God's glory in the heavens/The Observatory
Transit Circle at Cambridge Observatory, U.S.
XVI.
THE OBSERVATORY.
We have endeavoured to give our readers an idea of the principle, construction, and function of the telescope; but before passing from the subject, we cannot but advert to the remarkable circumstance, that the chief improvements are due to the labours of men busily engaged in trades and professions, which might at first sight seem incompatible with scientific pursuits. We have already instanced the case of Sir William Herschel, who was not deterred by the many engagements of his musical profession from devoting himself to the grinding of specula. But this is only one of many similar cases. The greatest imxprovement in the telescope, since the date of its invention, is due to a Spitalfields silk-weaver, John Dolland. His family were exiled from France by the revocation of the Edict of Nantes, and found a home in the suburbs of London. The devotion of the Spitalfields weavers to mathematics is one of the most curious as well as the most gratifying passages in the history of science; and John Dolland shone as one of the most distinguished of the number. He did not scruple to break a lance with the illustrious Euler; and for his improvements in the telescope, he received the highest honour of the Royal Society,—viz., the award of the Copley medal. He was the founder of the fortunes of his family, though it was his son Peter who amassed the wealth that flowed from the achromatic, arrangement. Peter, like his father, plied the shuttle in his youth, but he soon abandoned it for the more promising field of optics. Ramsden, who gained so much celebrity for his skill in dividing astronomical instruments, was a Yorkshire clothier. It would be easy to mention the names of many others, who, while they did not abandon the trade or profession in which they were trained, yet found time to improve the telescope, and advance the cause of astronomy. It is pleasant to record such cases, in which the daily toil of life has been lightened and dignified by science. It is satisfactory, too, to note, that science in those cases was wooed without any pecuniary loss. Their love of science seemed to make them only more prosperous in business. It is frequently very different when the mechanic acquires a taste for light literature. Instead of strengthening his arm, like science, for daily toil, it too often enervates him, by fostering a disrelish for the stern duties of life. It is an encouraging fact, viewed in connexion with the elevation of the labouring classes, that in our large cities, especially in London, there is a steady demand amongst this class for the higher class mathematical works. The men that haunt old book-stalls in rusty coats or moleskin jackets, are not always bent, as we are apt to suppose, on the purchase of the ephemeral literature of former days; they are often pondering over the purchase of some work in mathematics which formerly was in repute, but which now, from changes in educational methods, may be purchased for a trifle. Many a mechanic is working hard at fluxions in his garret, ignorant of the improved notation of modern days. He finds his great reward in the delight which the exercise of his intellectual faculties affords, and, with no thought of scientific celebrity, he revels in the profundities of the higher calculus. Such pure and disinterested love of science is one of the most hopeful features of the labour question. It proves that the highest intellectual labour is perfectly compatible with daily toil. The decay of mechanics' institutes is usually quoted as a proof that the intellectual elevation of the labouring classes is hopeless. But it admits of doubt whether the result is not due more to an under than an over-estimate of the mechanic's capabilities. The mechanic who could relish fluxions, would not long find advantage in attending the showy and superficial lectures usually given at such institutions. A system better calculated to develop his capabilities would most probably meet with more success.
