we notice
in the arms (which we see edge-on) of the nebula in Coma Berenices. The
hypothesis, therefore, that our universe is a spiral nebula has much to
be said for it. If it be accepted it greatly increases our estimate of
the size of the material universe. For our central, lens-shaped system
is calculated to extend towards the Milky Way for more than twenty
thousand times a million million miles, and about a third of this
distance towards what we have called the poles. If, as we suppose, each
spiral nebula is an independent stellar universe comparable in size with
our own, then, since there are hundreds of thousands of spiral nebulae,
we see that the size of the whole material universe is indeed beyond our
comprehension.

[Illustration: _Photo: Mount Wilson Observatory._

FIG. 26.--A SPIRAL NEBULA SEEN EDGE-ON

Notice the lens-shaped formation of the nucleus and the arm stretching
as a band across it. See reference in the text to the resemblance
between this and our stellar universe.]

[Illustration: _Photo: H. J. Shepstone._

100-INCH TELESCOPE, MOUNT WILSON

A reflecting telescope: the largest in the world. The mirror is situated
at the base of the telescope.]

[Illustration:

________________________________________________________________
| |
| THE SOLAR SYSTEM |
|________________________________________________________________|
| | | | | |
| | MEAN DISTANCE | PERIOD OF | | |
| NAME | FROM SUN (IN | REVOLUTION | DIAMETER | NUMBER OF |
| | MILLIONS OF | AROUND SUN | (IN MILES) | SATELLITES |
| | MILES) | (IN YEARS) | | |
|_________|_______________|____________|____________|____________|
| | | | | |
| MERCURY | 36.0 | 0.24 | 3030 | 0 |
| VENUS | 67.2 | 0.62 | 7700 | 0 |
| EARTH | 92.9 | 1.00 | 7918 | 1 |
| MARS | 141.5 | 1.88 | 4230 | 2 |
| JUPITER | 483.3 | 11.86 | 86500 | 9 |
| SATURN | 886.0 | 29.46 | 73000 | 10 |
| URANUS | 1781.9 | 84.02 | 31900 | 4 |
| NEPTUNE | 2971.6 |