# Spectacles and Eyeglasses, their forms, mounting, and proper adjustment/Chapter 4

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Ordinary prudence demands that the prescriber of glasses make a careful examination of the manner in which his directions have been carried out, since neglect of this precaution may nullify the results of the most painstaking correction of the refraction. If the surgeon himself furnish the spectacles, it is doubly incumbent on him to make a thorough inspection of glass and frame, and to carefully adjust the latter so as to be entirely comfortable to the wearer. Then, too, it is not enough that the frames correctly perform their function at first; they must continue to do so. Should there be no optician in his neighborhood, the surgeon will be called upon to bring to a proper shape frames which have passed through all sorts of accidents, and it is better that he should do this work than entrust it to less competent hands.
Proving the Strength of Lenses.—The focal length of a convex lens may be directly measured by finding the distance at which it brings the sun’s rays to a focus. To do this, the rays which have passed through the lens are simply caught upon a piece of paper or other screen, the two being held in such relationship that the image of the sun formed on the screen is round. The screen is then to be moved back and forth until the point is found at which this image is smallest, and the distance of such point from the lens is the focal length of the lens. To learn the strength of the lens in diopters, we divide 100 centimeters (one meter) by the focal length expressed in centimeters, or 40 inches (about one meter) by the focal length expressed in inches. For instance, if we found the focus of the lens ​under examination to be distant 10 in., or 25 cm., from the lens, 40 in. divided by 10 in., or 100 cm. divided by 25 cm., will alike give a quotient of 4, and the lens measured was, therefore, a + 4. D.
The focal length of a concave lens may be similarly measured by combining it with a stronger convex lens and then measuring the strength of the resulting weaker convex. The strength of the original convex used being known, we have only to subtract from it the weak convex resultant to find the strength of the concave with which we are dealing. The focal length of convex and concave cylindrical lenses may be measured in the same way as the corresponding sphericals, it being only necessary to observe that the parallel rays of light after passing through a convex cylindrical lens are arranged in the form of a line at the focus of such lens; not brought to a point, as is the case with convex sphericals.
Phacometers.—Such methods as the one described above are, however, too tedious for ordinary use, though quite elaborate contrivances called phacometers have been devised on this principle. A lens measure constructed on an entirely different idea has appeared, the invention of Mr. J. T. Brayton, of Chicago. Fig. 41 shows the size and appearance of the instrument, as well as the method of its use. Of the three steel pins which project from its top the two outer ones are fixed, while the central one moves up and down easily but is held up by a spring. On pressing the surface of a spherical lens squarely against these points the central one will be depressed until they all three touch the glass, the curvature of the surface of the lens determining the amount of such depression. The motion being transferred through a rather simple mechanism to the hand upon the dial, this travels over a scale which shows in diopters the strength of the lens corresponding to the surface tested. The other surface is then to be explored in the same way. If the lens is bi-convex ​or bi-concave, the results of measuring each surface separately are added together; if periscopic, the less is deducted from the greater. When used upon a cylindrical surface the hand will stand at zero when the three points are in line with the axis of the cylinder. When the points are placed at right angles to the axis the strength of the cylinder is shown.
Since this instrument indicates the refractive value of a lens from the curvature of its surfaces only, leaving out of account the index of refraction of the material, it is evident that it can be accurate for only one variety of glass. To adjust the instrument turn the midmost of the three points to either the right or left until the index properly registers the value of a known lens. It will then be adjusted for all lenses made of a like glass.
Neutralization of Spherical Lenses.—The method of determining the strength of spectacles which is of most general utility is the well-known one of neutralization. If a convex spherical lens be held about a foot from the eye, and any object, say that part of a window sash where a ​vertical and horizontal line cross, be viewed through it, any motion given the lens will result in an apparent motion in the opposite direction of the object sighted. That is, if the lens is moved to the right, the object appears to move to the left; if the lens is raised the object appears to sink. If the same maneuver be employed with a concave spherical glass, the object again appears to move, but this time in the same direction as the motion imparted to the lens; if the lens is moved to the right, the object appears to move to the right also. Here we have the readiest possible means of distinguishing between a convex and a concave lens. Moreover, one gets in this way an idea of the strength of a lens, as the stronger the lens the more rapid is the apparent motion of the object seen through it.
If, continuing the experiment, the two lenses be placed together, with their curved surfaces in apposition, and a trial be made of the effect of moving them before an object, as was done previously with each lens singly, the object will appear: 1 (if the concave lens is the stronger), to move in the same direction as the motion of the glass, but more slowly than before; 2 (if the convex lens is the stronger), to move in the opposite direction to the motion of the glass, but more slowly than before; 3 (if the lenses are of equal strength), to have no motion.
Therefore, to find the strength of a spherical lens it is only necessary to combine it in this way with successive lenses of known strength and of the opposite sign until that one is found which neutralizes the apparent motion of objects seen through it. This lens is the measure of the strength of the one tested. This method is accurate within an eighth diopter, or less, for plano-convex and plano-concave lenses; with bi-convex, bi-concave, and toric glasses it is only possible to neutralize the apparent motion near the center of the lens; toward the edges motion will still be visible when the lenses are strong.
Cylindrical Lenses may be recognized by viewing through them some object presenting a straight line, say ​the vertical line of a window sash. If the cylindrical lens be rotated about the visual axis, the portion of the vertical line seen through the glass will appear to be oblique, as compared with that seen above and below the glass (Fig. 42). This oblique displacement takes place in a direction contrary to the rotary motion given the lens if the latter is convex, and in the same direction as the motion if the lens is concave. To ascertain the position of the axis of a cylindrical lens it should be rotated slowly in this manner until the line seen through it appears continuous with that above and below (Fig. 43).
This line will then lie either in the axis or at right angles to it. To ascertain which of the latter is the case, the effect of motion from side to side is to be tried. If the axis of the cylinder corresponds with the vertical line looked at, motion from side to side produces apparent motion of the object; if, however, the axis lies at right angles to the vertical line no such motion results. In other words, in the direction of its axis a cylindrical lens acts as a piece of plain glass; across its axis it acts as a spherical lens of the same strength. If it is desired to know upon which surface of a lens the cylinder is ground, this may be ascertained by holding the lens nearly horizontally between the eye and a window, so that the line ​of sight strikes its upper surface very obliquely. One can thereby see the lines of the window reflected upon the upper surface of the lens. By rotating the lens about its optic axis these lines appear broken if the surface is cylindrical, but retain their continuity if the reflecting surface is spherical. The direction of the axis of a cylindrical lens having been ascertained, its strength may be determined by neutralizing it with a cylinder of the opposite sign, as was explained when speaking of spherical lenses. Care must be taken that the two lenses are so placed that their axes coincide.
A Sphero-cylindrical Lens is equal in refractive effect to two cylindrical lenses with their axes perpendicular to each other. Having found that axis across which motion is least rapid, we may neutralize the motion with a spherical lens and, holding these two together, proceed to neutralize the motion across the other axis just as if dealing with a simple cylinder. When our object is not to determine the strength of an unknown lens, but to see if the lenses of a pair of spectacles agree with the prescription previously written, we may, of course, shorten the above procedures by picking out from the test case the glass, or glasses, which will neutralize the spectacles if the latter are of the proper strength, and observing whether the apparent motion of objects ceases when they are held together.
Locating the Optical Center.—Every glass before being worn should be examined with regard to the position of the optical center of each lens and the distance of these from each other, as inaccuracy in this important particular is not uncommon. Indeed, in the cheap spectacles which some persons unfortunately buy, proper centering is the exception. In grinding large numbers of lenses by machinery a certain number in each batch are, I believe, always found to be badly centered. These are not returned to the wheel or the furnace by the thrifty manufacturer, but are graded as second class, or if very bad indeed as third class, ​and with those which will not pass inspection in other particulars go to make up the trash sold by peddlers. A simple way to find the location of the optical center is to hold the lens about a foot above the corner of a rectangular card lying on the table. The corner seen through the lens will only appear complete and continuous with the rest of the card when its tip is opposite the optical center.
In Fig. 44, A represents a lens so held that its optical center is marked by the corner of the underlying card; B is a lens improperly held. The center first found may be marked with a speck of ink, the center of the other spectacle glass found in the same way, and the distance between them measured. If care is taken to hold the glass exactly level and the eye directly over it this method will give results accurate enough for most purposes.
The Apex of a Prism may be determined by viewing through the glass fine lines crossed at right angles, holding the prism so that its edge and supposed apex just touches one line at the point of intersection. When the real apex of the prism coincides with the intersection of the lines, the appearance presented is that shown in Fig. 45; when, however, the apex is to one side of the point of intersection, the line seen through the prism appears broken, as in Fig. 46. In this case the prism is to be rotated until the line appears ​continuous, when the point of intersection of the lines will mark the apex of the prism.
The Strength of a Prism may be expressed in two ways; either in degrees of the refracting angle, which is the angle forming the edge and separating the two refracting surfaces of the prism, or by means of some formula which denotes the power of the prism to turn a ray of light from its course. This power is usually expressed in degrees of the angle of deviation, which is the angle separating the course of a ray of light after having passed through the prism from that which it would have pursued had its course been unobstructed.
| Fig. 36. | Fig. 37. |
| Figs. 45 and 46.—Method of Finding the Apex of a Prism. (After Maddox.) ||
The obvious advantage of the latter mode of expression, which gives directly the optical strength of the prism, over the former, which merely states the value of a physical angle from which the strength can be more or less accurately inferred, has called forth several suggestions for an improved method of numbering ophthalmological prisms. Dr. Edward Jackson was the first to point out that the prism had escaped attention when the numeration of our other glasses was reformed. He pro posed that in harmony with the mode of stating the value of angles which is commonly accepted in other departments of science, they be marked in degrees of their angles of deviation. With the idea of conforming their numera tion to the dioptric system of numbering lenses, Mr. C. F. Prentice proposed to adopt as a unit that prism having the power necessary to produce one centimeter of deviation in the course of the ray after having passed through and the distance of one meter beyond the prism. Dr. ​S. M. Burnett proposes that this unit be called the prism diopter, and that the centimeter of deviation be measured upon a plane surface—that is, upon a tangent of the arc whose radius is one meter.
Within practical limits the objections which have been raised to the prism diopter are few and of little moment, and it has great simplicity to recommend it. In a series of prisms so numbered, however, the higher prisms are not simple multiples of the lower ones. Twenty prisms of two P. D. each would not be equal to a prism of 40 P. D., but to a prism of 42 P. D.
The centrad as a unit of measurement of prism power was suggested by Dr. W. S. Dennett. After mature consideration this unit has been formally recommended by the American Ophthalmological Society, and will doubtless in a few years entirely, as it has already to a great degree, displace the old system of numbering.
The term radian denotes in mathematics a portion of the arc equal to the radius. The centradian is the one hundredth part of the radian. The centrad is such a prism as, held with one surface perpendicular to the incident ray, causes a deflection equal to a centradian. If the measurement be made at one meter, then, the radius and radian being each one meter long, the centradian will equal a centimeter, measured on the arc, and the centrad is such a prism as will produce this amount of deflection. If the measurement be made at two meters—a very convenient distance—one centrad will produce a deflection of one hundredth of two meters, or two centimeters.
It will be seen that the practical difference between a centrad and a prism diopter consists in this, that in the former the amount of deflection is measured on the arc, while in the latter it is measured on the tangent. For ophthalmological prisms, which are of necessity weak, the difference between centrads and prism diopters is so slight as to be of no moment. The numeration of prisms by ​centrads has the advantage that it is founded on a method of stating the value of the angle which is used in other departments of physics. Its higher numbers in the scale are, moreover, simple multiples of the unit.
