CHAPTER XIX.

TELESCOPES-REFRACTORS AND REFLECTORS.

683. A TELESCOPE is an optical instrument employed in view. ing distant objects, especially the heavenly bodies. The term telescope is derived from two Greek words, viz., tele, at a distance, and skopeo, to see. So far as is now known, the ancients had no knowledge of the telescope. Its invention, which occurred in 1609, is usually attributed to Galileo, a philosopher of Florence, in Italy.

The discovery of the principle upon which the refracting telescope is constructed was purely accidental. The children of one Jansen, a spectacle-maker of Middleburgh, in Holland, being at play in their father's shop, happened to place two glasses in such a manner, that in looking through them, at the weathercock of the church, it appeared to be nearer, and much larger than usual. This led their father to fix the glasses upon a board, that they might be ready for observation; and the news of the discovery was soon conveyed to the learned throughout Europe. Galileo hearing of the phenomenon, soor discovered the secret, and put the glasses in a tube, instead of on a board; and thus the first telescope was constructed.

684. The telescope of Galileo was but one inch in diameter, and magnified objects but 30 times. Yet with this simple. instrument he discovered the face of the Moon to be full of inequalities, like mountains and valleys; the spots on the Sun; the phases of Venus; the satellites of Jupiter; and thousands of new stars in all parts of the heavens.

Notwithstanding this propitious commencement, so slow was the progress of the telescope towards its present state, that in 1816, Bonnycastle speaks of the 30-fold magnifying power of the telescope of Galileo as "nearly the greatest perfection that this Kind of telescope is capable of!"

685. If he be the real author of an invention who, from a knowledge of the cause upon which it depends, deduces it from one principle to another, till he arrives at the end proposed, then the whole merit of the invention of the telescope belongs to Galileo. The telescope of Jansen was a rude instrument of mere curiosity, accidentally arranged; but Galileo was the first who constructed it upon principles of science, and showed the practical uses to which it might be applied.

It is said that the original telescope constructed by Galileo is still preserved in the British Museum. A pigmy, indeed, in its way, but the honored progenitor of a race of giants!

686. The discovery of the telescope tended greatly to sustain

688. Subject of Chapter XIX.? Telescope? Derivation? Ancient or modern? Inventor? Incidents of discovery? 684. Galileo's telescope? Discoveries with it? Progress in telescope making? 685. Is Galileo entitled to the honor of inventing the telescope? Where is bis? 686. Relation of discovery to Copernican theory? Effects

the Copernican theory, which had just been promulgated, and ot 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: "I, 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 advaucing, 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.

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 to the size of the lens at A. But it would be very difficult to direct such a telescope toward celestial objects, or to get the eye in the focus after it was thus directed.

upon Galileo? His renunciation? Death? 687. Kinds of telescopes? Deccrib Dow 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 us to be capable of entering the eye.

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 purposes to which they are devoted.

B

A is the object-glass, B the eye-piece, and C the place where the tube, in which the eyepiece is set, slides in and out of the large tube, to adjust the eye-piece to the focal distance. 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 of eye-picces?

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

equator.

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 coneave sides

692. How refractors mounted, and why? When equatorial, and why? 606. Ho abject-glasses made? What a leno? 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 Burlow 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 instruments 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 18 inches-one for the Dudley Observatory, 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, 51, 6, 9, and 11 inches aperture; the last, an instrument of remarkable defining power, is now mounted in Mr. Rutherford's Observatory, in 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.

2. For a list of telescopes in this country, with the names of their respective makers. focal length, 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?)