ing the comparative degrees of the Magnifying Power of several Eye Glasses, although, gentle Reader, you may think that the 1st-is one of the completest Paradoxes you ever met with.

1st. "The more a Glass Magnifies, the more it Diminishes;” — i. e. if you have two Concaves, or Convexes, and wish to know which magnifies most - hold one in each hand, about one foot from your Eye, and about five feet from a window frame the Lens through which the panes of Glass appear least,- magnifies most: - this is the readiest way of ascertaining the comparative power of various Lenses.

2nd. The further the Eye-glass requires to be removed from the Object-glass, and the more the Inner tube must be drawn out the more the Eye-glass magnifies. If an Opera Glass magnifies 3, and the Eye-glass is changed for a concave which is a little deeper to make it magnify 4 times, to obtain distinct vision the tube will require to be drawn out further.

The Double Object-glass before-mentioned is an Inch and a Half in Diameter; its length, when in use, when it magnifies 4 times, is about 4 Inches; and with its tubes weighs

some

3 and Ounces; a larger Glass is cumberto carry, an unsightly machine to use; and the additional Illuminating and Magnifying power gained by a larger aperture and longer focus-are in a Theatre, of very little, indeed I think of no use.

There is no need of a Magnifying power of more than 4 times—nor of a pencil of rays of more than three tenths and three quarters in diameter, which is given by an Object-glass of an inch and a half in diameter.

That the Field of View is considerably larger through an Object-glass of Two inches in diameter, if the Magnifying power be as much as 4 times, is a Vulgar Error.

The increase of the field of view in the larger Object-glasses is comparatively very trifling, and much more than counterbalanced by their unwieldy weight and length. Moreover, the difficulty of making Object-glasses, as their diameter is increased, increases in so high a ratio, that those of Two inches, very rarely define Objects so perfectly and sharply, as those of an Inch and a Half in diameter.

In every department of Art, the acme of

perfection is always partly accidental, and is not to be attained with undeviating certainty by any Rules; and as there are 100 of 1 made for 1 of 2 inches aperture—it is in the like proportion, more easy to select a Fine One.

Imperfections in the Object-glasses of Opera Glasses, like those of Telescopes, are magnified and become evident, as the Magnifying power of the Eye-glass is increased.

Defects in an Object-glass which, when it magnifies only Twice, are almost imperceptible when it Magnifies Four or Five times, become too glaring to pass muster before a good Eyethe Vision, (especially the margin of the field of view,) becoming less Sharp, and the edges of the Objects being fringed with Colour.

Opticians charge £2. 12s. 6d. for the Best Achromatic Opera, in a plain mounting, with an Object-glass of 1 inch in diameter and which magnifies 4 times - £2. 2s. for the Common Achromatics, which magnify 21and £1. 1s. for those of like power with Single Object-glasses.

I must here caution my Reader, that the real, or the relative powers, of various Opera

Glasses, can only be accurately appreciated by actual trial in the Theatre, in which they are to be used-especially, comparisons of Achromatic and Single Object-glasses; and unless particular care be taken that they are glassed with precisely the same kind of Glass-with Concaves which give precisely the same Magnifying power and the Glasses are directed to the same Object, at, as nearly as can be, the same time; and the Ocular Glasses and Objectglasses are all perfectly clean- Comparison

will be in vain.

The difference of even 3 and 4 in the degree of Magnifying, will, with some objects, give quite a different character to an Opera Glass,even if the Object-glasses and Eye-glasses are equally good.

An inexperienced person, will say that the Opera Glass which magnifies 4 times, defines some objects more distinctly than that which magnifies only 34 - but that there is a greater degree of brightness about the latter, and that the vision in it appears clearer :-This, is thus to be accounted for, the lower the Power, the clearer and brighter objects appear-and up'

to a certain maximum, (which I think for use in a Theatre is about 4 times,) the higher the Power the better minute objects will be defined, and the sharper and more distinct the Vision will appear.

The Achromatic Object-glass above-mentioned, is composed of a Plano Concave lens, and a double Convex, which, combined, are generally (in every part) of the thickness of about ths of an Inch.

The thickness of the usual Plano Convex Single Object-glass of the like focus, in the central and thickest part of it is seldom more than ths of an Inch, and in the thinner parts, not half that: however, such is the

* Sometimes a Film or Fog forms between the Objectglasses, or, as the Optical phrase is, "the Glasses sweat:'

--

when this happens, they must be taken out of their cell and wiped with a bit of soft Leather or of very fine Silver Paper - but never do this but when it is absolutely needful - and then, take care to replace them in the same position; it is seldom requisite oftener than once or twice in a Year. Nor wipe the Object or Eye-glass except they really require it - as often as you wipe them - you scratch them a little.