# Spectacles and Eyeglasses/Chapter 3

page
| Philadelphia, 189 |
| Name of Patient, |
| O. D. |
| O. S. |
| Unless otherwise specified, furnish the following: Medium length temples; saddle bridge; No. 2 eyes. Dimensions are given to middle of wires. Dimensions given are in millimeters. |
| Frames of gold steel | Frames of gold steel | Frames of gold steel | Catalogue No. | Catalogue No. |
| Interpupillary Distance |
| Bridge | | Height | Height | Top in out |
| Bridge | | Width of Base |
| M.D. |
A prescription blank such as that here given indicates what measurements are required, and will be found useful in practice. The upper part is for the lenses, the lower part for the frames.
To Obtain the Interpupillary Distance, with which the first dimension of the frame, the distance between the ​geometrical centers (A to B, Fig. 24), is generally identical, the physician seats himself facing the patient in a good light, the latter being directed to look straight before him
at some distant object. The measuring rule is placed before the patient's eyes, as close to them and as far from the physician’s eyes as possible. The zero of the scale being placed opposite the center of one pupil, the center of the
other may be marked by the physician's thumb nail, as represented in Fig. 25, and the distance between them read off the scale. This distance seldom varies more than 5mm. ​from 60 mm., or 2$^3$⁄$_8$ in. It will be observed that as the physician's eyes are less than the length of his arm away from the patient's face when this measurement is taken, in fact, about two feet away, the marks upon the rule, though apparently opposite the pupils, will in reality be a little within the centers; so that the distance obtained will be a little less than it should be. When the physician's eyes are two feet away from those of the patient, and the rule is one inch away from them, the error in measuring an interpupillary distance of 60 mm. by this method is almost exactly 2 mm. This amount should, therefore, be added to the apparent interpupillary distance to obtain the true one.
The measurement obtained in this way is sufficiently accurate for most purposes, but if a greater degree of accuracy be desired in any case it may be attained by means of the little device suggested by Dr. Maddox, which is represented in Fig. 26. This is to be placed before one of the patient's eyes in an ordinary trial frame having a graduated bar for showing the distance of each geometrical center from the middle of the bridge. The gaze of the observed and that of the observing eye being directed to ​each other's pupils, the two sights of the implement are brought into line between them, as shown in Fig. 27. The same procedure is then gone through with for the other eye, and the distance of the second pupil from the median line of the face, as registered by the trial frame, is added to
that of the first, to obtain the interpupillary distance. This procedure is also of advantage in revealing and measuring any difference in the distance of the pupils from the median line, due to asymmetry of the face. The use of a trial frame for making accurate measurements requires the ​bestowal of considerable attention to see that the support of the nose piece is vertical, the joints close and tight, and the markings correct; otherwise it may readily introduce the errors its use is intended to obviate. There are, in the shops, many special forms of the "pupillometer" constructed on the principle of a rule held before the eyes and a single sight for each pupil. Two of these are shown in Figs. 28 and 29. The interpupillary distance as registered by them requires, of course, the same correction as docs that obtained by the simple graduated rule.
Height of the Bridge.—This is the distance of the top of the bridge above a line joining the centers of the lenses. In Fig. 24, it is the distance from E to F, which is the height of E above a line joining A and B; not the height of E above a line joining C and D, which is sometimes erroneously supposed to represent the height of the bridge.
If a rule be held horizontally before the patient's eyes, with the lower edge touching the nose at the natural position for the spectacle bridge, the height of this edge of the rule above the pupil on either side will show at a glance about how high the top of the future bridge must be. We may then select from our sample frames that one whose bridge corresponds most nearly with this supposed height, and being sure to place it in the natural position, we carefully note whether the pupils are above or below the centers of the eyes of the frame. If they are below these centers, sufficient must be added to the height of the bridge ​now upon the face to allow them to coincide; if the pupils are above the centers, a corresponding subtraction from the height of the trial bridge must be made. Each sample frame may have its dimensions attached to it, or any frame may be used as a fitting frame and afterward measured. To measure the height of a bridge the glasses are laid upon a sheet of ruled paper, or other object offering a convenient straight line, in such a way that the line passes through the geometrical centers of the eyes, or, what is the same thing, through the joints of the end pieces on each side (Fig. 24). The height to which the bridge projects above this line is then readily measured. It is seldom greater than 10 mm., and in rare cases may be a minus quantity, the top of the bridge being below the level of the centers of the lenses.
