e
which this brother and sister had arranged to carry out as the object of
their life-long devotion. The discoveries which Herschel was destined to
make were to be reckoned not by tens or by hundreds, but by thousands.
The records of these discoveries are to be found in the "Philosophical
Transactions of the Royal Society," and they are among the richest
treasures of those volumes. It was left to Sir John Herschel, the only
son of Sir William, to complete his father's labour by repeating the
survey of the northern heavens and extending it to the southern
hemisphere. He undertook with this object a journey to the Cape of Good
Hope, and sojourned there for the years necessary to complete the great
work.

[Illustration: Fig. 97.--The Constellation of Orion, showing the
Position of the Great Nebula.]

As the result of the gigantic labours thus inaugurated and continued by
other observers, there are now about eight thousand nebulae known to us,
and with every improvement of the telescope fresh additions are being
made to the list. They differ from one another as eight thousand pebbles
selected at random on a sea-beach might differ--namely, in form, size,
colour, and material--but yet, like the pebbles, bear a certain generic
resemblance to each other. To describe this class of bodies in any
detail would altogether exceed the limits of this chapter; we shall
merely select a few of the nebulae, choosing naturally those of the most
remarkable character, and also those which are representatives of the
different groups into which nebulae may be divided.

[Illustration: PLATE XIV.

THE GREAT NEBULA IN ORION.]

We have already stated that the great nebula in the constellation of
Orion is one of the most interesting objects in the heavens. It is alike
remarkable whether we consider its size or its brilliancy, the care with
which it has been studied, or the success which has attended the efforts
to learn something of its character. To find this object, we refer to
Fig. 97 for the sketch of the chief stars in this constellation, where
the letter A indicates the middle one of the three stars which form the
sword-handle of Orion. Above the handle will be seen the three stars
which form the well-known belt so conspicuous in the wintry sky. The
star A, when viewed attentively with the unaided eye, presents a
somewhat misty appearance. In the year 1618 Cysat directed a telescope
to this star, and saw surrounding it a curious luminous