Page images
PDF
EPUB

the Copernican theory, which had just been promulgated, and c which Galileo was an ardent disciple. Like Copernicus, how ever, his doctrines subjected him to severe persecutions, and he was obliged to renounce them.

The following is his renunciation, made June 28, 1683: Galileo, in the seventieth year of my age, on bended knees before your eminences, having before my eyes and touching with my hands the Holy Gospels, I curse and detest the error of the Earth's movement." As he left the court, however, after this forced renunciation, he is said to have stamped upon the Earth, and exclaimed, "It does move, after all?" Ten years after this, he was sent to prison for the same supposed error; and soon, his age advancjag, the grave received him from the malice of his persecutors.

DIFFERENT KINDS OF TELESCOPES.

687. Telescopes are of two kinds-Reflectors and Refractors. Refracting telescopes are made by refracting the light to a focus with a glass lens (675); and reflecting telescopes, by reflecting it to a focus with a concave mirror (681). Besides this general division, there are various kinds both of reflectors and refractors.

Telescopes assist vision in various ways-first, by enlarging the visual angle under which a distant object is seen, and thus magnifying that object; and, secondly, by converging to a point more light than could otherwise enter the eye--thus rendering objects distinct or visible that would otherwise be indistinct or invisible.

All the light falling upon a six or a twelve inch lens may be converged to a focus, so as to be taken into the human eye through the pupil, which is but one-fourth of an inch in diameter. Our vision is thus made as perfect by art as if nature had given us ability to enlarge the eye till the pupil was a foot in diameter.

688. Refracting telescopes may consist of a double-convex lens placed upon a stand, without tube or eye-piece. Indeed, a pair of ordinary spectacles is nothing less than a pair of small telescopes, for aiding impaired vision.

REFRACTING TELESCOPE WITH A SINGLE LENS.

A

B

Here the parallel rays are seen to pass through the lens at A, and to be so converged to a point as to enter the eye of the beholder at B. His eye is thus virtually enlarged te the size of the lens at A. But it would be very difficult to direct such a telescope toward cclestial objects, or to get the eye in the focus after it was thus directed.

apon Galileo? His renunciation? Death? 687. Kinds of telescopes? Describ How assist vision? Illustration. 688. Simplest form of refracting telescope?

689. The Galilean telescope consists of two glasses—a doub convex next the object, and a double-concave near the eye. The former converges the light till it can be received by a small double-concave, by which the convergency is corrected (502), and the rays rendered parallel again, though in so small a beam as to be capable of entering the eye.

JALILEAN TELESCOPE.

B

Here the light is converged by the lens A, till it can be received by the double-concave lens B, by which the rays are made to become a small parallel beam that can enter the eye at C. This was the form of the telescope constructed by Jansen, and improved by Galileo; on which account it is called the Galilean telescope. In the cut, the two lenses are represented as fastened to a board, as first exhibited by Jansen.

690. The common astronomical telescope consists of two glasses-viz., a large double-convex lens next the object, called the object-glass; and a small double-convex lens or microscope next the eye, called the eye-piece. For the greater convenience in using, they are both placed in a tube of wood or metal, and mounted in various ways, according to their size, and the pur. poses to which they are devoted.

LENSES PLACED IN A TUBE.

B

A is the object-glass, B the eye-piece, and C the place where the tube, in which the eye piece is set, slides in and out of the large tube, to adjust the eye-piece to the focal dis tance. By placing the lenses in a tube, the eye is easily placed in the focus, and the object-glass directed toward any desired object.

691. The object-glass of a telescope is usually protected, when not in use, by a brass cap that shuts over the end of the instrument; and the eye-pieces, of which there are several, of differ

689. Galilean telescope? Why called Galüean? 690. How common astronomica telescopes made? Why in tube? 691. How object-glass protected? What said f syo-piccou?

[blocks in formation]

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

[blocks in formation]

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 te move westward just as fast as the Earth turns eastward; so that the celestial object being unce 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 mator.

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? 696, 1157 shiect-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 Burlon 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 a number of very fine instraments have been made in this country, most of them by Mr. Henry Fitz, Jun., formerly of New York City. Several very good instruments have also been made by Alvan Clark, Esq., of Boston, and others still by Charles A. Spencer, Esq., of Troy, N. Y. Mr. Fitz died in New York, November 27, 1863.

1. The author was personally well acquainted with Mr. Fitz, and during his life gave favorable descriptions of his instruments in these pages, and did all that he could to make his capabilities known to the American public. He made his first telescope in 1835. In the Winter of 1844 he invented a method of perfecting object-glasses for refracting telescopes, making the first one of the bottom of an ordinary tumbler. In the Fall of 1845 he exhibited, at the Fair of the American Institute, an instrument of 6 inches aperture, which, although made of common American material, in the way of flint glass, was a very excellent instrument. Continually progressing in size, he finally succeeded in making instruments of 16 inches aperture, one of which is now in the possession of Mr. Van Duzee, of Buffalo, N. Y. He made two of 13 inches-one for the Dudley Observa tory, at Albany, and the other for an association of gentlemen, at Alleghany City, Pa. Of 12 inches aperture, he produced one for the Observatory at Ann Arbor, Michigan, and another for the Vassar Female College. He made for M. L. Rutherford, of New York. at various times, telescopes of 4, 5, 6, 9, and 11 inches aperture; the last, an instrument of remarkable defining power, is now mounted in Mr. Rutherford's Observatory, ir Eleventh Street, New York City. Mr. Vickers, of Baltimore, has a 10-inch. Several of the size of 8 and 9 inches are scattered over the country. The British Chargé d'Af faires at Montevideo has a 9-inch. Mr. Campbell, of New York, has an 8-inch. Of a large number of 6 inches aperture, one very fine instrument was ordered by the United States Government, for Lieut. Gillies's expedition to Chili; it is still in the Observatory of the Chilian Government. At about the same time, he made another of the same size for Mr. Robert Van Arsdale, of Newark, N. J. Mr. Thomas F. Harrison, Principal of the Public Grammar School in Greenwich Avenue, New York, has another mounted on that building. [Removed on the rebuilding of the school edifice in 1865.-Ed.]

2. For a list of telescopes in this country, with the names of their respective makers, focal lengths, size of object glasses, &c., see table on subsequent page.

695. What said of the manufacture of telescopes? What other Americans have made them (What said of Mr. Fitz? Telescopes?)

[graphic][merged small]

696. The above cut represents an equatorial telescope manu factured 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 Finder is seen attached to the lower end of the large instrument. It takes in s large field of view in the heavens than the latter, and enables the observer to look us obets with facility, and bring them into the field of the larger instrument

« PreviousContinue »