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Spectacles And Their Adjustment

Spectacles And Their Adjustment.

A SPECTACLE LENS should be so placed that, in use, the line of sight passes through the optical center perpendicular to the plane in which tile glass lies. These simple conditions would be extremely easy to satisfy were. it not for the fact that the organ of' vision must of necessity be extremely active and mobile. The eyeball may, in fact, be rolled in its socket about 60' in every direction from a point immediately in its front, while movements of the head, or even of the entire body, are constantly called in requisition as greater range of vision is required. These facts destroy at once the possibility of so placing a glass that its center may be coincident with, and its plane be perpendicular to, the line of sight under all circumstances. Only a glass fastened to the eve and moving with it could fulfil these conditions.

Though the glass cannot be attached to the eye, it can be and is attached to the head, which, as has been noted, is nearly constantly in motion, seconding the activity of the eyes. The necessity of looking through the center of the glass limits for the wearer the range of the eyes in their sockets, and increases in a corresponding degree the excursions made by the head. This augmented head motion, which can be noticed in almost all wearers of glasses, arises partly also from the effort to bring the plane of the lenses perpendicular to the line of sight. The only exception to these statements is in tile case of a person who is wearing a glass which under corrects his ametropia, and who looks through it obliquely in order to increase its refractive effect. The more exactly the glass and frame are fitted to the requirements of the case, the less of this auxiliary head movement will be required ; some increase in it must, however, be accepted as one of the concomitants of wearing spectacles. When a person with glasses raises his head continually, markedly elevates or depresses his chin, or forcibly twists his spectacle frame in his fingers, he is instinctively seeking to correct faulty refraction or faulty frame fitting.

Spectacles are ordered to be worn either constantly, or for near work only, or for distant vision only. It will be readily understood that the circumstances under which near work is usually done admit of the most exact adjustment of the glass. Stich work is usually held in the bands or occupies a desk or beach having a fixed position relative to the workman. It is below the level of his eyes and within reach of his bands, and only slight excursions of the eyes are required in its performance. As the line of sight is directed downward, the " near" glass (n, Fig. 166) must be placed below the level of the eye; at least its optical center must be so placed. It must face strongly downward in order to bring its plane perpendicular to the line of sight (b). It should face slightly inward for the same purpose, since the visual axes converge in near vision. This convergence necessitates, further, that the optical centers of' the glasses shall be placed from 4 to 6 mm. nearer together 236


than are the centers of the pupils, since the visual axes would otherwise pass to their inner sides. If an isosceles triangle is constructed with the interpupillary distance as its base, and the visual axes, directed toward a near object, as its remaining sides, it will be apparent that the farther from the eyes a pair of glasses stand and the nearer to the eyes the work is situated, the less should be the distance between the optical centers of the glasses. The precise distance between optical centers which any given case may require may thus be determined.

In " distant" vision the gaze may be directed toward any point of the hori or firmament, and yet, practically, the relation of the line of sight to the face, and consequently to glasses at to the face, does not vary greatly. A distant object would have to change its position considerably in order to move through five degrees of one's field of view. Hence rapid changes in the direction of the line of' sight are seldom required. Ample time is afforded for whatever adjustments of the head and trunk may be necessary. Distant vision usually takes place, therefore, with the visual axes directed forward perpendicular to the plane of the face (a, Fig. 166). When glasses are ordered for this use alone they should have optical centers separated by the same interval as that between the pupils (since they will not be used during convergence), and should face directly forward, lying in a plane parallel to the general plane of the face (d, Fig. 166). The optical centers of the lenses should stand at the same height as the pupils.

