APPENDIX. FROM THE "MECHANISM OF THE HEAVENS," BY DENISON OLMSTED, LL.D., PROFESSOR OF NATURAL HISTORY AND ASTRONOMY AT YALE COLLEGE. THE TELESCOPE. HE Telescope, as its name implies, is an instrument employed for viewing distant objects.* It aids the eye in two ways; first, by enlarging the visual angle under which objects are seen, and, secondly, by collecting and conveying to the eye a much larger amount of the light that emanates from the object, than would enter the naked pupil. A complete knowledge of the telescope cannot be acquired without an acquaintance with the science of optics; but one unacquainted with that science may obtain some idea of the leading principles of this noble instrument. Its main principle is as follows: By means of the telescope, we first form an image of a distant object, as the moon for example, and then magnify that image by a microscope. The invention of this noble instrument is generally ascribed to the great philosopher of Florence, Galileo. He had heard that a spectacle-maker of Holland had accidentally hit upon a dis * From two Greek words, Tλ, (tele,) far, and oxozew (skopeo,) to see. covery, by which distant objects might be brought apparently nearer; and, without further information, he pursued the inquiry, in order to ascertain what forms and combinations of glasses would produce such a result. By a very philosophical process of reasoning, he was led to the discovery of that peculiar form of the telescope which bears his name. Although the telescopes made by Galileo were no larger than a common glass of the kind now used on board of ships, yet, as they gave new views of the heavenly bodies, revealing the mountains and valleys of the moon, the satellites of Jupiter, and multitudes of stars which are invisible to the naked eye, the discovery was regarded with infinite delight and astonishment. Reflecting telescopes were first constructed by Sir Isaac Newton, although the use of a concave reflector, instead of an object-glass, to form the image, had been previously suggested by Gregory, an eminent Scottish astronomer, whose name is still employed to designate the Gregorian telescope. The first telescope made by Newton was only six inches long, and its reflector was little more than an inch in diameter. Notwithstanding its small dimensions, it performed so well, as to encourage further efforts; and this illustrious philosopher afterwards constructed much larger instruments, one of which, made with his own hands, was presented to the Royal Society of London, and is now carefully preserved in the library of the Society. Newton was induced to undertake the construction of reflecting telescopes, from the belief that refracting telescopes were necessarily limited to a very small size, with only moderate illuminating powers; whereas the dimensions and powers of the former admitted of being indefinitely increased. Considerable magnifying powers might, indeed, be obtained from refractors, by making them very long; but the brightness with which telescopic objects are seen, depends greatly on the dimensions of the beam of light which is collected by the object-glass, or by the mirror, and conveyed to the eye; and therefore small objectglasses cannot have a very high illuminating power. The experiments of Newton on colours led him to believe, that it would be impossible to employ large lenses in the construction of telescopes, since such glasses would give to the images they formed the colours of the rainbow. But later opticians have found means of correcting these imperfections, so that we are now able to use object-glasses a foot or more in diameter, which give very clear and bright images. Such instruments are called achromatic telescopes, a name implying the absence of prismatic or rainbow colours in the image. It is, however, far more difficult to construct large achromatic than large reflecting telescopes. Very large pieces of glass can seldom be found that are sufficiently pure for the purpose; since every inequality in the glass, such as waves, tears, threads, and the like, spoils it for optical purposes, as they distort the light, and produce confused images. The achromatic telescope (that is, the refracting telescope, having such an object-glass as to give a colourless image) was invented by Dollond, a distinguished London artist, about the year 1757. He had in his possession a quantity of glass of a remarkably fine quality, which enabled him to carry his invention at once to a high degree of perfection. It has ever since been a matter of the greatest difficulty, with the manufacturers of telescopes, to find pieces of glass, of a suitable quality for object-glasses, more than two or three inches in diameter. Hence, large achromatic telescopes are very expensive, being valued in proportion to the cubes of their diameters; that is, if a telescope whose aperture (as the breadth of the object-glass is technically called) is two inches, cost twenty-four pounds, one whose aperture is eight inches would cost one thousand five hundred and twenty pounds. Since it is so much easier to make large reflecting than large refracting telescopes, it may be asked, why the latter are ever attempted, and why reflectors are not exclusively employed? I answer, that the achromatic telescope, when large and well constructed, is a more perfect and more durable instrument than the reflecting telescope. Much more of the light that falls on the mirror is absorbed than is lost in passing through the objectglass of a refractor; and hence the larger achromatic telescopes afford a stronger light than the reflecting, unless the latter are made of an enormous and unwieldy size. Moreover, the mirror is very liable to tarnish, and will never retain its full lustre for many years together; and it is no easy matter to restore the lustre when once impaired. The three most celebrated telescopes hitherto made, are Herschel's forty-feet reflector, the great Dorpat refractor, and the still more remarkable telescope recently completed by Lord Rosse. Herschel was a Hanoverian by birth, but settled in England in the younger part of his life. As early as 1774, he began to make telescopes for his own use; and, during his life, he made more than four hundred, of various sizes and powers. Under the patronage of George III., Le completed, in 1789, his great telescope, having a tube of iron forty feet long, and a speculum forty-nine and a half inches, or more than