Over the system of numbering prisms in degrees of the refracting angle the use of the centrad has all the advantages possessed by the modern numeration of spherical lenses over the old. Its use, moreover, involves no perplexity in the mind of one who has become habituated to the former method, since, as shown in Table VI the difference in value of one of the old and one of the new prisms of the same number is slight for the weaker, more used prisms. The one, however, represents a definite, fixed value; the other does not.
As the surgeon has a choice of two essentially different methods of numbering, so, also, he has at his command several modes of determining the strength of unknown prisms, and may select that one which is simplest and involves least calculation for the numeration which he uses. The refracting angle may be readily found by means of Table III, introduced when speaking of the prismatic equivalent of decentered lenses. The situation of the optical center is to be marked upon a spherical lens of convenient strength, and the prism to be tested superimposed. By viewing the corner of a card through these two glasses, as was directed in describing the method of finding the optical center, this center will be found to have been carried toward the base of the prism. The position of this apparent optical center is to be likewise marked upon the spherical lens, and its distance from the true one measured. In the left-hand column of Table III find the strength of the lens used, and on a level with this across the page the distance in millimeters between the true and apparent optical centers. At the head of the column in which this measurement is found will stand the strength of the prism with which the lens was combined, this strength ​being expressed in degrees of the refracting angle. For instance, if having combined an unknown prism with a +
7. D. lens we find the apparent displacement of the optical center to be 4 mm., the table shows at a glance that the refracting angle of the prism tested had a value of 3°. ​The refracting angle may be directly measured by adapting the legs of a pair of compasses to the two refracting surfaces and then laying the compasses on an ordinary protractor.
The surgeon is, however, very little concerned with the refracting angles of prisms, except as they are the basis of the old system of numbering, which is now almost superseded by one in which the number of the prism indicates in centrads the power which that prism possesses of causing deviation in a ray of light. One of the simplest and most convenient devices for measuring this power is that suggested by Dr. Maddox. It consists of a strip of cardboard suspended horizontally on the wall on a level with the eyes of the observer.
The upper border of the card (Fig. 47) is marked from right to left with a scale of degrees, or rather tangents of degrees, proper to the distance at which the prism is to be held from the card. In Table VII is given the distance from the right-hand border of the card of the mark for each degree of deviating angle. With the help of this table one may readily construct the scale, using column A if he elect to work at six feet, or column B if a two-meter range be preferred.
To practice this method of prismetry, the glass to be tested is held at the proper distance from the card, its apex to the left, and its upper border just below the figures of the scale, as in Fig. 47. The observer’s eye being placed behind the prism, the right vertical border of the card appears displaced toward the observer’s left and points upward to the number expressing the strength of the prism in degrees of the angle of deviation. During this ​maneuver care must be taken that the prism is held at precisely the distance from the card for which the scale of the latter is arranged; also that the apex of the prism points exactly to the left.
This latter requirement may be secured by rotating the prism until the line of the bottom of the card appears unbroken, as at A, in
In adapting this method of prismetry to centrads or prism diopters, the scale at the top of the card should simply be laid off in centimeters, and the prism be held at the ​ance of one meter. Each centimeter that the right border of the card is apparently moved to the left, on viewing it through the prism, will then represent one centrad, or one prism diopter.
Scratches, Specks, Bubbles, Flaws, etc., in the glass will hardly escape detection if they are carefully looked for while the lens is held in different lights. Placing the glass against a dark background and allowing a bright light to fall obliquely upon it will perhaps bring them out as plainly as any other maneuver.
Irregularity of the Surface may be discovered by reflecting from that surface any object having regular outlines. The observer should stand facing a window, holding the lens against a dark background in his left hand, and pass a straight-edged piece of paper held in his right hand between his eyes and the lens. Two images of the paper will be reflected from the lens—one formed by each surface. Any irregularity of these surfaces will make the images appear broken, or with wavy outlines.