We shall not, however, delay longer by signalising the names of those who have contributed most to the improvement of astronomical instruments, but at once introduce the reader to the observatory itself. All observatories have a great family likeness; but, to be more special, we shall suppose that the observatory in question is the one erected, about twenty years ago, in Glasgow, in connexion with the University, and over which Professor Nichol presided with so much distinction. The handsome building, on the summit of the hill above Partick, owes its erection and completeness very much to the munificence of the merchants of Glasgow, who came forward with liberal donations. If you wish to see it in full operation, you must visit it by night. The transit-room is the principal one; and when you are ushered into it, you find that it is a large sombre apartment, its walls painted black, and many bright instruments faintly gleaming in the light of dimly-shaded lamps. The room requires to be as dark as possible, and the lamps are used merely for illuminating the interior of the telescope and the face of the clock. As your eye gets accustomed to the gloom, you probably find the observer stretched on his back upon a couch, which is movable, so as to bring his eye close to the telescope. He is situated between two huge stone pillars that serve as supports for the transit-circle, which is the grand instrument of the regular observatory. You may form a pretty fair conception of the instrument, if 3^ou suppose a pair of carriage-wheels, with their connecting-axle laid across the tops of the pillars, the axle resting upon two metal supports on which it turns. The telescope is then to be conceived as fixed across the middle of the axle, so that it is hung precisely like a cannon on its carriage. It can only move on its axis, up and down; it can turn neither to the right nor the left. On examining the rim of the wheel, you will find an inlaid narrow band of gold all round, and on this are engraved very minute lines, with intervals of two seconds. When the telescope is elevated to a particular star, the circle, of course, turns round, being fixed to the axis; and the observer, when he has placed the star exactly on a spider's line in the centre of the field of view, leaves the eye-glass of the telescope, and views, with a powerful compound microscope, the divided limb. He marks what particular division comes under a spider's line stretched in the field of view of the fixed microscope, and this gives him the required altitude of the star. The measuring apparatus is so perfect, that the position can be read off to the fraction of a second. The transit-circle is one of the most perfect productions of art, both in regard to its optical and measuring powers. It was executed at Munich, the metropolis of art. Most of the finest instruments of European and American observatories have been sent forth from the workshops of this city. It is, however, satisfactory to note, that the tide seems to be once more turning in favour of our own country, for the recently-executed transit-circle at Greenwich is entirely of home manufacture, and its performance is quite unmatched.
When reference is made to the delicate measurements of the astronomer, it is satisfactory to have a clear conception of what the minute divisions mean. Now, what is meant when it is said, that such an instrument reads to the fraction of a second? How far distant from one another must the slender lines be which include a second of space? Some notion of this extreme closeness may be formed, when we state, that about six thousand lines would be crowded into the space of an inch on the limb of a circle six feet in diameter, and yet the astronomer has to deal with even minute fractions of the intervals between these lines. The distances of the fixed stars depend on the measurement of quantities so minute. It was not till within these few years that Ave could with certainty determine the distance of any of the stars, just because we had not till then the means of dealing with quantities so minute as a second; but so remarkable is our advance in this respect, that one star—viz., Capella—has a parallax (on which the distance depends) of only one-twenty-fifth of a second, or, on the circle in question, the one-twenty-fifth of the sixthousandth of an inch; and yet astronomers speak of its distance as certainly determined. And what renders the thing all the more wonderful is, that these small quantities must be extricated from errors far greater. No instrument, as well as no observer, is supposed to be faultless. The axis of the telescope may not be perfectly level—-it may not be precisely east and west; the telescope maybe set wrong on the axis; the observer may have some obliquity; and the atmosphere may turn the ray of light out of its straight course;—and each of these sources of error will occasion an amount of deviation far greater than the quantity to be ascertained. Yet the astronomer, by his formulae, hunts out truth so ingeniously amidst a maze of error, that he at last inevitably runs it dowi/. He has a ton of sand and gravel from which to extract a single shining grain of gold; and he sets to work so systematically, that, minute as it is, it cannot slip through his fingers. We have endeavoured to give a conception of a second on the rim of a brass circle; but it is also satisfactoryto have some notion of what a second is on the circle of the heavens. And, as in the one case we took an inch as our unit, we shall now take the apparent breadth of the moon, the most familiar of the heavenly bodies. Suppose a string stretched from one border of the moon to the opposite one, how many stars could be strung upon it in order that they would be a second apart from each other, the diameter of the moon being about half a degree? No fewer than two thousand would be required, each star beiug regarded as a mere point of light. Of course this string of individual stars would appear to the eye as a perfectly continuous line of light. Yet the astronomer Strüve, with the great equatorial of Dorpat, could not only individualise each star, but though one hundred more were strung on between any two of the stars, he could still measure the intervals between these interpolated stars; or, in other words, he could, with his micrometer, measure, with certainty, a space in the heavens so minute as the one-hundredth part of a second.
