Page:Encyclopædia Britannica, Ninth Edition, v. 11.djvu/803

HERSCHEL 767 quent motions and appearance, turned out to be a new planet, moving outside of the orbit of Saturn. To this plaueb he in due time assigned the name of Georgium Sidus ; but this name has by general consent been laid aside in favour of Uranus. It was discovered with a favourite 7-foot reflector having an aperture of 6 inches ; subsequently, when he had provided himself with a much more powerful telescope, of 20 feet focal length, he discovered what he believed to be no less than six satellites. Modern observations have shorn this gloomy planet of four of these supposed attendants, but at the same time have added two others apparently not observed by Herschel. No less than seven memoirs on the subject were communicated by him to the Royal Society, extending from the date of the discovery in 1781 to 1815. There is a peculiarity worthy of notice in Herschel s mode of obser vation which led to the discovery of this planet. He had observed that the spurious diameters of stars are not much affected by increasing the magnifying powers, but that the case is different with other celestial objects ; hence if anything in his telescopic field attracted his notice by peculiarity of appearance, he immediately varied the magnifying power in order to decide the nature of the object. Thus Uranus was discovered ; and had a similar method been applied to Neptune, that planet would have been discovered at Cambridge some months before it was recognized at Berlin. We now come to the commencement of Herschel s most important series of observations, culminating in what ought probably to be regarded as his most capital discovery. A material part of the task which he had set himself as the work of his astronomical life embraced the determination of the relative distances of the stars from our sun and from each other. Now, in the course of his scrutiny of the heavens, he had observed many stars in apparently very close contiguity, but often greatly differing in their relative brightness. He concluded that, on the average, the brighter star would be the nearer to us, and the smaller enormously more distant. He considered that an astronomer on the earth, in consequence of its immense orbital displacement of some 180 millions of miles every six months, would see such a pair of stars under different perspective aspects ; and this variety of perspective aspect observed and measured would, he thought, lead to an approximate determination of their distance. With this view he mapped down the places and aspects of all the double stars that he met with, and communicated in 1782 and 1785 very extensive cata logues of the results. Indeed, the very last scientific memoir that he ever wrote, sent to the Royal Astronomical Society in the year 1822, at the time when he was its first president and already in the eighty-fourth year of his age, related to these investigations. In the first of these memoirs he throws out the hint that these apparently con tiguous stars must, if constituted after the material laws of our solar system, circulate round each other through the effects of gravitation ; but he significantly adds that the time had not yet arrived for settling the question. Thus the philosopher abides his time in patience and confidence, and a dozen years afterwards (1793) he remeasures the relative positions of many of these contiguous pairs, and we may conceive what his feelings must have been at find ing the verification of his prediction. For he found that some of these stars had circulated round each other, after the manner required by the laws of gravitation. Thus Herschel had demonstrated the action of the same mechanical laws among the distant members of the starry firmament which bind together the harmonious motions of our solar system. This sublime discovery would of itself suffice to immortalize his memory in the respectful homage of all future races of intelligent men. If only Herschel had lived long enough to learn the approximate distances of some of these binary combinations, he would at once have been able to calculate their masses when com pared with that of our own sun ; and thus, knowing, as we now do, that these stars in their weights are strictly comparable with the weight of our own sun, he would have found another of his analogical conjectures realized. In the year 1782, in consequence of his fame, Herschel was invited to Windsor by Geoi-ge III., and then accepted the offer made by the king to become his private astronomer, and henceforth devote himself wholly to a scientific career. The salary offered and accepted was .200 per annum, to which an addition of 50 per annum was subsequently made for the astronomical assistance of his faithful sister. Dr Watson, to whom alone the amount of this salary was mentioned, made the natural remark, " Never before was honour purchased by a monarch at so cheap a rate." In this way the great philosophical astronomer removed from Bath first to Datchet and soon afterwards permanently to Slough, within the easy access of his royal patron at Windsor. The old pursuits at Bath were soon resumed at Slough, but with renewed vigour and without the old pro fessional interruptions. The greater part, in fact, of tlie papers already referred to are dated from Datchet and Slough, for the magnificent astronomical speculations in which he was engaged, though for the most part conceived in the earlier portion of his philosophical career, required years of patient observation before they could be fully examined and realized. It was at Slough in 1783 that he wrote his first memor able paper on the " Motion of the Solar System in Space," a sublime speculation, yet through his genius realized by considerations of the utmost simplicity. He returned to the same subject with fuller details in 1805. It was also after his removal to Slough that he published his first memoir containing his speculative ideas on the construction of the heavens, which from the first had been the chief aim of his toils both of mind and body. In a long series of remarkable papers, addressed as usual to the Royal Society, and extending from the year 1784 to 1818, when he was eighty years of age, he demonstrated the fact that our sun is a star situated not far from the bifurcation of the Milky Way, and that all the stars visible to us lie more or less in clusters scattered throughout a comparatively thin stratum, but in the other two dimensions stretching immeasurably further into space. At one time he imagined that his powerful instruments had pierced through this stellar stratum of the Milky Way, and that he had approximately determined the form of some of its boundaries. In the last of his memoirs he had convinced himself of his error, and he admitted that to his telescopes our stratum of stars is "fathomless." Over this stratum of stars and their planetary attendants, the whole being in ceaseless motion round some common centre of gravity as the resultant point of the combined gravitation, Herschel discovered on either side a canopy of discrete nebulous masses, from the condensation of at least a part of which the whole stellar universe had been formed, a magnificent conception, pursued with a force of genius and put to the practical test of observation with an industry almost incredible. It was the work of a single mind, carried to its termination with no assistance beyond that of a loyal sister, almost as remarkable a personage as himself. Hitherto we have said nothing about that grand reflect ing telescope, of 40 feet focal length and 4 feet aperture, which is too often regarded as the chief effort of his genius and his perseverance. The full description of this cele brated instrument will be found in the 85th volume of

the Transactions of the Royal Society. Gigantic as it really