Adjusting Spectacle Frames.—It requires some little practice to enable one to tell at a glance just where such an irregularly shaped object as a spectacle frame has been ​wrongly bent; having found the error, it is a more simple matter to correct it. For the latter purpose small pliers are should have narrow but strong required. They jaws, round in one pair and square in another. (Fig. 48.) For different parts of the frames and for making special bends many special forms of pliers are in use. Fig. 49 shows the shapes of the jaws of a few of them. A small, stout screw-driver with a point suited to the screws of spectacles will also be necessary. A special pencil which makes a white, easily erasable mark on glass is useful for marking the position of centers, and of axes of cylinders. Such marks are a great aid in fitting frames.
Eye wires are generally of such light material as to take their shape from the contained glass, and are, therefore, not liable to become misshapen. The popular round lenses frequently rotate within the eyewire, which is disastrous in case they contain a cylinder or prism. On such a lens near its margin two minute diamond marks should be placed to show the horizontal diameter. Sometimes the long axis of an oval eye gets rotated within the eye wire (Fig. 50), so that it no longer stands squarely across the face. By loosening the screws it can readily be re-adjusted. Abnormal crookedness about the bridge is best disclosed by placing a straight edge (indicated by the ​line S E in Figs. 50, 51, 53, 54 and 55) in such a position as to enable one to compare the two sides of the frame. If the bridge is bent at its junction with the eye wire a rotation results, looking very much like that just mentioned, but dependent upon an entirely different fault (Fig. 51). It is readily corrected with pliers or fingers.
The planes of the glasses may cross each other (Fig. 52), in consequence of a twist in almost any part of the bridge though the trouble is, usually, that the angle of the bridge at A is not of the same size as its fellow of the opposite ​side. The bridge is inclined, as shown in the cut, more to one glass than to the other. It requires application to the patient’s face to determine which is the proper inclination, and in order that the glasses may be equalized at this and not at the improper one.
In Fig. 53 the bend is at the junction of the eye wire with the bridge, rendering corresponding angles of the two sides of the frame unequal. The diagram shows the change necessary to correct the trouble. A similar fault is shown in Fig. 54. This appears at first sight to be just like the last; it is, however, a neighboring angle of the bridge which needs equalizing with its fellow.
In the frame represented in Fig. 55 the glasses lie in the same plane, but one of them is nearer the center of the bridge than the other, due to the fact that, of the angles of the bridge which can be seen by viewing the frame in this position, the two which lie on one side of the curved portion are too much open, while the two on the other side are too little so. Of course, the bridge may be misshapen in any portion of its extent, but the illustrations given are sufficient to show the sort of faults one may expect.
Having rectified all want of symmetry in the “front,” the defects in the fit of the temples can best be corrected by trying the frames on the patient’s face. If on doing so it is found that their temples cut into the temples of the wearer, instead of just touching the skin, as they should do, the trouble is obviously that the distance between the temples is too small, and they must be bent out at the hinges, so as to throw them, when open, farther apart. This is done with the square-jawed pliers, seizing the wire close up to the hinge. When the opposite condition pertains, that is, when the distance between the temples is too great, leaving a space between each wire and the side of the wearer’s head, they require to be bent in. To do this, take the end of each side in turn in the square-jawed pliers, in such a way that the edge of one jaw shall be in contact with ​the temple as close to the hinge as possible and the latter be held rigidly open. The temple may then be pressed in with the fingers, and will bend at the point where it is pressed against the edge of the pliers. If the latter are rightly placed this does not make an angle in the wire forming the temple, but simply alters the angle already formed at A in Fig. 55, by the expansion of the end of the temple to help form the hinge. Care must be taken that one temple is not bent out more than the other, or, as is apt to be the case, become so during use. When this happens the effect is quite different from what might be expected. The glass on the same side as the temple the more bent out will be brought closer to the eye, while its fellow will be carried farther forward and the bridge will ride obliquely across the nose. To remedy this it is only necessary to equalize the divergence of the temples.
