=======+==========+=========+============+==============+===========
| Distance | Radius | Mass | Period | Density
-------+----------+---------+------------+--------------+-----------
Nebula |6 × 10⁶ LY|6 × 10³LY|1.6 × 10¹⁰ S|5.4 × 10⁷ year| 5.6 × 10⁻¹
Galaxy | |6 × 10³ | 10⁹ | 2.5 × 10⁸ | 1.2 × 10⁻²
-------+----------+---------+------------+---------------+----------
The velocities of escape would be about 280 km for the nebula and 170
km for the galaxy. Considering the problematic nature of the data,
the agreement is such as to lend some color to the hypothesis that
the spirals are stellar systems at distances to be measured often in
millions of light-years. The computations by O. H. Truman[8] and by R.
K. Young and W. E. Harper[9] of the motion of our system with respect
to the spirals, based on the radial velocities of the spirals, are
another and stronger argument for the hypothesis.
[8] _Popular Astronomy_, =24=, 111, 1916.
[9] _Journal of the R.A.S., Canada_, =10=, 134, 1916.
The principal objection lies in their apparently systematic
distribution with respect to the Milky Way. The matter is usually
stated in the form that “spirals avoid the Milky Way.” There are less
than 300 nebulae _known_ to be spiral in form. The greatest of them all
is just on the border. It is suggested that the spirals seem to follow
the distribution of the small faint nebulae. If this is true, the most
that can be definitely stated is that they tend to cluster in certain
regions of the sky, in one of which the north pole of the galaxy is
located.
As the small nebulae and the spirals inhabit the same regions of the
sky, it is probable that the order of distance of the two classes is
the same. Classes e, f, may even turn out to be spirals themselves.
This, however, is a question for large instruments, and is outside the
scope of the present paper.