  The selective light absorption in space has been raised in astronomical
literature. The substance producing the absorption must have some mass; thus
the question is how large it is. We develop a dynamical model of the Milky Way
system, assuming that it can be represented by a flattened ellipsoid of
rotation. We use the spatial distribution of $\delta$-Cephei and Algol type
variable stars, and mean velocities of stars according to Campbell to calculate
the dynamical density of the Milky Way near the Sun, $0.100\,M_\odot/pc^3$. We
find that the dynamical density is equal to the mean density of stars in the
vicinity of the Sun. Our conclusion is that the intrinsic gravity of stars
fully explains their motion, and the existence of any other matter in any
significant quantity seems unlikely. Therefore, the existence of noticeable
selective absorption seems to be absolutely improbable, unless one admits the
existence in the space of particles much smaller than atoms of elements known
to us. Normal absorption may exist if the particle diameter is of the order of
a millimetre or less, and their mass is comparatively small. This absorption
has not yet been reliably detected; the fact that the number of stars increases
with stellar magnitude more slowly than theory requires in case of uniform
distribution of stars in space, can be equally explained by both light
absorption and decrease in number of stars with distance.
