in sixty seconds, another in sixty minutes, and
another in twelve hours; so certain may we be
that Saturn's nearest satellite rushes round its
parent planet in less than a day, at the very
short distance of not two of the planet's
diameters, while Jupiter's corresponding moon takes
more than a day and a half (brief space enough),
at the height of three diameters in the sky, to
complete the tour of her central world. The
revolutions of those satellites, we feel quite
assured, are no more a deception, an optical
illusion, than the balls which a juggler keeps
circling in the air a couple of yards in front of
us.

To extend our discoveries beyond the solar
system, the reader here shall be reminded of
what lie may have read before.* Light, we
know, may be dissected. A sunbeam darting
through a keyhole, and falling on a triangular
prism of glass, is resolved into separate rays,
which give the respective colours of the rainbow.
The ray, so far analysed, is called the prismatic
spectrum, and forms an oblong stripe or image
of coloured light. From Newton's time until
quite lately, the dissection of the spectrum went
no further.
* See Recent Discoveries concerning Light, in
vol. v., page 270.

By skilful observations with prisms, Dr.
Wollaston, and afterwards Herr Fraunhofer,
discovered that the oblong image, obtained by
decomposing sunlight, was crossed in various places
by dark lines, which always occupy the same
relative position when the prismatic spectrum
is obtained from the sun; while the light, either
from fixed stars or from artificial sources, gives
a spectrum either without lines, or crossed by
lines occupying different positions. The important
point was, that the position and number of
the lines were invariably the same when the
light was obtained from the same source. These
lines were called Fraunhofer's lines, and so the
matter rested for a while.

But recently, the discovery was made that
various mineral substances, entering into flame
or fire, not only alter the colour of the flame, but
cause a variation in the spectrum obtained from
the light given out. Such variations were found
so constant and unfailing in betraying the
presence of that substance in the flame, that
they allowed the practice of a new mode of
analysis, consequently called spectral analysis.
Thus sodium, the metallic basis of common salt,
mixed with burning combustibles, gives a ghastly
yellow colour to the flame, and a peculiar marking
to the spectrum. It shows a single vivid
yellow band, due to the combustion of the metal.
Every other combination in which the same
metal exists, always produces the same result;
so that by the presence or absence of the
particular band of colour which belongs to
sodium, the presence or absence of that metal
can be detected with perfect certainty. The
same is true of calcium, the metallic base of
lime, and other like substances; also of copper
and the other ordinary metals, each of which
communicates a colour to flame, and exhibits its
own proper bands in the spectrum.

Spectral analysis, thus briefly sketched, has
been employed to examine the light of the sun;
and it reveals to us the fact that the sun
contains a number of metals identical with those
which are found on earth. This delicate but
certain mode of investigation, which we owe to
Messrs. Kirchhoff and Bunsen, has since been
applied to the still more difficult task of
ascertaining the constitution of the nebulæ.

The nebulæ are cloud-like luminous bodies, of
various shape, shining with a pale uncertain
glimmer, some of which have been calculated by
Sir John Herschel to be situated at such
enormous distances, that their light takes no less
than two millions of years to travel from them
to our earth. Some few (as the nebula in the
constellation Andromeda, and the Magellanic
clouds which revolve round the South Pole)
are visible to the naked eye; but the greater
majority are telescopic objects. The study of
the nebulæ followed the invention of the
telescope. What are they? That is the question.

In 1612, four years after the accidental
discovery of the telescope, Simon Marius described
the nebula in Andromeda. In 1656, Huyghens
traced the image of that which is observed in
the sword of Orion. These two clouds,
Humbolt says, might be regarded as a more or less
advanced condensation of vapoury matter and
cosmic nebulosity. Marius, when he compares
the nebula in Andromeda to the light of a
candle seen through a semi-transparent body,
clearly indicates the difference which exists
between the nebula; properly so called and the
clusters of stars more or less distinct, such
as the Pleiades and others, observed by
Galileo.

Galileo, not considering the nebula of Andromeda
worthy of any special attention—although
the most powerful instruments have not yet
discovered in it any stars—took all nebulæ, and
the Milky Way itself, for luminous clusters of
stars huddled close together. He made no
distinction between what is cloud and what
is stars, as Huyghens did in the nebula of
Orion.

Since those days, it is to William Herschel
that we owe the greater part of our knowledge
respecting the nebulæ.. According to him, they
cover one two-hundred-and-seventieth part of
the whole visible firmament. The number of
nebulæ whose place has been determined in
light ascension and declination, already exceeds
three thousand six hundred. All of these, seen
through a telescope, appear, at the first glance,
completely different from the other heavenly
bodies. But in proportion as the means of
observation are increased by the construction of
more and more powerful instruments, it is found
possible to resolve an increasing number of
nebulæ; i.e. to ascertain that many of these
luminous masses are, as Galileo supposed,
accumulations of stars crowded together, whose
minuteness (in consequence of their prodigious