We have considered the measurement of space; but that of time is still more difficult. On entering the transit-room, you will observe, on looking up, that there is a narrow slit in the roof running from north to south; and your view of the heavens is confined to the narrow strip of blue which is seen through this slit. Now, the astronomer, at his transit circle, ignores all the rest of the heavens. He has fixed his telescope so that it can point only to tins small portion of the sky. He draws an imaginary circle, called a meridian, and he will study the deportment of the stars only at the moment of crossing this line. He stations himself at this ideal barrier, and before allowing any stellar traveller to pass, he questions him minutely to determine his identity—the two essential points being, the time of day when he passes, and his distance north or south from the equator. He questions him on these points, because, if any discrepancy occurs on the occasion of any future transit, it is sure to bring out the secrets of his history. But how is the time-questioning effected? Put your eye to the telescope, and the process will at once be revealed to you. You are surprised, when you look in, to see a blaze of light, instead of darkness, as you expected. The light proceeds from a lamp, and its design is, to shew you clearly a slender kind of grating spread over the field of view. This consists of seven spiders' threads, stretched up and down at equal distances, and one crossing in the middle. The perpendicular line in the middle corresponds to the imaginary meridian line. You soon discover a star coming in at one side of the field of view, and, to your surprise, marching rapidly across the lines to the other side. The rapidity invariably startles when first observed; and it affords the most sensible proof of the motion of the earth. No doubt, you can persuade yourself of this motion by watching the heavens during any starry night, and observing how the stars that were in the east at one hour, are in the west at another. But this is a matter of inference. It is not a direct, sensible proof. You do not see the stars moving as you do the trees and houses when you travel along in a carriage. But in the transit instrument, so sensible is the motion, that it is a most nervous business to note the precise moment when the star passes across the various wires; but it is needful to do this, in order to determine, more accurately, the moment when it passes the central line. This applies, however, only to the most rapid stars; for, as you approach the pole, the motion becomes slower and slower, till it is imperceptible. The observer must be within hearing of the clock, and as he silently counts the seconds, he must note down when the star passes each line; and as the star may take but a very few seconds to travel from one line to another, he has no sooner noted the one transit, than he has to note another. This would be all comparatively easy if the star passed each line precisely at the beat of each second; but this rarely occurs, and, consequently, the observer has to make a hurried estimate of the fraction of a second; and the requirements of science are such, that he must be able to appreciate the tenth of a second. Listen to the beat of a clock, and if you attempt to divide the interval between two beats into ten smaller intervals, you will have some idea of the difficulty of transit observation.
One of the most recent and important improvements in astronomical observation is designed to obviate this difficulty; and we owe it to America, which is now beginning to make valuable contributions to science as well as literature. The principle of the contrivance in question, consists in the substitution of the sense of touch for the sense of hearing. It is found, that sight and touch will act in much closer concert than sight and hearing. Instead, then, of watching the clock with the ear, and the star with the eye, the observer, when he notes the transit with the eye, presses a key with his linger, which makes a record of the observation. The finger, it is found, acts in instantaneous concert with the eye. The key acts upon a pen, which makes a mark on a sheet of paper moved by machinery. The beauty of the contrivance lies much in the application of electricity, which is now made to do duty in every possible way, from the ringing of a bell in the servants' hall, to the exploding of a mine under a fortress. The pen is connected with the clock by an electric band, in such a manner that, though the observer be absolutely deaf, he can, on examining the sheet of paper after the observation, tell, to the hundredth part of a second, the instant when the star passed the wire. The sheet of paper need not be close to the observer. It may be at Paris, or St Petersburg, or wherever there is electric communication; and the moment the observer presses the key in this country the record may he made hundreds of miles distant.
