photograph some divisions
extending entirely round the nebula, showing that it seems to be
formed of a series of rings; and there are some outlying portions
which form part of the same system. Truly this is a marvellous object.
It is impossible for us to form any conception of the true dimensions
of this gigantic nebula; it is so far off that we have never yet been
able to determine its distance. Indeed, I may take this opportunity of
remarking that no astronomer has yet succeeded in ascertaining the
distance of any nebula. Everything, however, points to the conclusion
that they are at least as far as the stars.

[Illustration: FIG. 9. THE GREAT NEBULA IN ANDROMEDA.]

It is almost impossible to apply the methods which we use in finding
the distance of a star to the discovery of the distance of the
nebulae. These flimsy bodies are usually too ill-defined to admit of
being measured with the precision and delicacy required for the
determination of distance. The measurements necessary for this purpose
can only be made from one star-like point to another similar point. If
we could choose a star in the nebula and determine its distance, then
of course, we have the distance of the nebula itself; but the
difficulty is that we have, in general, no means of knowing whether
the star does actually lie in the object. It may, for anything we can
tell, lie billions of miles nearer to us, or billions of miles further
off, and by merely happening to lie in the line of sight, appear to
glimmer in the nebula itself.

If we have any assurance that the star is surrounded by a mass of this
glowing vapor, then it may be possible to measure that nebula's
distance. It will occasionally happen that grounds can be found for
believing that a star which appears to be in the glowing gas does
veritably lie therein, and is not merely seen in the same direction.
There are hundreds of stars visible in a good drawing or a good
photograph of the famous object in Andromeda, and doubtless large
numbers of these are merely stars which happen to lie in the same line
of sight. The peculiar circumstances attending the history of one star
seem, however, to warrant us in making the assumption that it was
certainly in the nebula. The history of this star is a remarkable one.
It suddenly kindled from invisibility into brilliancy. How is a change
so rapid in the lustre of a star to be accounted for? In a few days
its brightness had undergone an extraordinary incr