yet attained--a fact which wholly discredits all previous
inferences as to the limits of our sidereal system.
Hence, notwithstanding the wonderful instrumental
advances of the nineteenth century, knowledge of the
exact form and extent of our universe seems more
unattainable than it seemed a century ago.


The Structure of Nebulae

Yet the new instruments, while leaving so much
untold, have revealed some vastly important secrets of
cosmic structure. In particular, they have set at rest
the long-standing doubts as to the real structure and
position of the mysterious nebulae--those lazy masses,
only two or three of them visible to the unaided eye,
which the telescope reveals in almost limitless abundance,
scattered everywhere among the stars, but
grouped in particular about the poles of the stellar
stream or disk which we call the Milky Way.

Herschel's later view, which held that some at least
of the nebulae are composed of a "shining fluid," in
process of condensation to form stars, was generally
accepted for almost half a century. But in 1844, when
Lord Rosse's great six-foot reflector--the largest telescope
ever yet constructed--was turned on the nebulae,
it made this hypothesis seem very doubtful. Just as
Galileo's first lens had resolved the Milky Way into
stars, just as Herschel had resolved nebulae that resisted