The one pre-Herschelian problem in sidereal astronomy was the distance of the stars. Owing to its bearing on the Copernican theory, the problem was attacked by the astronomers of the seventeenth and eighteenth centuries. Herschel made numerous attempts to detect the parallax of the brighter stars, but failed. Meanwhile there had been many illusions. Piazzi believed that his instruments—which in reality were worn out and unfit for use—had revealed parallaxes in Sirius, Aldebaran, Procyon, and Vega; Calandrelli, another Italian, and _John Brinkley_ (1763-1835), Astronomer-Royal of Ireland, were similarly deluded; and in 1821 it was shown by _Friedrich Georg Wilhelm Struve_ (1793-1864), the great German astronomer, that no instruments then in use could possibly be successful in measuring the stellar parallax. A few years later, however, Fraunhofer brought the refractor to a degree of perfection surpassing all previous efforts. In 1829 he mounted for the observatory at Königsberg a heliometer, the object-glass of which was divided in two, and capable of very accurate measurements. This heliometer eventually revealed the parallax of the stars in the able hands of Friedrich Wilhelm Bessel. Friedrich Wilhelm Bessel was born at Minden, on the Weser, south-west of Hanover, on July 22, 1784. His father was an obscure Government official, unable to provide a university education for his son. Bessel’s love of figures, together with an aversion to Latin, led him to pursue a commercial career. At the age of fourteen, therefore, he entered as an apprenticed clerk the business of Kuhlenkamp & Sons, in Bremen. He was not content, however, to remain in that humble position. His great ambition was to become supercargo on one of the trading expeditions sent to China; and so he learned English, Spanish, and geography. But he never became a supercargo. In order to be fully equipped for such a position, he determined to learn how to take observations at sea, and his acquaintance with observation aroused a desire to study astronomy. He constructed for himself a sextant, and by means of this, along with a common clock, he determined the longitude of Bremen. Such enthusiasm could not be long without its reward. For several years Bessel remained a clerk, and the hours devoted to study were those spared from sleep. He studied the works of Bode, Von Zach, Lalande, and Laplace, and in two years was able to compute the orbits of comets by means of mathematics. From some observations of Halley’s comet at its appearance in 1607, Bessel calculated its orbit, and forwarded the calculation to Olbers, then the greatest authority on cometary astronomy. Olbers was delighted at this work, and he sent the results to Von Zach, who published them. The self-taught young astronomer had accomplished a piece of work which fifteen years before had taxed the skill and patience of the French Academy of Sciences.