It is possible, with all this precision, that the observer may err. He is only making a report of a picture painted on his retina, and we can have no absolute assurance that his report of that picture is perfectly accurate. Indeed, it is found that this is a most important source of error. It is found that each observer has his own individual obliquity of judgment; and this must be determined before absolute reliance can be placed on his observations. But might we not dispense with the observer altogether? Could we not, when we order a telescope, also order an eye to look through the telescope? Having the eye to examine after the observation, we would not be dependent on the errors of judgment at the moment of observation. It would be a great ease to the astronomer himself, as there is no task so comfortless as that of observing in a transit-room. The scientific martyr has to shiver the live-long night on his couch. A cascade of bitterly cold air, often far below the freezing point, is constantly pouring down upon his head, and, unfortunately, the most precious nights for observation are the most bitterly cold. If a sea-coal fire were permitted, it would be some consolation; but such a thing cannot be dreamed of. Even the heat of the observer's body, cold as it is, endangers the delicate adjustments of the instrument when there is too close a proximity and anything like a blazing fire would be quite destructive of nice observation. An artificial eye would, then, be an acquisition of no ordinary value. This idea, although it savours of the wild conception of Frankenstein, is already partially realised. The artificial eye consists of a surface sensitive to light, placed where the eye of the observer is now placed, and the image of the celestial object is drawn at any moment on this surface, instead of on the retina of the observer. The difference is, that the impression on the artificial eye is permanent, and we can examine it at our leisure, whereas the impression on the living eye is transitory, and we have to depend on a hurried, and perhaps erroneous report. An apparatus is erected at Kew Observatory on this principle, for the observation of the sun's disc. To gain this object, it is so arranged by clock-work that an artificial retina is presented at a certain moment, and after receiving the photographic image withdrawn. This plan has had only partial success; but we can readily conceive it to be so developed as to work a revolution in astronomy.
After examining the transit-room, the visitor will be ushered into the dome, where the equatorial instrument is fixed on a pillar. The dome, of sheet-iron, is a very conspicuous object for miles around. It serves no other purpose than that of a convenient shelter for the telescope. The equatorial, unlike the transit instrument, is made to turn in every direction. In the transit-room, the observer must wait till the star come round to the slit in the roof. In the dome, he turns the telescope to the object at once. The dome has also a slit like the transit-room; but, in order to accommodate the wider range of its inmate, it is made to turn round on its base, so that the slit may be always opposite the mouth of the telescope. An equatorial is a telescope so mounted that it keeps the object in view, and does not allow it to flit by as in the transit instrument. If, in travelling on a railway, you look at some near object through a telescope, you will, in order to keep the object in view, require to be constantly changing the direction of the telescope. This is precisely the case with the equatorial, only it is the motion of the earth, not the railway carriage, that requires to be compensated. The motion is produced by clock-work attached to the axis of the telescope.
We have now taken a very rapid glance at the principal instruments of an observatory; but before leaving the scene, we must bestow a little attention on the astronomer himself. What should be the most marked moral feature of his character? A distinguished Christian poet fixes on devotion:
"An undevout astronomer is mad."
But poetical sentiment does not always coincide with; stern fact. We fear that astronomers, as a class, are not marked very strongly by devotion. There is much grandeur in the following conception of Long-fellow's:—
"And over all is the sky, the clear and crystalline heaven.
Like the protecting hand of God inverted above them."
One would think that, as it is the business of their lives to look up to this inverted hand of God, they would be habitually impressed with His glorious presence. But an object of grandeur depends, for its effect, altogether on the point of view from which we contemplate it. A stone mason might have spent a good part of his life in helping to build St Paul's, and yet, though constantly on the building, with square and plumb-line in hand, he would not occupy so favourable a position for appreciating its proportions, and the sublime ideas which it embodies, as the man who might know nothing about hewing and polishing, but who contemplated it at a distance. The sailor on the mast-head of a ship-of-war, at the mouth of the Alma, was in a better position for forming a right judgment of the battle-field and the glory of the victory, than the man who was in the thick of the fight. In like manner, the mere unprofessional man may be in a much better position for drawing from astronomy its divine teachings, than the man who spends his days and nights in the details of the science. The latter may be so absorbed with these details, that he may never think of withdrawing to a proper distance to contemplate the grandeur of the temple on which he is engaged. It is only the man who can, from the height of Calvary, project the glorious fabric on the background of eternity, that can exclaim with deep, heartfelt devotion, "The heavens declare thy glory, and the firmament sheweth thy handiwork. Day unto day uttereth speech, and night unto night sheweth knowledge. There is no speech nor language, where their voice is not heard."