four feet in diameter. Let us endeavour to form a just conception of this gigantic instrument, which we can do only by dwelling on its dimensions, and comparing them with those of other objects with which we are familiar, as the length or height of a house, and the breadth of a hogshead. The reflector alone weighed nearly a ton. So large and ponderous an instrument must require a vast deal of machinery to work it, and to keep it steady; and accordingly the frame-work surrounding it was formed of heavy timbers, and resembled the frame of a large building. When one of the largest of the fixed stars, as Sirius, is entering the field of this telescope, its approach is announced by a bright dawn, like that which precedes the rising sun; and when the star itself enters the field, the light is insupportable to the naked eye. The planets are expanded into brilliant luminaries, like the moon; and innumerable multitudes of stars are scattered like glittering dust over the celestial vault. Its The great Dorpat telescope is of more recent construction. It was made by Fraunhofer, a German optician, of the greatest eminence, at Munich, in Bavaria, and takes its name from being attached to the observatory at Dorpat, in Russia. It is of much smaller dimensions than the great telescope of Herschel. object-glass is nine and a half inches in diameter, and its length fourteen feet. Although the price of this instrument was nearly one thousand two hundred pounds sterling, yet it is said that this sum barely covered the actual expenses. It weighs five thousand pounds, and yet is turned with the finger. In facility of management it has greatly the advantage of Herschel's telescope. Moreover, the sky of England is so frequently unfavourable for astronomical observation, that one hundred good hours (or those in which the higher powers can be used) are all that can be obtained in a whole year. On this account, as well as from the difficulty of shifting the position of the instrument, Herschel estimated that it would take about six hundred years to obtain with it even a momentary glimpse of every part of the heavens. This remark shows that such great telescopes are unsuited to the common purposes of astronomical observation. Indeed, most of Herschel's discoveries were made with his small telescopes; and although, for certain rare purposes, powers were applied which magnified seven thousand times, yet, in most of his observations, powers magnifying only two or three hundred times were sufficient. The highest power of the Dorpat telescope is only seven hundred, and yet the director of this instrument, Professor Struve, is of opinion that it is nearly or quite equal in quality, all things considered, to Herschel's forty-feet reflector.* *The largest and most remarkable telescope ever constructed, is that which science owes to the enterprise, public spirit, and enlightened love of knowledge which distinguishes the Earl of Rosse. This noble instrument, which is erected at Parsonstown, in Ireland, is a reflecting telescope. "The great speculum is 6 feet diameter 5 inches thick at the edges, and 5 inches at the centre, and its weight is about 3 tons. Its composition is copper and tin; 126 parts of copper to 57 of tin. The price of the copper alone was reckoned at about £100. By grinding and polishing, its thickness was reduced about one eighth of an inch. Its focal distance is about 54 feet. The casting of the speculum took place in April, 1842, which, with all th multifarious operations connected with it, was accomplished without any accident, and with a degree of success beyond expectation. The speculum has a reflecting surface of 4071 square inches, while that of Sir W. Herschel's 40 feet telescope had only 1811 square inches on its polished surface; so that the quantity of light reflected from this speculum is considerably more than double that of Herschel's largest reflector, and it is chiefly on the quantity of light either transmitted or reflected, that the power of telescopes to penetrate into space depends. The process of grinding this speculum was conducted under water, and the moving power employed was a steamengine of three horse power. The substance made use of to wear down the surface was emery and water, a constant supply of these was kept between the grinder and the speculum. It required six weeks to grind it to a fair surface. "The tube of this telescope is 56 feet long, including the speculum box, and is made of deal, 1 inch thick, hooped with iron. On the inside, at intervals of 8 feet, there are rings of iron, 3 inches in depth, and 1 inch broad, for the purpose of strengthening the sides. The diameter of the tube is 7 feet. It is fixed to mason-work in the ground, to a large universal hinge; which allows it to turn in all directions. At 12 feet distance on each side, a wall is built, 72 feet long, 48 high on the outer side, and 56 on the inner, the walls being 24 feet distance from each other, and lying exactly in the meridional line. When directed to the south, the tube may be lowered till it become almost horizontal; but when pointed to the north, it only falls till it is parallel to the earth's axis, pointing then to the pole of the heavens. Its lateral movements take place only from wall to wall: and this commands a view for half-an-hour on each side of the meridian; that is, the whole of its motion from east to west is limited to 15 degrees. It is ultimately intended to connect with the tube-end galleries, machinery which shall give an automaton movement, so that the telescope shall be used as an equatorial instrument. The tube and speculum, including the bed on which the speculum rests, weigh about 15 tons. "The telescope rests on a universal joint, placed on masonry about 6 feet below the ground, and it is elevated and depressed by a chain and windlass; and though it weighs 15 tons, the instrument is raised by two men with great facility. Of course it is counterpoised in every direction. The observer, when at work, stands in one of four galleries, the three highest of which are drawn out from the western wall, while the fourth, or lowest, has for its base an elevating platform, along the horizontal surface of which a gallery slides from wall to wall, by machinery within the observer's reach, but which a child may work. When the telescope is directed to an object near the zenith, the observer stands at an elevation at least 50 feet above the ground. The |