ling of the telescope.
The massive sections of the instrument, some of which weigh as
much as ten tons each, were constructed at Quincy, Mass., where
machinery sufficiently large to build battleships was available.
They were then shipped to California, and transported to the summit
of Mount Wilson over a road built for this purpose by the construction
division of the observatory, which also built the pier on which the
telescope stands, and erected the steel building and dome that
cover it.

[Illustration: Fig. 12. Large irregular nebula and star cluster
in Sagittarius (Duncan).

Photographed with the 60-inch telescope.]

[Illustration: Fig. 13. Faint spiral nebula in the constellation
of the Hunting Dogs (Pease).

Photographed with the 60-inch telescope.]

The parts of the telescope which are moved by the driving-clock
weigh about 100 tons, and it was necessary to provide means of
reducing the great friction on the bearings of the polar axis. To
accomplish this, large hollow steel cylinders, floating in mercury
held in cast-iron tanks, were provided at the upper and lower ends
of the polar axis. Almost the entire weight of the instrument is
thus floated in mercury, and in this way the friction is so greatly
reduced that the driving-clock moves the instrument with perfect
ease and smoothness.

The 100-inch mirror rests at the bottom of the telescope tube on
a special support system, so designed as to prevent any bending of
the glass under its own weight. Electric motors, forty in number, are
provided to move the telescope rapidly or slowly in right ascension
(east or west) and in declination (north or south), for focussing
the mirrors, and for many other purposes. They are also used for
rotating the dome, 100 feet in diameter, under which the telescope
is mounted, and for opening the shutter, 20 feet wide, through
which the observations are made.

A telescope of this kind can be used in several different ways.
The 100-inch mirror has a focal length of about 42 feet, and in
one of the arrangements of the instrument, the photographic plate
is mounted at the centre of the telescope tube near its upper end,
where it receives directly the image formed by the large mirror. In
another arrangement, a silvered glass mirror, with plane surface,
is supported near the upper end of the tube at an angle of 45 deg., so
as to form the image at the side of the tube, where the photographic
plate can be placed. In this case, the