robable that these continuous spectra are really an aggregate of very
faint luminous lines.

A list of all the nebulae known to have a gaseous spectrum would now
contain about eighty members. In addition to the planetary nebulae, many
large and more diffused nebulae belong to this class, and this is also
the case with the annular nebula in Lyra and the great nebula of Orion.
It is needless to say that it is of special interest to find this grand
object enrolled among the nebulae of a gaseous nature. In this nebula
Copeland detected the wonderful D3 line of helium at a time when
"helium" was a mere name, a hypothetical something, but which we now
know to be an element very widely distributed through the universe. It
has since been found in several other nebulae. The ease with which the
characteristic gaseous spectrum is recognised has suggested the idea of
sweeping the sky with a spectroscope in order to pick up new planetary
nebulae, and a number of objects have actually been discovered by
Pickering and Copeland in this manner, as also more recently by
Pickering by examining spectrum photographs of various regions of the
sky. Most of these new objects when seen through a telescope look like
ordinary stars, and their real nature could never have been detected
without the spectroscope.

When we look up at the starry sky on a clear night, the stars seem at
first sight to be very irregularly distributed over the heavens. Here
and there a few bright stars form characteristic groups, like Orion or
the Great Bear, while other equally large tracts are almost devoid of
bright stars and only contain a few insignificant ones. If we take a
binocular, or other small telescope, and sweep the sky with it, the
result seems to be the same--now we come across spaces rich in stars;
now we meet with comparatively empty places. But when we approach the
zone of the Milky Way, we are struck with the rapid increase of the
number of stars which fill the field of the telescope; and when we reach
the Milky Way itself, the eye is almost unable to separate the single
points of light, which are packed so closely together that they produce
the appearance to the naked eye of a broad, but very irregular, band of
dim light, which even a powerful telescope in some places can hardly
resolve into stars. How are we to account for this remarkable
arrangement of the stars? What is the reason of our seeing so few at the
parts of the heavens farthest from the