FEBRUARY 15, I917| 
These are castings of the best steel. The driving 
clock is at the lower end of the axis. The worm- 
wheel by which the axis is turned is g ft. in cir- 
cumference, and weighs 2 tons. When hoisted 
into its position the entire weight of the polar axis 
was about 14 tons. The approximate positions of 
its bearings had been determined beforehand by 
means of a wire stretched from one to the other 
and adjusted by observations with a surveyor’s 
transit. The declination axis is a steel forging 
5¢ tons in weight, 144 ft. long, and 154 in. in 
diameter. 
The telescope tube is composed of three sec- 
tions. At the bottom is the steel mirror cell, 
7% ft. in diameter, which, with mirror counter- 
poises and mirror, weighs 6 tons. It is bolted 
to the central section, a steel casting 6 ft. long 
Fic. 3.—The 72-in, reflector, October 22, 1916. 
and weighing 7 tons; while surmounting this 
is the skeleton tube, more than 23 ft. long, and 
weighing, with attachments, about 2 tons. 
Through the centre of the mirror there is a 
circular hole ro} in. in diameter, and behind this 
the eyepiece is placed when the telescope is used 
as a Cassegrain reflector. There are two 4-in. 
finders, one on each side of the tube, and one 
of aperture 7 in. and focal length 30 ft. To 
assist in the adjustment of the axis of the tele- 
scope, the long-focus finder was mounted centrally 
in the tube. The objective can be seen at the 
upper end of the tube (Figs. 3 and 4), and the eye- 
piece at the centre of-the mirror cell. The cor- 
rection required to be made to the upper end of 
the polar axis was 1/50 in. in altitude and 1/25 in. 
in azimuth. The observing platform is seen near 
NO. 2468, VOL. 98] 
NATURE 
473 
the upper end of the tube. It is moved up and 
down the shutter opening on rails fastened to the 
main ribs by cables operated by an electric motor. 
At each side of the platform are wings, one of 
which is shown in the figure, movable in and out, 
to embrace the tube at any convenient position 
for observing. 
In order to operate the telescope there are two 
similar switchboards on the south pier, one on 
each side, that one being used which happens 
to be most convenient. In operation the assistant 
will stand at either one of these switchboards and 
set the telescope and dome to the required ap- 
proximate position by means of the three operating 
switches shown on the switchboard. The one on 
the left turns the dome east or west at the rate 
of 60° per minute, the centre one moves the tube 
north or-south 45° per minute, and the switch 
The shutters are 
open their full width (r5 ft.), and the wind-shield is seen 
before the telescope. 
Fic. 4.—The Observatory, October 22, 1916. 
to the right moves the whole telescope east or 
west on the polar axis 45° per minute. The push- 
button switches above serve to illuminate the 
declination, sidereal, and hour circles, by which 
the approximate positions are obtained, and to 
clamp and unclamp the slow motions in R.A. and 
Declination. The fine setting and guiding are per- 
formed by the observer at either the upper or 
lower end of the tube by small keyboards carried 
by him when observing, the one at the lower end 
being shown attached tothe cable passing across 
the lower end of the cell. By pressing the keys 
on this board two motions are given in either 
direction in either co-ordinate, the fast one for 
fine setting 1° in five minutes, and the slow one 
for guiding at 1/20th of this speed. The tele- 
scope responds instantly to all these motions, and 
can be handled with as great ease as a small 
refractor. 
