[Ill.u.s.tration: THE 100-INCH HOOKER TELESCOPE, LARGEST REFLECTOR IN THE WORLD, ON MT. WILSON. (_Photo, Mt. Wilson Solar Observatory._)]
[Ill.u.s.tration: THE LARGEST REFRACTOR, THE 40-INCH TELESCOPE AT YERKES OBSERVATORY. DOME 90 FT. IN DIAMETER. (_Photo, Yerkes Observatory._)]
[Ill.u.s.tration: THE 150-FT. TOWER AT THE MT. WILSON SOLAR OBSERVATORY. At the left is a diagram of tower, telescope and pit. At the upper right is an exterior view of the tower; below a view looking down into the pit, 75 ft. deep.
(_Photo, Mt. Wilson Solar Observatory._)]
The focal lengths of object gla.s.s and eyepiece will determine just what distance apart the lenses must be in order to give perfect vision. But it is quite as important that the axes of all the lenses be adjusted into one and the same straight line, and then held there rigidly and permanently. Otherwise vision with the telescope will be very imperfect and wholly unsatisfactory. The distance from the objective, or object gla.s.s to its focal point is called its focal length; and if we divide this by the focal length of the eyepiece, we shall have the magnifying power of the telescope. The eyepiece will usually be made of two lenses, or more, and we use its focal length considered as a single lens, in getting the magnifying power. A telescope will generally have many eyepieces of different focal lengths, so that it will have a corresponding range of magnifying powers. The lowest magnifying power will be not less than four or five diameters for each inch of aperture of the objective; otherwise the eye will fail to receive all the light which falls upon the gla.s.s. A 4-inch telescope will therefore have no eyepiece with a lower magnifying power than about 20 diameters. The highest magnifying power advantageous for a gla.s.s of this size will be about 250 to 300, the working rule being about 70 diameters to each inch of aperture, although the theoretical limit is regarded as 100.
The reason for a variety of eyepieces with different magnifying powers soon becomes apparent on using the telescope. Comets and nebulae call for very low powers, while double stars and the planetary surfaces require the higher powers, provided the state of the atmosphere at the moment will allow it. If there is much quivering and unsteadiness, nothing is gained by trying the higher powers, because all the waves of unsteadiness are magnified also in the same proportion, and sharpness of vision, or fine definition, or "good seeing," as it is called, becomes impossible. The vibrations and tremors of the atmosphere are the greatest of all obstacles to astronomical observation, and the search is always in order for regions of the world, in deserts or on high mountains, where the quietest atmosphere is to be found.
Quite another power of the telescope is dependent on its objective solely: its light-gathering power. Light by which we see a star or planet is admitted to the retina of the eye through an adjustable aperture called the pupil. In the dark or at night, the pupil expands to an average diameter of one-fourth of an inch. But the object-gla.s.s of a telescope, by focusing the rays from a star, pours into the eye, almost as a funnel acts with water, all the light which falls on its larger surface. And as geometry has settled it for us that areas of surfaces are proportioned to the squares of their diameters, a two-inch object gla.s.s focuses upon the retina of the eye 64 times as much light as the una.s.sisted eye would receive. And the great 40-inch objective of the Yerkes telescope would, theoretically, yield 25,600 times as much light as the eye alone. But there would be a noticeable percentage of this lost through absorption by the gla.s.ses of the telescope and scattering by their surfaces.
The first makers of telescopes soon encountered a most discouraging difficulty, because it seemed to them absolutely insuperable. This is known as chromatic aberration, or the scattering of light in a telescope due simply to its color or wave length. When light pa.s.ses through a prism, red is refracted the least and violet the most. Through a lens it is the same, because a lens may be regarded as an indefinite system of prisms. The image of a star or planet, then, formed by a single lens cannot be optically perfect; instead it will be a confused intermingling of images of various colors. With low powers this will not be very troublesome, but great indistinctness results from the use of high magnifying powers.
