he mathematician to determine the amount of that probability. The
case, then, is as follows:--Among all the various systems one must be
true. We cannot lay our finger for certain on the true one, but we can
take that which has the highest degree of probability in its favour, and
thus follow the precept of Butler to which we have already referred. A
mathematician would describe his process by calling it the method of
least squares. Since Herschel's discovery, one hundred years ago, many
an astronomer using observations of hundreds of stars has attacked the
same problem. Mathematicians have exhausted every refinement which the
theory of probabilities can afford, but only to confirm the truth of
that splendid theory which seems to have been one of the flashes of
Herschel's genius.

CHAPTER XXII.

STAR CLUSTERS AND NEBULAE.

Interesting Sidereal Objects--Stars not Scattered uniformly--Star
Clusters--Their Varieties--The Cluster in Perseus--The Globular
Cluster in Hercules--The Milky Way--A Cluster of Minute Stars--The
Magellanic Clouds--Nebulae distinct from Clouds--Number of known
Nebulae--The Constellation of Orion--The Position of the Great
Nebula--The Wonderful Star th Orionis--The Drawing of the
Great Nebula in Lord Rosse's Telescope--Photographs of this
Wonderful Object--The Great Nebula in Andromeda--The Annular Nebula
in Lyra--Resemblance to Vortex Rings--Planetary Nebulae--Drawings of
Several Remarkable Nebulae--Nature of Nebulae--Spectra of
Nebulae--Their Distribution; the Milky Way.

We have already mentioned Saturn as one of the most glorious telescopic
spectacles in the heavens. Setting aside the obvious claims of the sun
and of the moon, there are, perhaps, two other objects visible from
these latitudes which rival Saturn in the splendour and the interest of
their telescopic picture. One of these objects is the star cluster in
Hercules; the other is the great nebula in Orion. We take these objects
as typical of the two great classes of bodies to be discussed in this
chapter, under the head of Star Clusters and Nebulae.

The stars, which to the number of several millions bespangle the sky,
are not scattered uniformly. We can see that while some regions are
comparatively barren, others contain stars in profusion. Sometimes we
have a small group, like the Pleiades; sometimes we have a stupendous
region of the heavens strewn over with stars, as in