In the greater number of cases the spectacles prescribed are intended for constant use that is, the wearer will need them as well in viewing distant objects as in .work near at hand. It is evident that to place the lenses in the exact position desirable for either of these purposes would render their use awkward for the other. In the height of the optical centers as well as in their distance from each other, and in the facing of the glass, we are therefore forced to place 11 constant" glasses in a position intermediate between that best for distant vision and that best for near work. This intermediate position is selected, not at all because these glasses are used at an intermediate distance, but because from this position they may be readily shifted, at least approximately, by a motion of the head into either of the other positions. The distance between the optical centers of "near" glasses should be from 4 to 6 rum. less than are the centers of the "distant " glasses intended for the ame patient. This dimension is, of course, unaffected by movements of the bead. Nevertheless, in order to reduce the unavoidable discrepancy to the mininium, the distance between the centers of the "constant" glasses should he 2 or 3 m m. less than that proper for the " distant " glasses. By a similar concession the " constant " glass is faced moderately downward and its centers placed somewhat lower than those of the " distant " glass, but not so low as those of the 11 near" glass.

In this connection the occupation of the patient should be considered. A eamstress or bookkeeper, for instance, if wearing a glass constantly should have it adj Listed almost like a 11 near" glass, while persons engaged in outdoor FIG. 166. Showing position of lenses before eye.


occupations will require an adjustment much nearer that proper for a " distant " glass.
The greater the strength of the prescribed lens the more necessary is attention to these details, since the effect of slight obliquity of the lens to the visual axis is greater in stronger lenses, as is also the effect of decentration. In bifocal glasses, therefore, in which there is both a stronger and a weaker lens ' the former must dominate the position of the spectacles. Convex bifocals in which the " near " element is the stronger should, therefore, approach the " near " spectacles in position, while concave bifocals are placed more nearly like a " distant " glass, as the 11 distant " element is here the stronger.

A s ectacte frame is a kind of tripod, its points of support being the top P of each ear and the bridge of the nose. It is not possible to make an indifferently selected point on the bridge of the nose serve as the support of spectacles. Nearly always it will be found that there is one particular point at which they tend to rest. In adapting spectacles to any given face, therefore, the problem is to bring the optical centers to the position previously determined that they should occupy with reference to the eyes, while at the same time their support is placed at this best adapted point on the crest of the nose. The spectacle bridge known as the " saddle " bridge is the only one which allows of unlimited variation in the relation of these two points.

In fitting a frame to the face the curved portion of the bridge between a and b, Fig. 167, should be adapted to the bones of the nose at the point at FiG. 167. Saddle bridge.
FiG. 168. Saddle bridge.
which it is supported. Having once received the proper shape, this portion of the bridge should not be altered, as its only function is to furnish a firm, equally pressing support for the "arms " c and d by means of which the centers of the glasses may be carried higher or lower on the face or the distance between them varied. These variations are accomplished by alterations in the angles of the wire at a and b. The length of the arms c and d governs the distance of the glasses from the eyes.

In prescribing or recording the measurements of a spectacle frame it is sufficient to give the distance between the centers of the glasses, with the height, depth, and width of the bridge. The height is the distance of h., Fig. 168 (the top of the bridge) above the line o o joining the centers of the glasses; hence the distance from h to g The depth is the distance between the top of the bridge (f, Fig. 167) and the point e on the plane in which the glasses lie. This distance maybe a negative one that is, f may be back of e. In the former instance the measurement is recorded as out, in the latter instance as in. The width of the base of the bridge is the distance between a and b. The measurements of a spectacle front may, therefore, be recorded in a single line, for example :

60 mm. X 5 mm. up X 3mm. out X 20 mm. base.

The direction in which the front of a spectacle faces depends on the angle which it forms with the side pieces or temples. If these latter are inclined


toward the bottom of the frame, the glasses when in use will face downward. It should be remembered that hook temples are simply hooks. They cannot, with comfort, be made to exert the force of a spring or a clamp upon the skin. They should touch the skin throughout the greatest possible portion of their extent, so as to distribute the weight they carry, and should not be allowed to press unequally owing to inequalities of the surface. Their proper form is a straight line from the hinge of the frame to the top of the ear, where a sharp curve joins that portion which is accurately fitted to the back of the ear,' with which it is in contact.

