consequently become
heated to incandescence in its passage, very much like what happens to a
meteor when moving through our atmosphere. The illumination thus set up
temporarily in one point, being transmitted through the nebulous wastes
around with the ordinary velocity of light, had gradually rendered this
surrounding matter visible. On the assumptions required to fit in with
such a theory, it was shown that Nova Persei must be at a distance from
which light would take about three hundred years in coming to us. The
actual outburst of illumination, which gave rise to this temporary star,
would therefore have taken place about the beginning of the reign of
James I.

Some recent investigations with regard to Nova Persei have, however,
greatly narrowed down the above estimate of its distance from us. For
instance, Bergstrand proposes a distance of about ninety-nine light
years; while the conclusions of Mr. F.W. Very would bring it still
nearer, _i.e._ about sixty-five light years.

The last celestial objects with which we have here to deal are the
_Nebulae_. These are masses of diffused shining matter scattered here and
there through the depths of space. Nebulae are of several kinds, and have
been classified under the various headings of Spiral, Planetary, Ring,
and Irregular.

A typical _spiral_ nebula is composed of a disc-shaped central portion,
with long curved arms projecting from opposite sides of it, which give
an impression of rapid rotatory movement.

The discovery of spiral nebulae was made by Lord Rosse with his great
6-foot reflector. Two good examples of these objects will be found in
Ursa Major, while there is another fine one in Canes Venatici (see Plate
XXII., p. 314), a constellation which lies between Ursa Major and
Booetes. But the finest spiral of all, perhaps the most remarkable nebula
known to us, is the Great Nebula in the constellation of Andromeda, (see
Plate XXIII., p. 316)--a constellation just further from the pole than
Cassiopeia. When the moon is absent and the night clear this nebula can
be easily seen with the naked eye as a small patch of hazy light. It is
referred to by Al Sufi.

[Illustration: PLATE XXII. SPIRAL NEBULA IN THE CONSTELLATION OF CANES
VENATICI

From a photograph by the late Dr. W.E. Wilson, D.Sc., F.R.S.

(Page 314)]

Spiral nebulae are white in colour, whereas the other kinds of nebula
have a greenish tinge. They are also by far the most numerous; and the
late P