heless, to astronomers it still remained in 1656, when Huygens
discerned, "as it were, an hiatus in the sky, affording a glimpse of a
more luminous region beyond."[42] Halley in 1716 knew of six nebulae,
which he believed to be composed of a "lucid medium" diffused through
the ether of space.[43] He appears, however, to have been unacquainted
with some previously noticed by Hevelius. Lacaille brought back with him
from the Cape a list of forty-two--the first-fruits of observation in
Southern skies--arranged in three numerically equal classes;[44] and
Messier (nicknamed by Louis XV. the "ferret of comets"), finding such
objects a source of extreme perplexity in the pursuit of his chosen
game, attempted to eliminate by methodising them, and drew up a
catalogue comprising, in 1781, 103 entries.[45]

These preliminary attempts shrank into insignificance when Herschel
began to "sweep the heavens" with his giant telescopes. In 1786 he
presented to the Royal Society a descriptive catalogue of 1,000 nebulae
and clusters, followed, three years later, by a second of as many more;
to which he added in 1802 a further gleaning of 500. On the subject of
their nature his views underwent a remarkable change. Finding that his
potent instruments resolved into stars many nebulous patches in which no
signs of such a structure had previously been discernible, he naturally
concluded that "resolvability" was merely a question of distance and
telescopic power. He was (as he said himself) led on by almost
imperceptible degrees from evident clusters, such as the Pleiades, to
spots without a trace of stellar formation, the gradations being so well
connected as to leave no doubt that all these phenomena were equally
stellar. The singular variety of their appearance was thus described by
him:--

"I have seen," he says, "double and treble nebulae variously arranged;
large ones with small, seeming attendants; narrow, but much extended
lucid nebulae or bright dashes; some of the shape of a fan, resembling an
electric brush, issuing from a lucid point; others of the cometic shape,
with a seeming nucleus in the centre, or like cloudy stars surrounded
with a nebulous atmosphere; a different sort, again, contain a
nebulosity of the milky kind, like that wonderful, inexplicable
phenomenon about Theta Orionis; while others shine with a fainter,
mottled kind of light, which denotes their being resolvable into
stars."[46]

"These curious objects" he consid