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I prefer a well-hammered SILVER FRAME with DOUBLE JOINTS, the Second joint of which may be turned on its pin over the First, so that they may be occasionally used with the Single joint only—they sit close and steady on the Head, and are convenient to wear under a Hatdo not press either on the Nose or on the Temples-but their pressure is general and equal, and as it may be varied, may be rendered more agreeable than any other Frame. Spectacles with only a Single Joint, must press hard somewhere.
TORTOISE-SHELL SPECTACLES have a gloomy heavy appearance, are no lighter than Silver ones, and are very easily and very often broken :- however, if you will have a Shell frame, let the front be all Black-variegated Shell is bad for the Eye.
The BLUED STEEL FRAMES are good looking enough when new, but soon lose their
Azure lustre, and then look very shabby: there is a prejudice in favour of a Steel frame as being very light, and, from its elasticity, that its pressure on the Head is less than that
of a Silver frame. It may be for the first fortnight; but in the course of that time, such is the ductile nature of a Silver frame, which soon adapts itself exactly and comfortably to the Head, and becomes infinitely easier and pleasanter than the springy Steel; and the truth of the old saying, "as easy as an Old Shoe," is remarkably felt in " an Old Silver Spectacle Frame."
That the Frame should be Light, is the only point which either the Makers or the Wearers of it seem to pay any attention to and to Lightness, every other property of it is willingly, but ridiculously sacrificed.
The actual difference in Weight between a Silver Front of that proper degree of strength which I have recommended, and one of the silly flimsy fronts which are commonly so much admired, does not exceed Four Pennyweights.
Let the Frame be large enough not to press
on the Head, or Head-aches, &c. &c. will be the inevitable consequence.
THE LENGTH OF THE BRIDGE, i. e. the distance between the Glasses, must be regulated by the distance between the Eyes, and the Centres of the Glasses must come exactly before the Centres of the Eyes;— according to the coincidence of which, Vision will be perfect or imperfect.
The mean Distance between the centres of the Pupils of the Eyes of People in general, is about Two Inches and a Half.
The following are the usual proportions of Spectacle frames.
The Length of the Bridge, from an Inch to an Inch and ths.
Of the Openings which hold the Glasses, if they are Oval, the longest diameter should not be less than an Inch and th, the Shortest about ths.
The length of the Common Knuckle is about ths of an Inch: - in some peculiarly formed Faces, this must be wider, and in others nar
The Length of the Knub Knuckle (which is
decidedly the most elegant) is only ths of an Inch.
THE WIDTH OF A COMMON FRAME.
See Figure in the Plate fronting the Title. To assist the Optician to ascertain exactly what ought to be the breadth of the Bridge,-I recommend him to have a Trial Frame, with an adjusting Bridge which will separate half an Inch and such separating part graduated
1. 2. 3. 4. 5. for each th of an Inch these on, and adjust them till the centres of the Glasses come exactly before the centres of the Eyes.
If his Frames are numbered 1.-2.-3.4.-5. according to the width of the Bridgethe Eyes of his Customers may be immediately suited to a nicety.
The Form of the Bridge must be regulated by the Form of the Nose which it is to cross.
The closer the Glasses are brought to the Eye the better, provided they do not come so close as to be touched by the Eye-lashes - if they do, the Glasses will be continually dimmed by the moisture from the Eye-lashes; * and what is worse, the Eyelids will become irritated and inflamed.
In the course of time Spectacle frames get out of proper shape, and become too loose to keep the Glasses up to the Eyes: - this arises so imperceptibly, that I have found it occur to several persons who were unconscious of it.The Optician easily remedies this, by restoring the bend of the Sides to their original form, and new pinning the Joints of them.
Nothing can look more ridiculous, than the trick which some Idle persons have, of sus
* The quantity of TEARS spread over the Globe of each Eye in the space of 24 hours, amounts to Two Ounces and upwards; i. e. a common sized wine-glass full. -- People who make use of Spectacles have opportunities of observing, that the evaporation of Tears tarnishes very much the Circles which surround the Glasses. Dr. P. DEGRAVERS, on the Eye and Ear, 8vo. 1800, p. 116. Surely the subject on which the Doctor made this experiment must have been," like Niobe all Tears."