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ent magnifying powers, are so fixed as to screw into a small movable tube in the lower end of the instrument, so as to adjust them respectively, to the focus, and to the eyes of different observers. Such telescopes usually represent objects in an inverted position.
The adjoining cut represents the simplest form of a mounted refractor. The object-glass is at A, where the brass cap may be seen covering it. B is the small tube into which the eye-piece is screwed, and which is moved in and out by the small screw C. Two eye-pieces may be seen at D-one short one, for astronomical observations, and a long one, for land objects. For viewing the Sun, it is necessary to add a screen, made of colored glass. At E, a bolt goes into a socket in the top of the stand, in which it turns, allowing the tele
scope to sweep around the horizon; while the joint, connecting the saddle in which the telescope rests with the top of the bolt, allows it to be directed to any point between the horizon and the zenith. But such stands answer only for comparatively small instruments.
692. Refracting telescopes are mounted in various ways. So important is it that they should not shake or vibrate, that, in most observatories, the stand rests upon heavy mason-work in no way connected with the building, so that neither the wind nor the tread of the observer can shake it. They are then furnished with a double axis, which allows of motion up and down, or east and west; and two graduated circles show the precise amount of declination and right ascension.
They are often furnished with clockwork, by which the telescope is made to move westward just as fast as the Earth turns eastward; so that the celestial object being once found, by setting the instrument for its right ascension and declination, or by the aid of the Finder-a small telescope attached to the lower end of the large one-it may be kept in view by the clockwork for any desirable length of time. A telescope thus furnished with right ascension and declination circles is called an Equatorial, or is said to be equatorially mounted, because it sweeps east and west in the heavens parallel to the
693. The object-glasses of telescopes are not always made of a single piece of glass. They may be made of two concavo-convex glasses, like two watch crystals, with their concave sides
692. How refractors mounted, and why? When equatorial, and why? 698. How object-glasses made? What a lens? A Barlow lens?
towards each other, or with a thin double concave glass between them. They are thus double, or triple; but when thus constructed, the whole is called a lens, as if composed of a single piece.
Leuses have also been formed by putting two concavo-convex glasses together, and filling the space between them with some transparent fluid. These are called Barlow lenses, from Prof. Barlow, their inventor.
694. As a prism analyzes the light, and exhibits different colors, so a double convex lens may analyze the light that falls near its circumference, and thus represent the outside of the heavenly bodies as colored. But this defect is remedied by using discs made of different kinds of glass, so as to correct one refraction by another. Refracting telescopes thus corrected are called Achromatic telescopes.
Achromatic is from the Greek a chroma, which signifies destitute of color. Most refracting telescopes are now so constructed as to be achromatic.
695. It is but recently that any good refracting telescopes have been made in this country. The best have formerly been made in Germany and France; but they are now manufactured with success, and to considerable extent, by Mr. Henry Fitz, Jun., New York City. The glass used by him is obtained from Paris, because none suitable for large telescopes has yet been made in America. His telescopes are perfectly achromatic, and are sold much cheaper than imported ones of the same size and value.
Mr. Fitz has recently made a very valuable improvement in the mounting of telescopes -one which is not only much superior to the old method, but which costs only about one-half as much. This improvement consists in using a single piece of cast-iron in the place of several pieces of brass work. It is very simple, secures great steadiness to the instrument, and is easily adjusted.
The writer is fully satisfied of the value of this improvement, and would recommend it, as well as Mr. Fitz's instruments, to all institutions and amateur astronomers about to purchase either. Besides patronizing a worthy American optician, they will get as good a telescope and much better mounting than by sending abroad, and at far less expense The following is a list of telescopes manufactured by Mr. Fitz, with the prices attached : PRICES OF FITZ'S TELESCOPES, EQUATORIALLY MOUNTED, ETC.
694. What is an Achromatic telescope? Derivation of term? formerly made? Where and by whom now, in this country? tained? What said of Mr. Fitz's telescopes?
695. Where telescopes
Where is the glass ob
He will furnish a very good telescope of three inches aperture for $120, equatorially mounted, with eye-pieces, &c. The size priced at $225, is equal to that at Yale College A good revolving dome for an observatory building can be built for $100.
This note is inserted exclusively for the benefit of institutions using the work, and without any request or remuneration from Mr. Fitz. Orders or letters of inquiry may be addressed to Henry Fitz, Jun., 237 Third-street, New York.
696. The adjoining cut represents an equatorial telescope manufactured by Mr. Henry Fitz of New York-the one used by the author in making most of his observations. Its object-glass is six inches in diameter, and its focal length eight feet. It is perfectly achromatic, and performs all the tests laid down in Dick's Practical Astronomer, as evidence of a good instrument, with perfect ease. Under favorable circumstances, it shows the sixth star in the trapezium of Orion, and to show Polaris double is a very easy test indeed.
A is the declination circle, and B the circle of right ascension. The two sticks hanging from these circles are used to move the instrument in right ascension or declination, while the observer is at the eye-piece.
The Finder is seen attached to the lower end of the large instrument. It takes in a larger field of view in the heavens than the latter, and enables the observer to look up objects with facility, and bring them into the field of the larger instrument. This instrument has no clockwork attached. It rests upon a pillar of heavy masonwork, the top of which may be seen in the cut; and in the hands of its present owner, Lewis M. Rutherford, Esq., has already rendered very efficient service.
697. The adjoining cut represents one of the largest telescopes in the United States. It is located in the observatory on Mount Adams, near Cincinnati, Ohio, and was for a time under the direction of Prof. Mitchel, by whose instrumentality it was purchased and mounted.
The object-glass is about 12 inches in diameter, with a focal distance of 17 feet. It was purchased in Munich, Germany, in 1844, at an expense of nearly ten thousand dollars. There is but one
larger than this in the United States, and but two larger in the world.
A FITZ REFRACTOR.
REFRACTING TELESCOPE AT CINCIN-
696. Mr. Rutherford's Telescope? By whom made? where located? By whom purchased? Where? When? tance? Comparative size?
667. Cincinnati refractorCost? Size and focal dis
THE GREAT CRAIG TELESCOPE, WANDSWORTH COMMON, NEAR LONDON.
698. This is the largest refracting telescope ever constructed. The object-glass is two feet in diameter, with a focal distance of 76 feet. The tube is of heavy sheet iron, and shaped somewhat like a cigar. It is 13 feet in circumference in the largest place, and weighs about three tons.
This telescope is suspended from a brick tower, 65 feet high, 15 feet in diameter, and weighing 220 tons. The top of the tower, from which the telescope is suspended, revolves; and by a chain running over pulleys, and a weight and windlass, it is balanced, and raised or lowered. The lower end rests on a small carriage, that runs upon a circu lar railroad around the tower, at the distance of 52 feet from its center. By these means it is directed to almost any point in the heavens. It is called the "Craig" telescope, in honor of the Rev. Mr. Craig, under whose direction, and at whose expense, it was constructed. It is located at Wandsworth Common, near London.
Focal distance? Tube? How
698. Describe the Craig telescope. Object glass? mounted? Why called "Craig" telescope? Where located?
699. Besides this monster refractor, there are several other very fine instruments in Europe; as the Dorpat telescope, Sir James South's, the Northumberland refractor, the Oxford telescope, &c. Several colleges and seminaries in the United States have observatories connected with them, and telescopes of greater or less value. The largest is at Cambridge, near Boston.
700. Quite a number of very respectable private observatories are also in operation in different parts of the country. The following table includes most of them: