During a year’s work at the Cape, Henderson undertook a series of observations on the bright southern star, α Centauri, with a view to determining its parallax. These observations were made in 1832 and 1833, but were not reduced until Henderson’s return to Scotland. At length, on January 3, 1839, he announced to the Royal Astronomical Society that he had succeeded in measuring the parallax of α Centauri, which he determined as about one second of arc, corresponding to a distance of about twenty billions of miles. This result was confirmed by the observations of _Thomas Maclear_ (1794-1879), his successor at the Cape, and by those of later observers, notably Sir David Gill, who has reduced the parallax to 0·75″. Other determinations of stellar parallax, some genuine and others illusory, were made soon after these successful observations. C. A. F. Peters and Otto Struve at Pulkowa were among the most famous parallax-hunters in the middle of the century. One of the most successful searchers after parallax was the German astronomer _Friedrich Brünnow_ (1821-1891), who was employed from 1865 to 1874 as Astronomer-Royal of Ireland. He determined the parallax of Vega as 0·13″, and this was confirmed in 1886 by Hall at Washington: while he measured the parallax of the star Groombridge 1830, which turned out to be 0·09″. He resigned his post in 1874, and his successor at Dublin Observatory proved to be his successor also in this branch of astronomy. _Robert Stawell Ball_, born in Dublin in 1840, was astronomer to Lord Rosse in 1865 and 1866, and became in 1874 Astronomer-Royal of Ireland in succession to Brünnow, a position which he filled until his appointment in 1892 as Professor of Astronomy at Cambridge, and director of the observatory there. During his term of office in Dublin he undertook, in 1881, a “sweeping search” for large parallaxes, thereby disproving certain ideas as to the proximity to the Earth of red and temporary stars; while he also determined the parallax of the star 1618 Groombridge.