The curve of hook temples given them by the maker will rarely be found to fit comfortably behind the ear. As was pointed out by Dr. Charles H. Thomas, the proper form for hook temples is a straight line from the hinge to the top of the ear, where a sharp curve should join this part of the temple to the easy curve which corresponds to the back of the ear (Fig. 56). Where the curve given the hook is too wide and is extended upon that part of the wire resting against the patient’s temple, as shown by the dotted line in Fig. 56, there is a constant tendency of the spectacles to slide forward. The wire, moreover, touches ​the back of the ear for a short distance only, where its pressure is further increased by the fact of the whole temple being put upon the stretch and acting as a spring. Especially at first should the frames not fit too tightly, as the skin is then more easily irritated by the wire than when it becomes accustomed to its presence.
In persons whose ears stand out far from the head a certain ridge upon the cartilage of the ear is thrown into prominence. Since the curve of a hook temple is a regular one, it will rest upon this ridge and be very uncomfortable; indeed, it may cut through the skin and into the cartilage. Under such circumstances the portion of the wire which is behind the ear should be made to follow every depression and elevation of the surface with which it is in contact; as it should in any case where the auricle is deformed or irregular in any way.
If one lens stands higher upon the face than the other, so that the patient looks through the upper part of one glass and the lower part of the other, it will be found that the temple on the side which stands the higher is turned down more than its fellow. It should be raised, or more frequently its fellow should be lowered. The fault may lie in the bridge, as shown in Fig. 52, or in the end piece, or in the temple itself. In the first instance, bringing the lenses into the same plane removes the difficulty; in the second, take the end piece in the round-jawed pliers, the jaws being applied to its edges close up to the eye wire. Holding these pliers in the left hand, apply the square jaws of the other pliers to the surfaces of the end piece; when, by twisting the latter about its long axis, the temple may be turned down to any desired extent. Thus, the temple is not bent at all, but the end piece between the hinge and the eye wire. Nearly the same effect may be produced by bending the wire of the temple close up to the hinge. As was remarked before, in speaking of the facing of the glasses, the effect of turning down both temples is not to make both ​lenses stand higher upon the face but to make the glasses face more downward.
Sometimes when the glasses do not sit properly the trouble will be found to be not in the frames but in the wearer. A considerable amount of asymmetry of the two sides of the face is not uncommon. One ear or one eye may be higher than its fellow, either of which conditions will make the glasses seem awry, and render necessary a compensating asymmetry of their frames.
Adjustment of Eyeglasses.—The starting-point in adjusting eyeglasses is at the nose-pieces, whose free surfaces should be made to conform accurately to the bones of the nose by which they are supported. When received from the maker they are generally curved, presenting a convexity toward the nose.
As the bones of the sides of the nose at the point where the guards are to rest are usually more or less convex also, the bearing obtained is ​a most insecure and uncomfortable one, as a glance at Fig. 57 will show. In Fig. 58 this glass is shown with its nosepieces properly adapted to the sides of the nose. Any conformation may be required, but that shown in Fig. 58 is the one most frequently needed. These changes in the shape of the nose-pieces are readily effected by means of the pliers, especially if the nose-pieces recommended in Part I are used. When the guards are of cork, care must be taken that they are not scarred and broken by the pliers, and a special tool with a longitudinal groove in the jaws for grasping the sides of the nose-pieces is here of service.
It will be readily seen, moreover, that the nose-pieces must incline equally to a vertical plane passing through the center of the nose; otherwise the glasses will stand awry; that is, provided the nose is straight and its two sides alike. In a large proportion of cases, however, a plane through the middle of the nose is only approximately vertical, so that one nose-piece must be inclined more than the other. (Fig. 59.) It is even necessary, sometimes, to incline the top of one nose-piece toward the vertical plane and the other away from it.
