Part 26

Certain it is that a great deal of valuable work is done with comparatively small telescopes, ranging from six to fifteen inches aperture, and this in all departments of astronomical research. Hence, some of the most active observatories do not figure in the above list; among them may be mentioned the observatories of Harvard College, Potsdam, Paris, Heidelberg, Cape of Good Hope, Edinburgh, South Kensington, Stonyhurst College, and the observatory of Dr. Isaac Roberts at Crowborough, England.

HOUSING OF EQUATORIALS.—The building which accommodates an equatorial telescope must evidently be designed to admit of giving a clear opening to any part of the sky. Usually this is accomplished by making the roof, or dome, with a circular base, provided with wheels, which run on rails. It is then only necessary to open a narrow portion of the dome, extending from top to base, and to turn the dome until this aperture is in the required direction. One of the most elaborate domes now in existence is that built by M. Eiffel for the great refractor of the Nice Observatory. The lower part of the building is in the form of a square, having a side of about eighty-seven feet and a height of about thirty feet. The dome itself is seventy-four feet in diameter, and the moving parts alone weigh ninety-five tons.

There are two shutters, each a little wider than half the possible opening; these run on short rails, and are moved simultaneously by means of an endless rope. The whole of the dome is built up of steel angle iron, covered with very thin sheet steel. In order to facilitate the manipulation of the dome, its great weight is buoyed up by means of a float attached to its base and immersed in a circular tank of water of a little greater size than the base of the dome. If any mishap occurs with this gigantic tank, the dome rests on wheels which run on a circular rail, so that the work need not be interrupted. The whole arrangement is very easily turned with the aid of a winch by one man when the dome is floating, but when resting on the wheels several men are required at the winch.

This brief description will serve to illustrate some of the problems which confront the possessor of a very large telescope. For smaller instruments, the observatories follow pretty nearly the same plan, except that it is unnecessary to provide an arrangement for floating the dome.

The observatory which shelters a reflecting telescope need not differ very greatly from one which contains a refractor. If the instrument be a Newtonian, it is generally convenient to sink the polar axis below the level of the floor in order that the observer may not be at too great a height from the ground, and in that case, the dome, or its equivalent, is all that is necessary. For his five-foot reflector, Dr. Common designed an observatory which is not of the ordinary form, but gives the necessary opening partly by means of large shutters and partly by a revolution of the whole house. It is not every one who is able to lay out $40,000 on such a dome as that erected at Nice by M. Bischoffeim.

The varying position of the eye end of a telescope, when it is turned to different parts of the sky, makes it necessary to provide comfortable and safe seating accommodation for the observer, more especially when the telescope is a very large one. In the case of the Yerkes telescope, the eye-piece is thirty feet higher when observing near the horizon than when observing near the zenith, and the observer must necessarily follow the telescope. The most convenient arrangement in such a case is to raise or lower the floor of the observatory as occasion demands. The floor of the Yerkes Observatory is seventy-five feet in diameter, and by means of electric motors it can be given a vertical motion of twenty-two feet. A similar arrangement was provided for the Lick telescope from the designs of Sir Howard Grubb. With smaller instruments, observing ladders and adjustable chairs of various forms are employed.

THE EQUATORIAL COUDÉ.—A form of equatorial telescope which has possibly a great future before it is one introduced at Paris under the name of the equatorial coudé, or elbowed telescope. Its practical advantage is that the observer remains in a constant and comfortable position, so that revolving domes and elevating floors, or other arrangements serving similar purposes, are no longer necessary. The telescope tube is of two parts of nearly equal length, and what is ordinarily the lower half of the tube forms part of the polar axis, while the other half is attached to it at right angles. At the point of intersection of the two halves of the tube is a plane mirror, and there is another mirror in front of the object-glass. If the latter mirror were removed, such a telescope would only enable the observer to see objects lying along the celestial equator, but by its means objects in all parts of the heavens can be brought within range to an observer gazing down the hollow polar axis. The largest instrument is that at the Paris Observatory, which has an object-glass 23½ inches in diameter for visual observations, and another of the same size for photographic purposes.

