were visible to the naked eye, we should see them encircling the
heavens as a narrow girdle forming perhaps the base of our whole
system of stars. This arrangement of the gaseous or vaporous stars
is one of the most singular facts that modern research has brought
to light. It seems to show that these particular stars form a
system of their own; but how such a thing can be we are still
unable to see.

The question of the form and extent of the Milky Way thus becomes
the central one of stellar astronomy. Sir William Herschel began
by trying to sound its depths; at one time he thought he had
succeeded; but before he died he saw that they were unfathomable
with his most powerful telescopes. Even today he would be a bold
astronomer who would profess to say with certainty whether the
smallest stars we can photograph are at the boundary of the
system. Before we decide this point we must have some idea of the
form and distance of the cloudlike masses of stars which form our
great celestial girdle. A most curious fact is that our solar
system seems to be in the centre of this galactic universe,
because the Milky Way divides the heavens into two equal parts,
and seems equally broad at all points. Were we looking at such a
girdle as this from one side or the other, this appearance would
not be presented. But let us not be too bold. Perhaps we are the
victims of some fallacy, as Ptolemy was when he proved, by what
looked like sound reasoning, based on undeniable facts, that this
earth of ours stood at rest in the centre of the heavens!

A related problem, and one which may be of supreme importance to
the future of our race, is, What is the source of the heat
radiated by the sun and stars? We know that life on the earth is