This section is from "Scientific American Supplement Volumes 275, 286, 288, 299, 303, 312, 315, 324, 344 and 358". Also available from Amazon: Scientific American Reference Book.
By EDGAR L. LARKIN, New Windsor Observatory, New Windsor, Illinois.
Within the past five years, scientific men have surpassed previous efforts in close measurement and refined analysis. By means of instruments of exceeding delicacy, processes in nature hitherto unknown, are made palpable to sense. Heat is found in ice, light in seeming darkness, and sound in apparent silence. It seems that physicists and chemists have almost if not quite reached the ultimate atoms of matter. The mechanism must be sensitive, as such properties of matter as heat, light, electricity, magnetism, and actinism, are to be handled, caused to vanish and reappear, analyzed and measured. With such instruments nature is scrutinized, revealing new properties, strange motions, vibrations, and undulations. Throughout the visible universe, the faintest pulsations of atoms are detected, and countless millions of infinitely small waves, bearing light, heat, and sound, are discovered and their lengths determined. Refined spectroscopic analysis of light is now made so that when any material burns, no matter what its distance, its spectrum tells what substance is burning. When any luminous body appears, it can be told whether it is approaching or receding, or whether it shines by its own or reflected light; whence it is seen that rays falling on earth from a flight of a hundred years, are as sounding lines dropped in the appalling depths of space. We wish to describe a few of these intricate instruments, and mention several far-reaching discoveries made by their use; beginning with mechanism for the manipulation of light. Optics is based on the accidental discovery that a piece of glass of certain shape will draw light to a focus, forming an image of any object at that point. The next step was in learning that this image can be viewed with a microscope, and magnified; thus came the telescope revealing unheard of suns and galaxies. The first telescopes colored everything looked at, but by a hundred years of mathematical research, the proper curvature of objectives formed of two glasses was discovered, so that now we have perfect instruments. Great results followed; one can now peer into the profound solitudes of space, bringing to view millions of stars, requiring light 5,000 years to traverse their awful distance, and behold suns wheeling around suns, and thousands of nebulæ, or agglomerations of stars so distant as to send us confused light, appearing like faint gauze like structures in measureless voids. The modern telescope has astonishing power, thus: When Mr. Clark finished the great twenty-six-inch equatorial, now at Washington, he tested its seeing properties. A photographic calligraph, whose letters were so fine as to require a microscope to see them, was placed at a distance of three hundred feet. Mr. Clark turned the great eye upon the invisible thing and read the writing with ease. But a greater feat than this was accomplished by the same instrument-- the discovery of the two little moons of Mars, by Prof. Asaph Hall, in 1877. They are so small as to be incapable of measurement by ordinary means, but with an ingenious photometer devised by Prof. Pickering of Harvard College, he determined the outer satellite to be six and the inner seven miles in diameter. The discovery of these minute bodies seems past belief, and will appear more so, when it is told that the task is equal to that of viewing a luminous ball two inches in diameter suspended above Boston, by the telescope situated in the city of New York. (Newcomb and Holden's Astronomy, p. 338.)
Phobos, the nearest moon, is only 4,000 miles from the surface of Mars, and is obliged to move with such great velocity to prevent falling, that it actually makes a circuit about its primary in only seven hours and thirty-eight minutes. But Mars turns on its axis in twenty-four hours and thirty-seven minutes, so the moon goes round three times, while Mars does once, hence it rises in the west and sets in the east, making one day of Mars equal three of its months. This moon changes every two hours, passing all phases in a single martial night; is anomalous in the solar system, and tends to subvert that theory of cosmic evolution wherein a rotating gaseous sun cast off concentric rings, afterward becoming planets. Astronomers were not satisfied with the telescope; true, they beheld the phenomena of the solar system; planets rotating on axes, and satellites revolving about them. They saw sunspots, faculæ, and solar upheaval; watched eclipses, transits, and the alternations of summer and winter on Mars, and detected the laws of gravity and motion in the system to which the earth belongs. They then devised the micrometer. This is a complex mechanism placed in the focus of a telescope, and by its use any object, providing it shows a disk, no matter what its distance, can be measured. It consists of spider webs set within a graduated metallic circle, the webs movable by screws, and the whole instrument capable of rotating about the collimation axis of the telescope. The screw head is a circle ruled to degrees and minutes, and turns in front of a fixed vernier in the field of a reading microscope. One turn of the screw moves the web a certain number of seconds; then as there are 360° in a circle, one-three-hundred-and-sixtieth of a turn moves the web one-three-hundred and-sixtieth of the amount, and so on. Thus, when two stars are seen in the field, one web is moved by the screw until the fixed line and the movable one are parallel, each bisecting a star. By reading with the microscope the number of degrees turned, the distance apart of the stars becomes known; the distance being learned, position is then sought; the observance of which led to one of the greatest discoveries ever made by man. The permanent line of the micrometer is placed in the line joining the north and south poles of the heavens, and brought across one of the stars; the movable web is then rotated until it bisects the other, and then the angle between the webs is recorded. Double stars are thus measured, first in distance, and second, their position. After this, if any movement of the stars takes place, the tell tale micrometer at once detects it.