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considered the best, as it has a much larger field of view, and a greater quantity of light-their prices, differing according to their size. There is also another Night-glass, called the erect Achromatic, which differs from the other in as much, that it shews the object in its proper position and free from colour, which renders it useful in the day-time; the field of view and magnifying power being the same as the Second size before described, this has been much approved by those sailors who have not the art of using the inverting Night-glasses.-Night-glasses have also been used by Astronomers for the purpose of discovering any novelties among the different arrangements of the Celestial bodies, and by those whose sight will not permit them to distinguish the arrangement of the stars without optical assistance. Nightglasses must be adjusted and marked for different distances in the Day-time; it is very difficult, indeed it is almost impossible, to get the exact focus, unless on a bright Moonlight night.
THE THREE AND A HALF FEET AND THE FIVE
FEET ACHROMATIC TELESCOPES.
THE smallest Achromatic that can be used for Astronomical purposes, without overmuch teasing of your Eye, is the 3 feet, which are now usually made with Two object-glasses of 27 inches in diameter. With this Telescope many of the principal celestial phenomena may be perceived: for I would make a distinction between " Seeing" and " Perceiving"-I use the former term when an object is Seen plainly, and the latter when it is only scarcely Perceivable.
Some objects which are easily and perfectly visible in a 5 feet Achromatic, with an aperture of 3%, are hardly perceivable with a 3 feet, which has an aperture of only 27-the Illuminating power of the former exceeding that of the latter as 144 does 72.See the fifth column of the Table of Achromatics, at page 24, and the Chapter on Illuminating Power: — others, which do not require much Illuminating power, may be seen as well with the latter as with the larger Telescope.
THE THREE AND A HALF FEET.
The usual Day Eye-tube of this Telescope gives it
a Magnifying power of about 45 times-with this low power the field of view is large, and the vision is very brilliant and from the vivid impression which objects make upon the Eye, people say that it is very clear, and those who are unacquainted with the relative effects of various magnifiers, are in general better pleased with such a low power than with a higher power-the advantage of which is not at first so striking-until applied to the attentive examination of some minute object, when the advantage of a due degree of Magnifying Power becomes evident→→ and the Day Eye-tube usually put to a 2 feet Achromatic, which makes a 3 feet magnify about 60 times, will be found far superior,
A Pancratic Eye-tube is made for this Telescope, which produces in a very perfect manner all the intermediate degrees of Magnifying power between 60 and 240, for either Terrestrial or Celestial purposes. See the Chapter on the Pancratic Eye-tube.
The Magnifiers usually put to this Instrument for Astronomy, are 80, 130, and 180. To these I would recommend the addition of
A Comet Eye-piece, magnifying about 15 times,
A Compound Eye-tube of 250, for Double Stars, &e. and a Polycratic Wheel containing 6 Convex lenses, magnifying 80, 130, 180, 250, 350, 440.-These particular powers are recommended to give an opportunity of comparing the Single lenses with the Eye-tubes
which contain 2 glasses. Those who are anxious to see the effect of higher Magnifiers, may have another Circle, magnifying 5, 6, 7, 8, 900, and 1000 timesbut there is No Object that will be seen so well with them as with the power of 250 or at most 350.
OBS. If you wish to have an Object-glass that will bear a high power-ask no questions about the Price of any Instrument which you are at all anxious about the quality of,-consider yourself fortunate if you obtain it. This Advice applies still stronger to the 5 feet Telescope.
I would hope that the preceding observation is quite superfluous, had I not, in the course of my residence of nearly half a century on this Planet, seen so many instances of People being so extravagantly fond of Bargains-and did I not still remember the anecdote of the "Humorous Hosier," who caught crowds of Customers, by placing in his window the following placard—
"The Cheapest and Best Stockings sold here.”
This brought flocks of Bargain-hunters to his Counter-"Let us see some of your Cheapest and Best Stockings?"
Hos. "There is a pair of each, these, are a Shilling those are a Guinea a pair."
The Writer respectfully assures the Reader, that the Quality of Telescopes varies in quite as many degrees as that of Stockings.
Read the Chapter on Magnifying powers and Eye
pieces. For Tests of the goodness of Achromatic Telescopes-see Oвs. in the Chapter on Magnify
the Chapter on "Double Stars."
In the Table at the End of the Chapter on Illuminating Power, the Reader will find that a Reflector to be as light as the 3 Achromatic above described, must have an Aperture of 4 inches in diameter.
MR. DOLLON D'S DESCRIPTION OF THE FIVE FEET AND THE THREE AND A HALF FEET ACHROMATIC REFRACTING TELESCOPE, MADE BY HIM.
"The Telescope is supported in the centre of gravity, with its rack-work motions, and mounted on its mahogany stand, the three legs of which are made to close up together by means of a brass frame, which is composed of three bars, connected together in the centre by three joints, and also to the three legs of the mahogany stand by three other joints, so that the three bars of this frame lie close against the insides of the legs of the mahogany stand when they are pressed together, for convenience of carriage.
The brass pin, under the rack-work, is made to move round in the brass socket, and may be tightened by means of a finger-screw, when the Telescope is directed nearly to the object intended to be observed. This socket turns on two centres, by which means it may be set perpendicular to the