residence at Feldhausen, Herschel was fortunate
enough to witness one of those singular changes in the aspect of the
firmament which occasionally challenge the attention even of the
incurious, and excite the deepest wonder of the philosophical observer.
Immersed apparently in the Argo nebula is a star denominated Eta
Carinae. When Halley visited St. Helena in 1677, it seemed of the fourth
magnitude; but Lacaille in the middle of the following century, and
others after him, classed it as of the second. In 1827 the traveller
Burchell, being then at St. Paul, near Rio Janeiro, remarked that it had
unexpectedly assumed the first rank--a circumstance the more surprising
to him because he had frequently, when in Africa during the years 1811
to 1815, noted it as of only fourth magnitude. This observation,
however, did not become generally known until later. Herschel, on his
arrival at Feldhausen, registered the star as a bright second, and had
no suspicion of its unusual character until December 16, 1837, when he
suddenly perceived its light to be almost tripled. It then far outshone
Rigel in Orion, and on the 2nd of January following it very nearly
matched Alpha Centauri. From that date it declined; but a second
and even brighter maximum occurred in April, 1843, when Maclear, then
director of the Cape Observatory, saw it blaze out with a splendour
approaching that of Sirius. Its waxings and wanings were marked by
curious "trepidations" of brightness extremely perplexing to theory. In
1863 it had sunk below the fifth magnitude, and in 1869 was barely
visible to the naked eye; yet it was not until eighteen years later that
it touched a minimum of 7.6 magnitude. Soon afterwards a recovery of
brightness set in, but was not carried very far; and the star now shines
steadily as of the seventh magnitude, its reddish light contrasting
effectively with the silvery rays of the surrounding nebula. An attempt
to include its fluctuations within a cycle of seventy years[125] has
signally failed; the extent and character of the vicissitudes to which
it is subject stamping it rather as a species of connecting link between
periodic and temporary stars.[126]

Among the numerous topics which engaged Herschel's attention at the Cape
was that of relative stellar brightness. Having contrived an
"astrometer" in which an "artificial star," formed by the total
reflection of moonlight from the base of a prism, served as a standard
of comparison, he wa