Part 7 (1/2)
[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.