The early makers and users of telescopes in the latter part of the seventeenth century found that the troublesome effects of chromatic aberration could be much reduced by increasing the focal length of the objective. This led to what we term engineering difficulties of a very serious nature, because the tubes of great length were very awkward in pointing toward celestial objects, especially near the zenith, where the air is quietest. And it was next to impossible to hold an object steadily in the field, even after all the troubles of getting it there had been successfully overcome.
Bianchini and Ca.s.sini, Hevelius and Huygens were among the active observers of that epoch who built telescopes of extraordinary length, a hundred feet and upward. One tube is said to have been built 600 feet in length, but quite certainly it could never have been used. So-called aerial telescopes were also constructed, in which the objective was mounted on top of a tower or a pole, and the eyepiece moved along near the ground. But it is difficult to see how anything but fleeting glimpses of the heavenly bodies could have been obtained with such contrivances, even if the lenses had been perfect. Newton indeed, who was expert in optics, gave up the problem of improving the refracting telescope, and turned his energies toward the reflector.
In 1733, half a century after Newton and a century and a quarter after Galileo, Chester More Hall, an Englishman, found by experiment that chromatic aberration could be nearly eliminated by making the objective of two lenses instead of one, and the same invention was made independently by Dollond, an English optician, who took out letters patent about 1760. So the size of telescopes seemed to be limited only by the skill of the gla.s.smaker and the size of disks that he might find it practicable to produce.
What Hall and Dollond did was to make the outer or crown lens of the objective as before, and place behind it a plano-concave lens of dense flint gla.s.s. This had the effect of neutralizing the chromatic effect, or color aberration, while at the same time only part of the refractive effect of the crown lens was destroyed. This ingenious but costly combination prepared the way for the great refracting telescopes of the present day, because it solved, or seemed to solve, the important problem of getting the necessary refraction of light rays without harmful dispersion or decomposition of them.
Through the 18th century and the first years of the 19th many telescopes of a size very great for that day were built, and their success seemed complete. With large increase in the size of the disks, however, a new trouble arose, quite inherent in the gla.s.s itself. The two kinds of gla.s.s, flint and crown, do not decompose white light with uniformity, so that when the so-called achromatic objective was composed of flint and crown, there was an effect known as irrationality of dispersion, or secondary spectrum, which produced a very troublesome residuum of blue light surrounding the images of bright objects. This is the most serious defect of all the great refractors of the day, and effectively it limits their size to about 60 inches of aperture, with present types of flint and crown. It is expected by present experimenters, however, that further improvements in optical gla.s.s will do much to extend this limit; so that a refracting telescope of much greater size than any now in existence will be practicable.
Improvements in mounting telescopes, too, are still possible. Within recent years, Hartness, of Springfield, Vermont, has erected a new and ingenious type of turret telescope which protects the observer from wind and cold while his instrument is outside. It affords exceptional facilities for rapid and convenient observing, as for variable stars, and is adaptable to both refractors and reflectors.
The captivating study of the heavens can of course be begun with the naked eye alone, but very moderate optical a.s.sistance is a great help and stimulates. An opera-gla.s.s affords such a.s.sistance; a field-gla.s.s does still better, and best of all, for certain purposes, is a modern prism-binocular.
CHAPTER XIX
REFLECTORS--MIRROR TELESCOPES
Cherished with the utmost care in the rooms of the Royal Society of London is a world-famous telescope, a diminutive reflector made by the hands of Sir Isaac Newton. We have already mentioned his connection with the refractor; and how he abandoned that type of telescope in favor of the reflecting mirror, or reflector in which the obstacles to great size appeared to be purely mechanical. By many, indeed, Newton is regarded as the inventor of the reflector.
By the principles of optics, all the rays from a star that strike a concave mirror will be reflected to the geometric focal point, provided a section of that mirror is a parabola. Such a mirror is called a speculum, and is an alloy of tin, copper, and bis.m.u.th. Its surface takes a very high polish, reflecting when newly polished nearly 90 per cent of the light that falls upon it.
