Sir John Herschel, in his observatory at Feldhausen, at the base of
the Table Mountain, witnessed several curious optical effects, arising
from peculiar conditions of the atmosphere incident to the climate of
the Cape. In the hot season “the nights are for the most part superb;”
but occasionally, during the excessive heat and dryness of the sandy
plains, “the optical tranquillity of the air” is greatly disturbed.
In some cases, the images of the stars are violently dilated into
nebular balls or puffs of 15′ in diameter; on other occasions they
form “soft, round, quiet pellets of 3′ or 4′ diameter,” resembling
planetary nebulæ. In the cooler months the tranquillity of the image
and the sharpness of vision are such, that hardly any limit is set
to magnifying power but that which arises from the aberration of the
specula. On occasions like these, optical phenomena of extraordinary
splendour are produced by viewing a bright star through a diaphragm
of cardboard or zinc pierced in regular patterns of circular holes by
machinery: these phenomena surprise and delight every person that sees
them. When close double stars are viewed with the telescope, with a
diaphragm in the form of an equilateral triangle, the discs of the two
stars, which are exact circles, have a clearness and perfection almost
incredible.