FIXED TELESCOPES.—There is still another method of using a telescope. The telescope itself may be fixed, and the light of the heavenly bodies may be reflected into it by means of a mirror which is made to revolve so as to keep pace with their movements. Foucault devised an instrument called the siderostat for this purpose, and although it is not largely employed for telescopic observations, it is very widely utilized for spectroscopic work, where the spectroscope is of a kind not readily attached to a telescope.

Another instrument used for the same purpose has recently been brought forward under the name of the cœlostat. This is simply a mirror which is made to turn on a polar axis in its own plane, and since a reflected ray of light moves through twice the angle that the reflecting surface turns through, the mirror is made to revolve at the rate of one revolution in two days. As the name indicates, the whole heavens appear stationary in such an instrument, whereas in a siderostat only one star at a time appears at rest, while its neighbors slowly revolve round it.

PHOTOGRAPHIC TELESCOPES.—The application of photography to the study of the heavenly bodies marks one of the greatest advances of the present century. The instruments which are employed for this purpose range from the ordinary tourist camera to the largest telescope. Unlike a person sitting for a portrait, the heavenly bodies can not be made to stand still for the purpose, and as instantaneous photographs can only be obtained in the case of the sun and moon, it is usually necessary to make the camera follow the stars very exactly during the time of exposure in order that the images may fall on precisely the same parts of the photographic plate.

Some guiding arrangement is, therefore, essential, and generally the photographic camera or telescope is attached to an ordinary equatorial which is driven by clockwork, or very carefully by hand if the camera be a small one. In the guiding telescope are two spider-threads at right angles to each other, and it is by constantly keeping the image of a star at the intersection of these “wires” that the operator ensures the images remaining in a constant position upon the sensitive plate.

An ordinary portrait camera, in the hands of a skilled observer, yields very beautiful pictures, but they are naturally on a small scale. The field of view of such an instrument is so large that a whole constellation may be photographed with a single exposure.

Portrait lenses of six inches aperture in the hands of Dr. Max Wolf and Professor Barnard have given magnificent delineations of the Milky Way, and of the extremely faint nebulosities which are to be found in many parts of the heavens.

For many purposes, however, telescopes of greater power are required, and here it may be remarked that the distance between the images of any two adjacent stars will vary in direct proportion to the focal length of the telescope. In the same way the size of the image of a planet, the moon, or a comet, increases as the focal length of the objective is increased.

Refracting telescopes which are employed for photography require object-glasses which are specially “corrected” for the photographic rays. White light is compounded of light of all colors, but it is the blue and violet constituents which are effective in producing photographic action on an ordinary sensitive plate. Now, an object-glass which is intended for visual purposes is made to focus at the same point as many as possible of the rays which are most effective to the human eye, that is the green, yellow, and red, and usually there is a blue or purple halo round the images of the brighter objects, which is, however, too feeble as a rule to interfere with visual observations. This blue halo will evidently result in defective definition if the lens be employed for photography. By putting the plate at the point where the blue rays are most nearly focused, a better image is obtained; but for really good work a photographic object-glass must be so designed that all the blue and violet rays are brought to one and the same focus. Such a lens will consequently be a very poor one for visual observations.

The new “photo telescopic” object-glass now manufactured by Messrs. Cooke appears to be full of promise. In this lens all the colors of the spectrum are brought to almost exactly the same focal point, so that it serves equally well for photographic or visual purposes.

This difficulty in regard to achromatism does not exist in the case of the reflecting telescope, since rays of light of every color are reflected at precisely the same angles. For this reason reflectors, when properly managed, give the best photographic results. Dr. Isaac Roberts and Dr. Common are especially identified with the application of the reflecting telescope for celestial photography. The instrument employed by the former consists of a 20-inch reflector and a 7-inch guiding telescope of the refracting form. The two telescopes are mounted on the extreme ends of the declination axis of an equatorial.