But the focus where the eyepiece must be used is in front of the mirror, and if the eye were placed there, the observer"s head would intercept all or much of the light that would otherwise reach the mirror. Gregory, probably the real inventor of the reflector, was the first to dodge this difficulty by perforating the mirror at the center and applying the eyepiece there, at the back of the speculum; but it was necessary to first send the rays to that point by reflection from a second or smaller mirror, in the optical axis of the speculum. This reflects the rays backward down the tube to the eyepiece, or spectroscope, or camera.
Another English optician, Ca.s.segrain, improved on this design somewhat by placing the secondary mirror inside the focus of the speculum, or nearer to it, so that the tube is shorter. This form is preferable for many kinds of astronomical work, especially photography. Herschel sought to do away with the secondary reflector entirely and save the loss of light by tilting the speculum slightly, so as to throw the image at one side of the tube; but this modification introduces bad definition of the image and has never been much used.
A better plan is that of Newton, who placed a small plane speculum at an angle of 45 degrees in the optical axis where the secondary mirror of the Gregory-Ca.s.segrainian type is placed. The rays are then received by the eyepiece at the side of the upper end of the tube, the observer looking in at right angles to the axis. And a modern improvement first used by Draper is a small rectangular prism in place of the little plane speculum, effecting a saving of five to ten per cent of the light.
It is not easy to say which type of telescope, the refractor or the reflector, is the more famous. Nor which is the better or more useful, or the more likely to lead in the astronomy of the future. When the successors of Dollond had carried the achromatic refractor to the limit enforced by the size of the gla.s.s disks they were able to secure, they found these instruments not so great an improvement after all. The single-lens telescopes of great focal length were nearly as good optically, though much more awkward to handle. But the quality of the gla.s.s obtainable in that day appeared to set an arbitrary limit to that great amplification of size and power which progress in observational astronomy demanded.
Then came the elder Herschel, best known and perhaps the greatest of all astronomers. At Bath, England, music was his profession, especially the organ. But he was dissatisfied with his little Gregorian reflector, and being a very clever mechanician he set out to build a reflector for himself. It is said that he cast and polished nearly 200 mirrors, in the course of experiments on the most highly reflective type of alloys, and the sort of mechanism that would enable him to give them the highest polish. In all his work he was ably and enthusiastically aided by his sister, Caroline Herschel, most famous of all women astronomers.
Upward in size of his mirrors he advanced, till he had a speculum of two feet diameter with a tube 20 feet long. Twelve to fifteen years had elapsed when in 1781, while testing one of these reflectors on stars in the constellation Gemini, he made the first discovery of a planet since the invention of the telescope--the great planet now known as Ura.n.u.s.
Under the patronage of King George, he advanced to telescopes of still greater size, his largest being no less than forty feet in length, with a speculum of four feet in diameter. Two new satellites of Saturn were discovered with this giant reflector, which was dismantled by Sir John Herschel with appropriate ceremonies, including the singing of an ode by the Herschel family a.s.sembled inside of the tube, on New Year"s Eve, 1839-40.
We have record of but few attempts to improve the size and definition of great reflectors by the continental astronomers during this era. In England and Ireland, however, great progress was made. About 1860 La.s.sell built a two-foot reflector, with which he discovered two new satellites of Ura.n.u.s, and which he subsequently set up in the island of Malta. Ten years later Thomas Grubb and Son of Dublin constructed a four-foot reflector, now at the Observatory in Melbourne, Australia.
Calver in conjunction with Common of Ealing, London, about 1880-95 built several large reflectors, the largest of five feet diameter, now owned by Harvard College Observatory; and, rather earlier, Martin of Paris completed a four-foot reflector.
The mirrors of these latter instruments were not made of speculum metal, but of solid gla.s.s, which must be very thick (one-seventh their diameter) in order to prevent flexure or bending by their own weight. So sensitive is the optical surface to distortion that unless a complicated series of levers and counterpoises is supplied, to support the under surface of the mirror, the perfection of its optical figure disappears when the telescope is directed to objects at different alt.i.tudes in the sky. The upper or outer surface of the gla.s.s is the one which receives the optical polish on a heavy coat of silver chemically deposited on the polished gla.s.s after its figure has been tested and found satisfactory.
