"The PANCRATIC Eye-tube: Invented by WM. KITCHINER, M.D."

This remark is necessary, because certain persons have found it convenient to construct Counterfeits* made with only ONE Tube - which have not half the variety of powers the Pancratic has, and consequently, magnify not Half so low-nor Half so high as they ought.

* To construct this Eye-tube perfectly, requires All care and excellent workmanship-the Lenses must be All of exactly the right focus All without any blemish-and the Glasses and the Tubes containing them must be very truly centred with regard to each other, and to the Object-Glass.

CHAPTER XIV.

MAGNIFYING POWERS FOR ASTRONOMY.

You should not have less than Nine Eye-tubes for Celestial purposes, or your Economy will be as unwise as that of the Cook who "spoilt the Broth for want of a Halfpennyworth of Salt"-each Eye must have the Telescope adjusted to its own peculiar focus, Each Object must have its own peculiar magnifying power.

These Eye-tubes cost about £1. 1s. each.

15 for Comets.

45 for the Sun, or Moon, Nebula, &c.

60 for Ditto.

80 for Jupiter and his Moons.

130 for Jupiter and Saturn.

160 for Ditto.

200 for Ditto, and Double Stars. 300 for Double Stars.

400 for Ditto.

For more particular directions for the application of these Eye-pieces, see p. 47 of Chapter IV. on a 31⁄2 and a 5 feet Achromatic — and page 140 of Chapter VIII. on a 7 feet Newtonian-and the following Chapter.

When two or more Glasses are fixed into a tube,

at certain distances from each other, they are called An Eye-piece; and whether intended for erect or inverted vision, they are in fact compound Microscopes, whereby the image of an object, formed in the focus of the Object-Glass, is seen and magnified.

The Glasses must be perfectly well polished, and as thin as possible for their curvature; which, together with their distance from each other, must be so proportioned, that the visual angle, or field of view of the Telescope, be as large as possible, and uniformly distinct to the very edge of it: and it is absolutely necessary in the setting of them, that the centre of the Glasses be placed exactly in the axis of the Tube, and the surface of them fitted into the cells exactly parallel to each other.

The Astronomical Eye-pieces for Achromatic and Newtonian Telescopes, are usually of the Huygenian construction, and are composed of two plano-convex Glasses, whose plane sides are next to the Eye, and whose foci are as 1 to 3. If a plano-convex lens of one inch focus, be placed at two inches distance from one of three inches focus, their magnifying power will be equal to a single lens of one inch and a half focus: the lens next to the Eye is called the Eyeglass; that next to the Object is termed the Fieldglass.

The spherical and colorific aberrations are said to be better corrected by this combination; and it has

a larger and more uniformly distinct field of view than any other Eye-tube composed of two Glasses.See the end of Chapter V. on Single Concaves and Convexes.

If the two Lenses which compose an Huygenian Eye-piece, are of proper proportions-if you take away the 1st glass next to the Eye-the 2d glass alone will magnify only about half as much as the Two do when used together:-if you take away the 2d glass, and use the Glass next to the Eye by itself, it will magnify about 3d more than the two lenses do when used together-thus supposing them together to magnify 100 times-the 2d glass used alone will magnify 50-and the 1st glass used alone will magnify 150:—thus in each Huygenian Eye-tube, you have in fact three Magnifying Powers. See the account of the Eye-pieces to the 5 feet Achromatic, in p. 47 of Chapter IV.

1st. I recommend that each Eye-tube be attached to a lengthening piece, which, when fixed to the Telescope, will, when it is adjusted to one power, be very nearly adjusted to all-this will be found a great convenience, especially in applying extremely High and extremely Low powers, and save a great deal of work to the observer, and a great deal of wear to the Adjusting screw.

2d. It is much more convenient to have Eyepieces made to slide into the Tube, than to screw

in-they are more readily changed, especially in cold dark nights when our fingers are benumbed with cold.

3d. The Caps of Eye-pieces, instead of screwing on, should slip on like the covers of ObjectGlasses, and the Dark Glass of the Comet and Moon Eye-tube, should be of a very light Green colour-which will be found extremely useful in observing the planet Venus.

4th. The Second, or Field glass, requires a cap quite as much as the First or Eye-glass doesindeed the 2d is generally much more exposed than the 1st glass is—both ought to be shielded from dust and air, and kept as clean as possible-for as often as they are wiped, they are in danger of being scratched, &c.

5th. The Cells containing the Eye-glasses should be so contrived, that the glasses may be perfectly comeatable to be cleaned, especially the high powers, which I have seen set in such a deep cell, that it was impossible to wipe them properly-but the Rim of the cell of Eye-Glasses and of Object-Glasses should project so far beyond them, that they may be laid down without their surfaces touching any thing.

6th. The Eye-glasses should be so carefully proportioned to each other, and so placed that the Eye may view the whole of the field at once, without the Eye touching the Tube or moving before it. The

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