The slope of one side of the nose from the crest backward is frequently steeper than that of the other side. A symmetrical eyeglass on such a nose will stand with one lens close to the corresponding eye and the other standing forward away from the eye. One nose-piece will tend to ​rest on its anterior edge and the other on its posterior edge. By partially revolving each nose-piece around the long axis of its bearing surface the glasses are brought parallel to the general plane of the face and each nose-piece presses evenly over its whole bearing surface. Fig. 60 is intended to illustrate what is meant. It is a diagrammatic view from above, showing the ends of the nose-pieces as arranged for a case in which the bridge of the nose does not slope equally on the two sides. These changes in the inclination of the nose-pieces are brought about by bending or twisting the foot at the point B in Fig. 61.
Having conformed the nose-pieces to their bony support, the tension of the spring by which they are pressed against the sides of the nose is to be regulated, the object being to have just sufficient force exerted to keep the guards securely in place. If the latter are properly fitted the amount of pressure necessary is not great. Though this pressure should be evenly distributed over the surfaces of the nose-pieces, want of firmness in the ”pinch” of their tops is particularly fatal, as the lower ends then become the principal support of the weight of the glasses, rendering them prone to topple forward and fall. To increase the tension of the spring, and consequently the pinch of the frames, the curve of the spring included between the lines at A, in Fig. 61, should be made more arched and rounded. Conversely, the force of the spring is lessened by flattening this arch. Any alteration in the shape of the spring, however, while it does not, of course, change the shape of the nose-pieces, does change the angle at which ​they are inclined to each other. For instance, if the spring be made more arched, the nose-pieces are brought nearer together, but the bottoms are especially approached toward each other. When the spring is flattened the bottoms of the nose-pieces are thrown proportionately farther apart than the tops. It follows that with each adjustment of the tension of the spring the inclination of the nose-pieces must be rectified. This is easily accomplished by twisting the “foot” or support of the nose-piece at B in Fig. 61.
When the points mentioned have been properly adjusted, the long axis of one or both glasses may fail to stand squarely across the face as it should do. The remedy lies in an appropriate bend of the spring at the point C (Fig. 61). This also requires a slight re-adjustment of the inclination of the nose-pieces to each other.
The distance between the centers of eyeglasses is determined (the distance between the nose-pieces when in use being a fixed quantity) by the distance of the nose-piece on each side from the center of the corresponding eye. The intercentral measurement may therefore be varied by varying the size of the eye used, and by altering the distance of the nose-pieces from the edges of the lenses by an appropriate bend of the foot B (Fig. 61), or by using a longer or ​shorter stud. The distance of the glasses from the eye is controlled by the length of the foot B, and in the better grades of goods this part is made in two or three lengths.
Eyeglasses seldom stand too low upon the face but they frequently have the fault of standing too high, especially for near work. The neatest way of lowering them, but one which must be attended to when prescribing them, is to have the studs attached above the horizontal diameter of the lens, instead of at that diameter, as is usual. They may thus be lowered one, two or three millimeters; or a special form of nose-piece may be used. (Fig. 20.) It is sometimes necessary to combine these methods.
The Care of Spectacles.—Spectacle frames will last longer and perform their function better if the wearer is instructed to exercise care in handling them. In putting them on and off, the hooks should be lifted from or into their position behind the ears; both hands being used, so as to avoid straining the temples widely apart or otherwise bending them. They should be folded together as little as possible, and when not in use should be laid in a safe place, open, and resting on the edge of the lenses, to avoid scratching the surfaces of the latter. For cleansing them nothing is better than a piece of clean old linen, or, if very much soiled, a little ammonia and water may be used, except on cemented bifocal glasses. While cleansing, the frame should be grasped by the end piece and not by the bridge, and in replacing the glasses on the eyes care should be taken not to crush them against the lashes and thus soil the refracting surfaces at once. When cylindrical or prismatic glasses are worn, patients may return after a time with the statement that the spectacles are unsatisfactory, when the trouble will frequently be found to be due to bending of the frame; or a lens may have fallen out and been replaced upside down, or with the wrong edge inward. It is well to have such persons report periodically to have their glasses re-adjusted.