But far and away the most famous reflecting telescope of all is the "Leviathan" of Lord Rosse, built at Birr Castle, Parsonstown, Ireland, about the middle of the last century. His Lordship made many ingenious improvements in grinding the mirror, which was of speculum metal, six feet in diameter and weighed seven tons. It was ground to a focal length of fifty-four feet and mounted between heavy walls of masonry, so that the motion of the great tube was restricted to a few degrees on both sides of the meridian. The huge mechanism was very c.u.mbersome in operation, and photography was not available in those days; nevertheless Lord Rosse"s telescope made the epochal discovery of the spiral nebulae, which no other telescope of that day could have done.
In America the reflector has always kept at least even pace with the refractor. As early as 1830, Mason and Smith, two students at Yale College, enthused by Denison Olmsted, built a 12-inch speculum with which they made unsurpa.s.sed observations of the nebulae. Dr. Henry Draper, returning from a visit to Lord Rosse, began about 1865 the construction of two silver-on-gla.s.s reflectors, one of 15 inches diameter, the other of 28 inches, with which he did important work for many years in photography and spectroscopy, and his mirrors are now the property of Harvard College Observatory. Alvan Clark and Sons have in later years built a 40-inch mirror for the Lowell Observatory in Arizona, and very recently a 6-foot silver-on-gla.s.s mirror has been set up in the Dominion of Canada Astrophysical Observatory at Victoria, British Columbia, where it is doing excellent work in the hands of Plaskett, its designer.
The huge gla.s.s disk for the reflector weighs two tons, and it must be cast so that there are no internal strains; otherwise it is liable to burst in fragments in the process of grinding. It should be free from air-bubbles, too; so the gla.s.s is cast in one melting, if possible. This disk was made by the St. Gobain Plate Gla.s.s Company, whose works have been ruthlessly destroyed by the enemy during the war; but fortunately the great disk had been shipped from Antwerp only a week before declaration of hostilities.
Brashear of Allegheny was intrusted with the optical parts, which occupied many months of critical work. The finished mirror is 73 inches in diameter, its focal length is 30 feet, and its thickness 12 inches. A central hole 10 inches in diameter makes possible its use as a Gregorian or Ca.s.segrainian type, as well as Newtonian. The mechanical parts of this great telescope are by Warner and Swasey of Cleveland, after the well-known equatorial mounting of the Melbourne reflector by Grubb of Dublin. Friction of the polar and declination axes is reduced by ball bearings. The 66-foot dome has an opening 15 feet wide and extending six feet beyond the zenith. All motions of the telescope, dome shutters, and observing platform are under complete control by electric motors.
Spectroscopic binaries form one of the special fields of research with this powerful instrument, and many new binaries have already been detected.
The great reflectors designed and constructed by Ritchey, formerly of Chicago and now of Pasadena, deserve especial mention. While connected with the Yerkes Observatory he constructed a two-foot reflector for that inst.i.tution, with which he had exceptional success in photography of the stars and nebulae. Later he built a 5-foot reflector, now at the Carnegie Observatory on Mount Wilson, California, with which the spiral nebulae and many other celestial objects have been especially well photographed.
Ritchey"s later years have been spent on the construction of an even greater mirror, no less than 100 inches in diameter, which was completed in 1919, and has already yielded photographic results dealt with farther on, and far surpa.s.sing anything previously obtained. Theoretically this huge mirror, if its surface were perfectly reflective so that it would transmit all the rays falling upon it, would gather 160,000 times as much light as the unaided eye alone.
Whether a 72-inch refractor, should it ever be constructed, would surpa.s.s the 100-inch reflector as an all-round engine for astronomical research, is a question that can only be fully answered by building it and trying the two instruments alongside.
Probably three-quarters of all the really great astronomical work in the past has been done by refractors. They are always ready and convenient for use, and the optical surfaces rarely require cleaning and readjustment. With increase of size, however, the secondary spectrum becomes very bothersome in the great lenses; and the larger they are, the more light is lost by absorption on account of the increasing thickness of the lenses. With the reflector on the other hand, while there is clearly a greater range of size, the reflective surface retains its high polish only a brief period, so that mere tarnish effectively reduces the aperture; and the great mirror is more or less ineffective in consequence of flexure uncompensated by the lever system that supports the back of the mirror.
