cross one of them, while the most distant of these objects
may be more than 200,000 light-years from the earth. The spiral
nebulae, more than a million in number, are vast whirling masses
in process of development, but we are not yet certain whether they
should be regarded as "island universes" or as subordinate to the
stellar system which includes our minute group of sun and planets,
the great star clouds of the Milky Way, and the distant globular
star clusters.

[Illustration: Fig. 7. Section of a steel girder for dome covering
the 100-inch telescope, on its way up Mount Wilson.]

These few particulars may give a slight conception of the scale
of the known universe, but a word must be added regarding some
of its most striking phenomena. The great majority of the stars
whose motions have been determined belong to one or the other of
two great star streams, but the part played by these streams in the
sidereal system as a whole is still obscure. The stars have been
grouped in classes, presumably in the order of their evolutional
development, as they pass from the early state of gaseous masses, of
low density, through the successive stages resulting from loss of
heat by radiation and increased density due to shrinkage. Strangely
enough, their velocities in space show a corresponding change,
increasing as they grow older or perhaps depending upon their mass.

It is impossible within these limits to do more than to give some
indication of the scope of the new astronomy. Enough has been said,
however, to assist in appreciating the increased opportunity for
investigation, and the nature of the heavy demands made upon the
modern observatory. But before passing on to describe one of the
latest additions to the astronomer's instrumental equipment, a
word should be added regarding the chief classes of telescopes.

REFRACTORS AND REFLECTORS

Astronomical telescopes are of two types: refractors and reflectors.
A refracting telescope consists of an object-glass composed of
two or more lenses, mounted at the upper end of a tube, which is
pointed at the celestial object. The light, after passing through
the lenses, is brought to a focus at the lower end of the tube, where
the image is examined visually with an eyepiece, or photographed
upon a sensitive plate. The largest instruments of this type are
the 36-inch Lick telescope and the 40-inch refractor of the Yerkes
Observatory.

[Illustration: Fig. 8. Erecting the steel buildi