Milky Way, and so very many in or
near that wonderful belt? The first attempt to give an answer to these
questions was made by Thomas Wright, an instrument maker in London, in a
book published in 1750. He supposed the stars of our sidereal system to
be distributed in a vast stratum of inconsiderable thickness compared
with its length and breadth. If we had a big grindstone made of glass,
in which had become uniformly imbedded a vast quantity of grains of sand
or similar minute particles, and if we were able to place our eye
somewhere near the centre of this grindstone, it is easy to see that we
should see very few particles near the direction of the axle of the
grindstone, but a great many if we looked towards any point of the
circumference. This was Wright's idea of the structure of the Milky Way,
and he supposed the sun to be situated not very far from the centre of
this stellar stratum.

[Illustration: PLATE F.

o CENTAURI.

_From a Drawing in the Publications of Harvard College Observatory._]

If the Milky Way itself did not exist--and we had simply the fact to
build on that the stars appeared to increase rapidly in number towards a
certain circle (almost a great circle) spanning the heavens--then the
disc theory might have a good deal in its favour. But the telescopic
study of the Milky Way, and even more the marvellous photographs of its
complicated structure produced by Professor Barnard, have given the
death blow to the old theory, and have made it most reasonable to
conclude that the Milky Way is really, and not only apparently, a mighty
stream of stars encircling the heavens. We shall shortly mention a few
facts which point in this direction. A mere glance is sufficient to show
that the Milky Way is not a single belt of light; near the constellation
Aquila it separates into two branches with a fairly broad interval
between them, and these branches do not meet again until they have
proceeded far into the southern hemisphere. The disc theory had, in
order to explain this, to assume that the stellar stratum was cleft in
two nearly to the centre. But even if we grant this, how can we account
for the numerous more or less dark holes in the Milky Way, the largest
and most remarkable of which is the so-called "coal sack" in the
southern hemisphere? Obviously we should have to assume the existence of
a number of tunnels, drilled through the disc-like stratum, and by some
strange sympathy all directed towards the s