seous, as well as
those termed "irregular," which frequent the region of the Milky Way.
Their constitution usually betrays itself to the eye by their blue or
greenish colour; while those yielding a continuous spectrum are of a
dull white. Among the more remarkable of these are the well-known nebula
in Andromeda, and the great spiral in Canes Venatici; and, as a general
rule, the emissions of all such nebulae as present the appearance of
star-clusters grown misty through excessive distance are of the same
kind. It would, however, be eminently rash to conclude thence that they
are really aggregations of sun-like bodies. The improbability of such an
inference has been greatly enhanced by the occurrence, at an interval of
a quarter of a century, of stellar outbursts in the midst of two of
them. For it is practically certain that the temporary stars were
equally remote with the hazy formations they illuminated; hence, if the
constituent particles of the latter be suns, the incomparably vaster
orbs by which their feeble light was well-nigh obliterated must, as was
argued by Mr. Proctor, have been on a scale of magnitude such as the
imagination recoils from contemplating. Nevertheless, Dr. Scheiner, not
without much difficulty, obtained, in January, 1899, spectrographic
prints of the Andromeda nebula, indicative, he thought, of its being a
cluster of solar stars.[1515] Sir William and Lady Huggins, on the other
hand, _saw_, in 1897, bright intermixed with dark bands in the spectrum
of the same object.[1516] And Mr. Maunder conjectures all "white" nebulae
to be made up of sunlets in which the coronal element predominates,
while chromospheric materials assert their presence in nebulae of the
"green" variety.[1517]

Among the ascertained analogies between the stellar and nebular systems
is that of variability of light. On October 11, 1852, Mr. Hind
discovered a small nebula in Taurus. Chacornac observed it at Marseilles
in 1854, but was confounded four years later to find it vanished.
D'Arrest missed it October 3, and redetected it December 29, 1861. It
was easily seen in 1865-66, but invisible in the most powerful
instruments from 1877 to 1880.[1518] Barnard, however, made out an
almost evanescent trace of it, October 15, 1890, with the great Lick
telescope,[1519] and saw it easily in the spring of 1895, while six
months later it evaded his most diligent search.[1520] Then again, on
September 28, 1897, the Yerkes 40-inch disclos