Relation of the Top of the Bridge to the Plane of the Lenses.—The measurement required to express this relation is that from J to K in Figs. 30 and 31; not the distance of J in front of a line joining C and D, as might be supposed. This measurement is also shown at II I, Figs. 18 and 19; it is obtained by a procedure similar to ​that just described for obtaining the height of the bridge. The rule being placed across the nose at the natural point, and the patient requested to wink, it may readily be seen whether the lashes touch the edge of the rule. If they do, the top of the bridge of the future spectacles must be back of the plane of the glasses, or "in." If they do not, we note how much nearer, if any, the edge of the rule might be brought without their touching, and so obtain a guide to the distance the top of the bridge should be in front of the plane of the lenses, or "out." The fitting frame which comes nearest to the requirements of the case in this particular is then placed upon the face, when by viewing it from above or from the side it can quickly be seen just how much change, if any, is needed to place the glasses a little beyond the reach of the lashes. The method of measuring the distance of a bridge in or out is so plainly shown in Figs. 30 and 31 that special explanation is unnecessary. They seldom measure more than 4 mm. out or 3 mm. in.
Width of Base.—The measurement from C to D, Fig. 30, is obtained, like the others, by measuring a bridge which fits, or estimating the change necessary in one which does not. This dimension is usually from 16 mm. to 20 mm. ​This method of obtaining the dimensions of the bridge required may seem tedious and uncertain in the description; in use it is not so, and after trial I think will be found preferable to any special device so far invented for recording the measurements. These, after shifting of screws and bending of wires, leave one to estimate what changes are required just as might have been done without their aid. Moreover, the heavy parts and lost space in joints of trial frames may readily conceal an error of 2 mm., or even 3 mm. in some measurements; the large, round eyes with heavy rims will not go under the brows, so that the in-out measurement of the bridge must frequently be guessed at; and the relation of the upper part of the eye wires to the brows is not shown. In fact, they introduce, in my estimation, quite as many sources of error as they eliminate.
Where the face is unsymmetrical no exact rules of procedure can be given, and considerable ingenuity may be required to fit a frame to such a face. If the nose is very peculiar, or one side of its bridge markedly steeper than the other, it may be of advantage to take an outline of the bridge at the natural position by bending a piece of lead wire to fit accurately and marking the outline of this upon the prescription blank, or sending the wire itself to the spectacle maker. Sometimes the brows are overhanging and the eyes deep set; so that the glasses cannot be properly centered before the eyes and placed close to them without the upper part of the rims burying themselves in the brows. In such cases the glasses should be decentered upward in their frames and the bridge made sufficiently high to bring the optical centers opposite the pupils. Though the patient will then look through the upper part of his glasses, his field of vision will not be any more limited than is already the case because of the overhanging brows.
​Prescription of Eyeglasses.—The dimensions which it is usual to furnish in prescribing eyeglass frames are the interpupillary distance, of course, with the distance between the two upper and the two lower ends of the nose pieces when they are in place on the face (A to B, and C to D, Fig. 12). These measurements alone will not insure a good fit in the frames, since neither the contour of the sides of the nose to which the guards are applied, the vertical centering of the lenses, nor the distance of the latter from the eyes are taken into account; but the same remark applies here as to the minor dimensions of spectacle frames, namely: that it is more simple, certain, and expeditious for the surgeon to make these adjustments in the frames themselves than to prescribe what the manufacturer shall do for him. Fortunately, eyeglass frames admit of great variation by bending their different parts, and being put together with screws, these parts are quickly interchangeable. Almost the only thing about them which admits of no adjustment is the length of the spring, and it is well for one who prescribes many eyeglass frames to have a series of such springs at hand from which to replace one which may be found too long or too short.