Dr. Common does not employ a guiding telescope at all. The photographic plate which he places at the focus of the reflector is smaller than the field of view, so that by means of an eye-piece fitted with a cross wire at the side of the dark slide, he is able to watch a star near the edge of the field. Both eye-piece and dark slide are attached to a frame which can be controlled by two screws at right angles to each other. If the guiding star leaves the cross wire through errors in driving, or other causes, the eye-piece and dark slide are bodily moved after it by means of the adjusting screws. This method not only has the advantage of saving the cost of a guiding telescope, but reduces the effects of vibration consequent upon the correction of errors by moving the whole telescope.

For photographing the sun a special instrument called a photoheliograph is usually employed. This differs only from an ordinary photographic telescope in being provided with a secondary magnifier, by which means the focal image formed by the object-glass is amplified before falling upon the photographic plate. On a bright, clear day pictures of the sun eight inches in diameter can be taken with an exposure of about 1/500th of a second, and such a photograph will frequently record more facts as to the state of the solar surface than a whole day’s observation. Lenses or mirrors of very long focus are also occasionally employed in solar photography, and in this way a large image is obtained without the use of a secondary magnifier.

Photographs of the moon and planets may be taken either with or without a secondary magnifier, but in either case the exposures are longer than for the sun.

Finally, it may be added that the sensitive plates and processes used in astronomical photography do not differ from those employed by ordinary photographers.

FOOTNOTES:

The focal length of a lens is the distance from its centre at which an image of a very distant object, such as the sun, is formed.

METEORS.—SIR ROBERT S. BALL

Our present knowledge as to the natural history of the shooting stars has been mainly acquired during the last hundred years. The first important step in the comprehension of these bodies was to recognize that the brilliant flash of light was caused by some object which came from without and plunged into our air. This was known at the end of the Eighteenth Century, largely by the labors of the philosopher Chladni in 1794.

Where Four Mountain Ranges Meet]

Could an ordinary shooting star tell us its actual history, the narrative would run somewhat as follows:

“I was a small bit of material, chiefly, if not entirely, composed of substances which are formed from the same chemical elements as those you find on the earth. Not improbably I may have had some iron in my constitution, and also sodium and carbon, to mention only a few of the most familiar elements. I only weighed an ounce or two, perhaps more, perhaps less—but you could probably have held me in your closed hand, or put me into your waistcoat pocket. You would have described me as a sort of small stone, yet I think you would have added that I was very unlike the ordinary stones with which you were familiar. I have led a life of the most extraordinary activity; I have never known what it was to stay still; I have been ever on the move. Through the solitudes of space I have dashed along with a speed which you can hardly conceive. Compare my ordinary motion with your most rapid railway trains; my journey will be done ere the best locomotive ever built could have drawn the train out of the station. Pit me against your rifle bullets, against the shots from your one-hundred-ton guns; before the missile from the mightiest piece of ordnance ever fired shall have gone ten yards I have gone 1,000 yards. I do not assert that my speed has been invariable—sometimes it has been faster, sometimes it has been slower; but I have generally done my million miles a day at the very least. Such has been my career, not for hours or days, but for years and for centuries, probably for untold ages. And the grand catastrophe in which I vanished has been befitting to a life of such transcendent excitement and activity; I have perished instantly, and in a streak of splendor. In the course of my immemorial wanderings I have occasionally passed near some of the great bodies in the heavens; I have also not improbably in former years hurried by that globe on which you live. On those occasions you never saw me, you never could have seen me, not even if you had used the mightiest telescope that has ever been directed to the heavens. But too close an approach to your globe was at last the occasion of my fall. You must remember that you live on the earth buried beneath a great ocean of air. Viewed from outside space, your earth is seen to be a great ball, everywhere swathed with this thick coating of air. Beyond the appreciable limits of the air stretches the open space, and there it is that my prodigious journeys have been performed. Out there we have a freedom to move of which you who live in a dense atmosphere have no conception. Whenever you attempt to produce rapid motion on the earth, the resistance of your air largely detracts from the velocity that would be otherwise attainable. Your quick trains are impeded by air, your artillery ranges are shortened by it. Movements like mine would be impossible in air like yours.