Still, the desire to measure this parallax was only intensified by the practical certainty of its existence, and by repeated failures. The attempts of Bradley failed. The attempts of Piazzi and Brinkley,[1] early in the nineteenth century, also failed. The first successes, afterwards confirmed, were by Bessel and Henderson. Both used stars whose proper motion had been found to be large, as this argued proximity. Henderson, at the Cape of Good Hope, observed α Centauri, whose annual proper motion he found to amount to 3".6, in 1832-3; and a few years later deduced its parallax 1".16. His successor at the Cape, Maclear, reduced this to 0".92. In 1835 Struve assigned a doubtful parallax of 0".261 to Vega (α Lyræ). But Bessel’s observations, between 1837 and 1840, of 61 Cygni, a star with the large proper motion of over 5”, established its annual parallax to be 0".3483; and this was confirmed by Peters, who found the value 0".349. Later determinations for α2 Centauri, by Gill,[2] make its parallax 0".75—This is the nearest known fixed star; and its light takes 4 1/3 years to reach us. The lightyear is taken as the unit of measurement in the starry heavens, as the earth’s mean distance is “the astronomical unit” for the solar system.[3] The proper motions and parallaxes combined tell us the velocity of the motion of these stars across the line of sight: α Centauri 14.4 miles a second=4.2 astronomical units a year; 61 Cygni 37.9 miles a second=11.2 astronomical units a year. These successes led to renewed zeal, and now the distances of many stars are known more or less accurately. Several of the brightest stars, which might be expected to be the nearest, have not shown a parallax amounting to a twentieth of a second of arc. Among these are Canopus, α Orionis, α Cygni, β Centauri, and γ Cassiopeia. Oudemans has published a list of parallaxes observed.[4] _Proper Motion._—In 1718 Halley[5] detected the proper motions of Arcturus and Sirius. In 1738 J. Cassinis[6] showed that the former had moved five minutes of arc since Tycho Brahe fixed its position. In 1792 Piazzi noted the motion of 61 Cygni as given above. For a long time the greatest observed proper motion was that of a small star 1830 Groombridge, nearly 7” a year; but others have since been found reaching as much as 10”. Now the spectroscope enables the motion of stars to be detected at a single observation, but only that part of the motion that is in the line of sight. For a complete knowledge of a star’s motion the proper motion and parallax must also be known. When Huggins first applied the Doppler principle to measure velocities in the line of sight,[7] the faintness of star spectra diminished the accuracy; but Vögel, in 1888, overcame this to a great extent by long exposures of photographic plates.