In eye glasses (pince nez) the same adaptability to differently proportioned faces is found in the " offset guard," which in spectacles is attained by means of the 11 saddle bridge." The nose pieces of these guards should be accurately moulded in every case to the sides of the nose at the point where they obtain the best bearing surface. Fixed points of support for the lenses are thus obtained. The height of the lenses before the eyes will now depend on the point of attachment of the 11 arm " of the guard to the nose pieces. In Fig. 169, for

FiG. 169. Guards of eye glasses.

example, the guard marked b will carry the lenses higher than the one marked e. The direction in which the lenses face is controlled by the size of the angle in the arm of the guard. Thus, in the figure, at a the arm has a right angle and will render the plane of the lenses nearly vertical ; that is, the latter will face directly forward, while at b the angle is greater than a right angle, and the glasses will face more downward.

The distance of the glasses from the eyes depends upon the length of this arm of the guard. The longer it is, the farther forward the glasses will be held; d and g, in the figure, have longer arms than a or b. Variations in the distance between the centers of the lenses may to a limited extent be procured by an arm which is bent so that its free end does not lie in the same plane as the nose piece. If greater latitude is required it must be procured by variation in the transverse diameter of the lens used, or by alteration of the length of the " stud " which connects the lens with the guard.

Methods of Testing Lenses. To ensure accuracy and comfort, spectacles, before being worn, should invariably be critically examined as to the strength of the lenses and the fit of the frame.

The most convenient method of determining the strength of lenses is the well known one of neutralization by means of the test case lenses of known strength. In practising this maneuver the lens is held about a foot before the eye and an object several yards away is sighted. On moving the lens slowlv across the line of sight the object seen through it appears to move also. In tl;e case of convex lenses this apparent movement is in a direction contrary to the motion imparted to the lens, or, in the language of the refraction room, is 'A against it." With concave lenses the apparent movement of the object is in the same direction as the movement of the lens, or 11 with it." If a convex and a concave lens of equal strength are held together, all this apparent move ceases ; they 11 neutralize " each other. The surgeon is, therefore, able


to quickly discover the strength of an unknown spherical lens by trying it with lenses of the opposite sign until that one is found which causes all movement of the object to cease. The strength of this lens is the same as that of the unknown one.

A cylindrical tells is recognized by the fact that that portion of a vertical line seen through it assumes an oblique position when the lens is rotated about its optic axis (a, Fig. 170). If the rotation of the lens is continued, the motion of the displaced portion of the line is reversed and its continuity is restored, as at b. This appearance is, therefore, presented in two positions of the cylindrical lens. In one position the vertical line marks the axis of the cylinder; in the other the line is at a right angle to the axis. To locate the axis an object presenting crossed lines, as at c, Fig. 171, is selected; the lens is so held that each line appears unbroken and is first moved horizontally, then vertically. The line across which motion is apparent marks the axis of the cylinder. The cylindrical lens of the opposite' sign which neutralizes this

FiG. 170. Alethod of testing cylindrical lenses.

FIG. of testing and prismatic lenses.

motion discloses the strength of the cylinder under examination. Care must be taken that the axes of the two coincide.

In a lens the cylindrical element, is recognized by its causing on rotation an apparent obliquity of a portion of a vertical line, just as (lid the simple cylinder. On viewing the crossed lines,'c, however, and moving the lens first horizontally, then vertically, apparent motion of the object is imparted in both directions, but in one it is more rapid than in the other. In neutralizing, the least rapid movement may be first obliterated by means of a spherical lens. This gives the strength of the sphere in the com Holding these two together, one proceeds to neutralize the cylindrical element by means of a cylinder of opposite sign, precisely as though no sphere were present.

On rotating a prismatic lens about one's line of sight an apparent displacement of a vertical line takes place, as at d, Fig. 171. When the line is continuous it marks the base apex line of the prism. At right angles to this is the meridian of maximum displacement. The prism being held at one meter's distance from the object, each centimeter of apparent displacement of the line shows one centrad of strength in the prism.

The optical center of a lens is located by using crossed lines, as at c, Fig. 171, except that for this purpose the lens is held within about a foot and tile lines should be fine. When each of the lines is continuous their crossing point marks the optical center.

The distance between centers being found correct and a final inspection disclosing no flaws or scratches in the glass, DO bends of the frame, or want of symmetry between its two sides, the spectacles are ready for the wearer.

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