfactory, such an appliance is probably the best treatment we can advise in case the trouble is not outgrown.