other, it is manifest that, shifting greatly the position of the observer, the stars might be made to open out or close up on each other, or even revolve the one about the other. In employing this mode of investigation, the objects of comparison fell within the field of view of the same telescope, and almost all extraneous sources of error were eliminated.

Such was the plan devised, or rather perfected by Herschel, (for his predecessors had already suggested it), and on the prosecution of which he entered with the zeal which ever distinguished this great astronomer. When he commenced his researches, some half dozen double stars had been discovered and recorded. His first duty was to increase this number as rapidly as possible, and from his entire catalogue to select those best adapted to his purpose. Under his penetrating glance, the number of these curious duplex objects increased with astonishing rapidity, and he was himself startled with their frequent occurrence. However, with a mind fixed on his original design, he selected a large number of pairs, of such relative magnitudes, and in such positions, as promised the most certain success. Let it be remembered that many of these delicate objects were not divided from each other by a space greater than the thousandth part of the sun's diameter.

To ascertain the apparent changes in the relative positions and distances of the stars composing these pairs, Herschel measured, with every care, the distance which separated them, and took the direction. of the line drawn from the centre of the one to the centre of the other. Variations of distance and position, occasioned by parallax, were easily computed

in kind and character, and the great astronomer commenced and prosecuted his observations with sanguine hopes of success. One thing was certain :—all parallactic movements would have a period of one year, since they arise from the annual revolution of the earth in its orbit, and at the end of this period the stars composing the double sets ought to return to the position occupied at the outset. What was Herschel's astonishment to find that, in many instances, the stars composing these pairs were actually in motion; but the movement was certainly not of a parallactic kind, for it neither agreed in direction or in period with the effects of parallax. Here was another grand discovery! These double stars, which were scattered throughout the heavens with far greater profusion than accidental optical proximity could warrant, were found to be magnificent systems of revolving suns! They were united by the law of gravitation, and exhibited the wonderful spectacle of stupendous globes, moving in obedience to the same influences which hold the planets in their orbits, and guide the comets in their eccentric career.-This is not the place to enter into detail concerning these wonderful objects.

While a new field of investigation, boundless and magnificent, was opened up to the human mind; while the great discoverer of these far-sweeping suns was more than rewarded for his toil and labor, the original object of his research was not only left unattained, but the method selected with so much reasonable hope of success, became utterly inapplicable. The parallactic and absolute motions of the systems of stars became so inextricably involved, that

the imperfect micrometrical means of Herschel could not separate them.

Thus far, the efforts to obtain the distance of the stars had been unavailing.-A negative solution had indeed been reached. That their distance was enormous, was made evident, from the fact that the parallax had remained insensible, even under the most careful and delicate instrumental tests. Any absolute solution began almost to be despaired of, when hope was again revived by the magnificent refracting telescopes, for which the world was indebted to the skill and genius of the celebrated Frauenhofer, of Munich.This great artist, aided by the profound science of Bessel, contrived and executed an instrument of extraordinary power, and especially adapted to the research for the parallax of the fixed stars.

Armed with a micrometrical apparatus of wonderful perfection, and capable of executing measures of great, as well as minute distances, the telescope was so arranged as to be carried forward by delicate machinery, with a velocity exactly equal to the diurnal motion of the object under examination. To give some idea of the delicacy of the contrivances with which these great telescopes have been provided, it is only necessary to state that the micrometer of the great Refractor of the Cincinnati observatory is capable of dividing an inch into 80,000 equal parts !— When mechanical ingenuity failed to construct lines of mathematical minuteness, the spider lent his aid, and it is with his delicate web that these measures are accomplished. Two parallel spider's webs are adjusted in the focus of the eye-piece of the micrometer, and when the light of a small lamp is thrown

upon them, the eye, on looking through the telescope, sees two minute golden wires, straight and beautiful, drawn across the centre of the field of view, and pictured on the heavens. These are within the control of the observer. He can increase or decrease their distance at pleasure, and so revolve them as to bring them into any position, every motion being accurately measured by properly divided scales.

Suppose, then, it is desired to take the distance and position of the stars forming a pair. The telescope is directed to them, and they are brought to the centre of the field of view. The clock work is set in action; it takes up the ponderous instrument, weighing more than 2,500 pounds, and with the most astonishing accuracy it bears it onward, keeping its mighty eye fixed on the object under examination. The observer is thus left with both hands free to make his measures. He first revolves his micrometer spider's lines round until one of them shall exactly pass from centre to centre of the two stars. This position is noted, and from it is deduced the angle formed by this line with the meridian. He then revolves them a quarter of the circumference, and they are then perpendicular to their former position. He now separates the wires until the one shall exactly bisect one star, while the other wire passes through the centre of the second star, reading this distance on the proper scale. He has fixed, in these two observations, the position and distance of the two components of the double set.— Such is the precision attained in this work, that the most minute motions cannot escape detection. If the stars separate from each other at so slow a rate that a million of years would be required to perform the

circuit of the heavens, their motion would be detected. in half a year!

With machinery more delicate even than this, and better adapted to the purpose, and of a kind somewhat different, Bessel once more renewed the research after the unattainable parallax of the fixed stars. His great instrument, called the heliometer, was mounted as early as 1829, but a multitude of causes, and some unsuccessful efforts, delayed his principal operations up to August, 1837. Three great principles guided him in his selection of 61 in the Swan, as the star on which to perform his observations.First. It was affected by a very great proper motion, a characteristic which we will explain fully hereafter, and which indicated it to be among the nearest of all the stars. Second. Its duplex character adapted it especially to the instrument he was about to employ. Third. The region occupied by 61 Cygni contains a number of minute stellar points, close to the double star, and presenting admirable fixed points, to which the relative motions of the two components of the star to be measured might be referred.

With these advantages, and a magnificent instrument, Bessel commenced his observations. He measured the distance from the centre of the line joining the two stars, to two of the small stellar points, which served him as points of reference, and this kind of observation was repeated night after night, whenever the stars were visible, from the middle of August, 1837, up to the end of September, 1838. The entire series. of observations was then taken and corrected for every possible known error, and in case any appre