Both types of telescope still have their enthusiastic devotees; and the next great reflector would doubtless be a gratifying success, if mounted in some elevated region of the world, like the Andes of northern Chile, where the air is exceptionally steady and the sky very clear a large part of the year. The highest magnifying powers suitable for work with such a telescope could then be employed, and new discoveries added as well as important work done in extension of lines already begun on the universe of stars.
On the authority of Clark, even a six-foot objective would not necessitate a combined thickness of its gla.s.ses in excess of six inches.
Present disks are vastly superior to the early ones in transparency, and there is reason to expect still greater improvement. The engineering troubles incident to execution of the mechanical side of the scheme need not stand in the way; they never have, indeed the astronomer has but just begun to invoke the fertile resources of the modern engineer. Not long before his death the younger Clark who had just finished the great lenses of the 40-inch Yerkes telescope, ventured this prevision, already in part come true: "The new astronomy, as well as the old, demands more power. Problems wait for their solution, and theories to be substantiated or disproved. The horizon of science has been greatly broadened within the last few years, but out upon the borderland I see the glimmer of new lights that await for their interpretation, and the great telescopes of the future must be their interpreters."
Practically all the great telescopes of the world have in turn signalized the new accession of power by some significant astronomical discovery: to specify, one of Herschel"s reflectors first revealed the planet Ura.n.u.s; Lord Rosse"s "Leviathan" the spiral nebulae; the 15-inch Cambridge lens the c.r.a.pe, or dusky ring of Saturn; the 18-1/2-inch Chicago refractor the companion of Sirius; the Washington 26-inch telescope the satellites of Mars; the 30-inch Pulkowa gla.s.s the nebulosities of the Pleiades; and the 36-inch Lick telescope brought to light a fifth satellite of Jupiter. At the time these discoveries were made, each of these great telescopes was the only instrument then in existence with power enough to have made the discovery possible. So we may advance to still farther accessions of power with the expectation that greater discoveries will continue to gratify our confidence.
CHAPTER XX
THE STORY OF THE SPECTROSCOPE
Sir Isaac Newton ought really to have been the inventor of the spectroscope, because he began by a.n.a.lyzing light in the rough with prisms, was very expert in optics, and was certainly enough of a philosopher to have laid the foundations of the science.
What Newton did was to admit sunlight into a darkened room through a small round aperture, then pa.s.s the rays through a gla.s.s prism and receive the band of color on a screen. He noticed the succession of colors correctly--violet, indigo, blue, green, yellow, orange, red; also that they were not pure colors, but overlapping bands of color.
Apparently neither he nor any other experimenter for more than a century went any further, when the next essential step was taken by Wollaston about 1802 in England. He saw that by receiving the light through a narrow slit instead of a round hole, he got a purer spectrum, spectrum being the name given to the succession of colors into which the prism splits up or decomposes the original beam of white sunlight. This seemingly insignificant change, a narrow slit replacing the round hole, made Wollaston and not Newton the discoverer of the dark lines crossing the spectrum at various irregular intervals, and these singularly neglected lines meant the basis of a new and most important science.
Even Wollaston, however, pa.s.sed them by, and it was Fraunhofer who in 1814-1815 first made a chart of them. Consequently they are known as Fraunhofer lines, or dark absorption lines. Sending the beam of light through a succession of prisms gives greater dispersion and increases the power of the spectroscope. The greater the dispersion the greater the number of absorption lines; and it is the number and intensity of these lines, with their accurate position throughout the range of the spectrum which becomes the basis of spectrum a.n.a.lysis.
The half century that saw the invention of the steam engine, photography, the railroad and the telegraph elapsed without any farther developments than mere mapping of the fundamental lines, A, B, C, D, E, F, G, H of the solar spectrum. The moon, too, was examined and its spectrum found the same, as was to be expected from sunlight simply reflected.