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

rower.

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 - put these on, and adjust them till the centres of the Glasses come exactly before the centres of the Eyes.

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If his Frames are numbered 1.-2.-3. 4.-5. according to the width of the Bridge— the 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."

pending their Spectacles on the very tip of their Nose this is as injurious to their own Eyes, as it is absurd to the Eyes of others.

The Bridge of Spectacles for Long-sighted Persons, who wear them to read with, is best of the form, (shewn in the Plate) which when the Eyes are employed in Reading or Working, &c. brings the Glasses parallel to the centres of the Eyes.

For Short-sighted Persons, it should be straight - because the Glasses are required to be parallel to the Eyes when you look straight forwards horizontally.

The proper Shape for the Openings which hold the Glasses, is the true regular Oval* - which form is similar to the Opening of the Eye.The Vulgar opinion is, that Circular Eye Glasses are ground of the most perfect Figure — the Fact is, that the Oval Eye Glasses are ground Round at first and then clipped Oval.

The Front of the Frame should project beyond the Glasses far enough to protect the

Spectacle frame-makers complain that the Glass grinders have a barbarous custom of distorting their Ovals, by putting in ill-shaped Glasses and forcing the open ings to adapt themselves to them.

Glasses when people carelessly lay them down on their surfaces-and also to prevent their being injured by rubbing against the Case, when passing in or out thereof.

The Front must be Strong enough not to bend in the smallest degree or the Glasses will lose their parallelism with the Eyes, vision will be distorted, and the Sight distressed.

The whole of the elasticity of the Frame must be in the sides.

The First Joint should be of Silver wire of theth of an Inch in Diameter, and Four Inches and a half in length, and so curved as not to touch the Head except by the last half inch of it. The First Joint is seldom more than 44 inches in length-but the apparently trifling addition of a quarter of an Inch in its length gives it a much more steady and comfortable attachment to the Head, than can be imagined by those who have not tried it—especially when the Second Joint is turned down and you use it with the First Joint only.

The Second Joint should be flat, about the th of an Inch in breadth, and 23 Inches in length, with a loop end- and the Pin which fastens it to the First Joint should be kept tight enough to