COSMOLOGY OF THE UNIVERSE 
43 
one-sixth this distance in thickness; (3) that the solar system is 
not far from the center of the galactic system. According to this 
postulate our system is comparatively simple in structure and ex- 
tremely finite in dimensions. The classes of objects visible in 
our telescope and generally considered as constituting the universe 
consist of (1) stars with their attendant system of planets, in- 
cluding the double and multiple systems; (2) star clusters of 
two main types — globular and loose clusters; (3) nebulae, of 
which there are four types — (a) bright, diffuse, undifferentiated 
masses of enormous size and wholly gaseous in character, (b) dark 
nebulae of remarkable form and size and associated in practically 
every case with bright nebulae, (c) planetary nebulae including 
the ring form. These are very few in number and practically all 
in the “Milky Way.” They give a gaseous spectrum and have 
high radial velocities, (d) The spirals, with their peculiar space 
distribution, clustering about the poles of the “Milky Way.” Their 
numbers run into the hundreds of thousands if not millions. 
Their spectrum resembles that of the denser clusters and bright 
star clouds in our galaxy. Their space velocities are enormous. 
Astronomical research is as noted for its methods as it is for 
its results. We have seen on the instrumental side the most 
astounding development recorded by any science ; from the sextant 
of Tyco Brahe through the telescope of Galileo to the one-hund- 
red-inch Hooker ; from the spectroscope of Huggins to the spectro- 
heliograph of Hale; from the micrometer of Hershell to the inter- 
ferometer of Michelson. On the theoretical side the list of notable 
names is too long to mention more than the extreme limits, com- 
mencing possibly with Olbers and expanding into a great galaxy 
of present-day men. Indeed, very recently there has been de- 
veloped a new branch of science known as Statistical Astron- 
omy. As Theoretical Astronomy attacked the problems of the 
solar system, so statistical astronomy bids more than fair to solve 
the question of the extent and form of our siderial universe. 
Kapteyn in Europe and Russell in America are probably the 
pioneers of this new astronomy. Within the last ten years Dr. 
Russell has demonstrated a remarkable progression of absolute 
magnitude with spectral type and suggested the strong possibility 
of the existence of so-called giant and dwarf stars. Dr. Walter 
S. Adams of the Mount Wilson observatory has discovered a 
similar relation between the relative intensities of certain sets of 
spectral lines in stars of known distances and their absolute mag- 
nitude. We have here then a method which will determine the 
