Eye-it will perform as well, as if it was as big as the Dome of St. Paul's.

The Opening of the Pupil of the Eye is in inverse proportion to the Brightness of the Objects presented to it as the latter increase in Brightness, the former diminishes in Diameter, therefore the less the magnifying power, the less the Pupil of the Eyewhich is always in an inverse proportion to the bigness and brightness of the pencil of Rays from a Telescope.

The ordinary opening of the Pupil, when the Eye is turned to the Light, has been computed to very little exceed th of an Inch in Diameter; See Figure 3 in the Engraving, fronting the Title, and No. 10 of the AP


Under the idea, that the opening of the Pupil when before an Opera Glass, is of the like dimension, it has been assumed, that-no matter what be the Magnifying power, or what the Diameter of the Object Glass of an Opera Glass, an Eye-glass ofths of an Inch in Diameter, would be even larger than is requisite.

My Eye, had for some time suspected the Truth of this Theory:- wishing to avail myself of the amusement of a change of Magnifying power, and to have an opportunity of illustrating its effects to others, I had a Revolving Eye head made like the double Head of the French Opera Glasses, invented by Mr. CAUCHOIX, with two Glasses-one magnifying

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* I have had Three very good Achromatic Opera Glasses made for me by this Optician, of an Inch and a half, and One of Two Inches aperture; but the latter size is heavy, and inconveniently, and I think, uselessly large, its Double Object Glass weighs 5 ounces, and the Instrument altogether 9 ounces, and cost £5 in Paris.-See an account of Mr. CAUCHOIX'S Opera Glasses, in page 374 of the Edinburgh Review, for October, 1819.

MEM. I do not quote this, because I am of the same opinion as the erudite Editor of the paper referred to my own opinion of Opera Glasses, is impartially stated, at the termination of this note.

LEMIERE, No. 6, Palais Royal, has introduced an Opera Glass with a Screw adjustment, similar to what our One foot Achromatic Telescopes in a Brass Tube on a Stand have but the adjustment is so fine, that it is more difficult to find the exact focus with it-than it is by moving the Tube, provided that be kept clean and slides smoothly: his charge for an Opera Glass with this adjustment and an Achromatic Object Glass of an Inch and a half in diameter, magnifying 21, is £2.4s.

3, the other 4; the Eye-glasses were ths of an Inch in diameter-and I was surprised to find, that on trying an Eye-glass of half an Inch in diameter, Vision was considerably brighter and easier to my Eye.

The Magnifying Power of an Opera Glass may be varied several different ways:

1st, By having a Concave fixed in the Eyehead which magnifies 23, for viewing Pictures, &c. and another to screw on over that, which will increase the power to 4, for Theatrical purposes;


The machinery for adjusting the focus, may be as much too fine-as too coarse- The fine Screw adjustment, which still seems the best that can be applied to Gregorian Reflectors, was applied to the original 46 Inch Achro- but when even a power of 150 is applied to



it is not quick enough, and the focal point is not half so easily and exactly hit, as with the more modern invention of the Tooth and Pinion adjustment on the side of the Tube.

I do not mean to insinuate, that I think the productions of our English Opticians are not equal to those of the Parisian Artists—I have had several Dozens of Opera Glasses, made by MR. PIERCE and MR. DOLLOND, which are as elegantly formed and finished-and the Optical parts, are as perfect as Art can produce: - For a Description thereof, see pp. 168, 169, 172, &c.

2dly, By having a Concave fixed in the Eychead which magnifies 4- and a Convex to screw on over that, which will reduce the power to 24. I like this last plan best, because most light is required for theatrical purposes. The additional Glass may be attached to the Eyehead by a hinge on the side; and when only one of them is wanted, the other may be turned up. The opening of the Stop in the Eyehead must be regulated to suit the deepest power:- or when that is used, a smaller Stop must be brought before the larger one, in the manner in which the Sun Glass is brought before the Eye Glasses in the 4 Eye Glass Perspectives.

I am induced to offer it as an invariable maxim,—that although there may be no use in the Diameter of the Eye-glass being much larger, yet, it should be somewhat larger than the Pencil of Rays transmitted by the Object-glass — which, when an Object-glass of 1 Inch Diameter magnifies 3 times, will, if not cut off by the Stop, be an Inch. (See next page.)

* To ascertain whether any of the Object-glass is cut off by the Stop in the Eye-tube-adjust the Opera Glass to distinct Vision then, take out the Eye-glass, put your

As I have before observed, it has been assumed that the opening of the pupil of the Eye, when before an Opera Glass, is about th of an inch in diameter; I believe it does not exceed ths, that the Image is brighter

with a pencil of three tenths and three quarters than it is with one of three tenths, my Eye assures me is evidently true but why the larger pencil of rays makes a stronger impression on the Eye, I will not pretend to offer any reason*-I know, it is contrary to the accepted Theory - however, it is True.


“There are more things in Heaven and Earth, Horatio, Than are dreamt of in your Philosophy."


The Diameter of the Eye-glass, for any Opera Glass, and any Magnifying power-should be rather more than what is given, by reducing the diameter of the Object-glass into Tenths of Inches, and dividing that, by the Magnifying power

Finger on the edge of the outside of the Object-glass, and look down the tube-if you can see your Finger just peeping over the edge of the Object-glass · none is cut off.

* See Sir Wm. Herschel's observations on Night Glasses, in page 68 or 69 of Vol. 90. of the Phil. Trans.


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