preserve the Second Joint in whatever position the wearer finds easiest and most convenient. -If this becomes loose it is easily tightened

by placing the pin on which it turns, over a piece of Iron and giving it a gentle tap.

MEM. If the Pin which fastens the Second Joint to the First is not rivetted very nicely and smoothly every time You take off your Spectacles, you will find it an Infallible Depilatory—and if " Time has not thinn'd your flowing Hair," your Second Joint very soon will.

N. B. If the Second Joint be turned upwards to an Angle of about 30 degrees, it will be in the best position for preserving the Frame in its proper place.

PRICES OF SPECTACLES.

CHAPTER XV.

OF THE QUALITY OF SPECTACLE GLASSES, AND HOW TO MEASURE THEIR FOCAL LENGTH.

THE defects of Spectacle Glasses are either from Veins - Specks - Scratches-Colouror false Figure.

1st. To discover Veins in a Convex Glass place a Candle about 5 or 6 yards from you; then look through the Glass, move it from your Eye till you find it full of Light, and you will then clearly see every vein, &c. in it which renders Vision imperfect by distorting the Objects.

2dly. Specks or Scratches are not so mischievous as Veins - for they do not distort the object, but only intercept part of the Light; however, such defective Glasses will not be used, except by such persons who think that their Two Eyes are not worth Two Shillings.

3dly. False Figure. To prove the Figure, lay a Book before you, hold the Spectacles in your hand, and looking through them remove them gradually from the Book. If the figure

of the Glasses is false, the objects will appear distorted and confused. Thus if you view any cross lines which form small squares, through a falsely ground glass some of the squares will appear with strait sides distinct and all of a size; others with crooked sides of different sizes and confused; so that the whole will be neither like the original, nor all parts of it distinct at the same distance.

If you view it through a true glass, it will be exactly like the original figure and uniformly distinct, only magnified according to the degree of the power of the lens used; and as you gradually remove the Glass beyond the focus,

if you take care to move the Glass perpendicular to the plane of the Paper, all parts of the object will become equally indistinct at the same distance.

4thly. The Colour of the Glass That is the best material for looking through, through which Objects appear nearest to their natural Colour the easiest way of examining the Colour of a Glass is to lay it upon a piece of White Paper.

TO MEASURE THE FOCAL LENGTH OF SPECTACLE GLASSES. The usual manner

of measuring the focus of Spectacles, is to hold them opposite a window against the side of a room, and draw them gently away forwards until the frame of the window can be seen, making a small image through the Spectacle glass on the Wainscot or Side of the room. When the window frame appears most distinct, the number of inches, measured by a rule between the glass and the wall, or partition opposite, will shew the real focal length.

CHAPTER XVI.

PEBBLES.

SOME folks have a notion, that "PEBBLES are much cooler to the Eyes than GLASSES:" the relationship between " a Pebble" and "a Stone" and the Proverb " as Cold as a Stone" probably gave rise to this prejudice in favour of Pebbles.

It is quite impossible to distinguish between good Pebbles and good Glasses.

People call that Glass coolest to the Eye, which is of the most proper focus for it, and

which therefore irritates and fatigues it the least and through which they can see easiest and best.

The only superiority of Pebbles over Glasses is, that they are not so liable to be broken; and as they cannot be scratched by any thing softer than a Diamond, they may be carried in the pocket without a Case; which are certainly great advantages, especially to Travellers and Short-sighted Persons (See page 100) but I must warn my friend the Reader that Pebbles are so very veiny, that it is seldom a bit is found so perfect as it ought to be, and therefore 1 pair of the Best Pebbles cost 16s. i. e. as much as 8 pair of the Best Glasses.

OBSERVATIONS ON THE MATERIALS AND MANUFACTURE OF GLASSES FOR SPECTACLES,

By Mr. S. PIERCE, Optician.

THE best material for the formation of Spectacle-glasses, is the White Plate, commonly termed Dutch Glass ;* it is very clear and hard, with

* I am informed that THE BRITISH PLATE GLASS now made in East Smithfield, is very superior to any that was formerly made in this country.- Editor.