members of the Solar System bear to those of the sun. In this case, at
any rate, the conjecture hazarded about Algol, that the star revolves
around a dark central orb, would be insufficient to account for the
observed variation.

Nearly midway between [beta] and [gamma] lies the wonderful ring-nebula
57 M, of which an imperfect idea will be conveyed by the last figure of
Plate 3. This nebula was discovered in 1772, by Darquier, at Toulouse.
It is seen as a ring of light with very moderate telescopic power. In a
good 3-1/2-inch telescope the nebula exhibits a mottled appearance and a
sparkling light. Larger instruments exhibit a faint light within the
ring; and in Lord Rosse's great Telescope "wisps of stars" are seen
within, and faint streaks of light stream from the outer border of the
ring. This nebula has been subjected to spectrum-analysis by Mr.
Huggins. It turns out to be a gaseous nebula! In fact, ring-nebulæ--of
which only seven have been detected--seem to belong to the same class as
the planetary nebulæ, all of which exhibit the line-spectrum indicative
of gaseity. The brightest of the three lines seen in the spectrum of the
ring-nebula in Lyra presents a rather peculiar appearance, "since it
consists," says Mr. Huggins, "of two bright dots, corresponding to
sections of the ring, and between these there is not darkness, but an
excessively faint line joining them. This observation makes it probable
that the faint nebulous matter occupying the central portion is similar
in constitution to that of the ring."

The constellation Hercules also contains many very interesting objects.
Let us first inspect a nebula presenting a remarkable contrast with that
just described. I refer to the nebula 13 M, known as Halley's nebula
(Plate 3). This nebula is visible to the naked eye, and in a good
telescope it is a most wonderful object: "perhaps no one ever saw it for