The Caldwell Objects

of the heavens with it. For his sweeps he usually
used a magnification of 157x, which offered a
small 15' field of view. The eyepiece was
mounted on the upper side of the telescope's
octagonal tube, where it made a 45° angle with
the vertical when the telescope was horizontal.
Herschel preferred this observing position,
looking down as if at a reading desk, and used
this arrangement until September 22, 1786, in
Sweep 600, when he changed to the "front view,"
as he called it. By removing the small diagonal
mirror and placing the eyepiece at the front end
of the tube, he found the images to be just as
good and well defined as at the Newtonian focus
and the light "incomparably more brilliant." He
found this position very convenient.
Before starting a sweep, Herschel pointed the
telescope toward the meridian and observed
from a 9-foot-long movable gallery. The telescope
hung freely in the center of its mount, and the
observer moved it with a handle fastened near
the eyepiece. Drawing the telescope along by
hand in a lateral motion, Herschel would walk
backward and forward on the movable gallery.
This enabled him to perform slow oscillations,
12° to 14° in breadth, each taking generally 4 to 5
minutes. At the end of each oscillation he took
short notes on what he had seen. If he had found
a new nebula or star cluster, he carefully noted
the stars in the field of view of both the
finderscope and the main telescope, so as to be
able to locate the object again. After this
procedure, Herschel would then raise or lower
the instrument about 8' or 10' along a north-south
line and then perform another oscillation. He
then continued for 10, 20, or 30 oscillations
according to circumstances, and the whole sweep
was numbered and registered in his journal.
At first, Herschel took his own notes, but
after 41 sweeps the disadvantages of doing so
became obvious. Because he needed light to
write, his eye could never achieve the dark
adaptation required for his "delicate observations."
Furthermore, the gallery was not well
placed, and holding down the heavy telescope at
the ends of the oscillations, when it tended to
arch upward, led him to become fatigued.
Herschel therefore began to sweep by elevating
and lowering the telescope manually with a
vertical motion. This method increased the labor
of moving the telescope, so he hired workmen to
do the job. Caroline wrote down his observations,
repeating everything back to him so he could
verify the "picture before me" while preserving
his night vision. A few sweeps (numbers 42 to 45)
were made experimentally, and with Sweep 46
on December 18, 1783, Herschel's long survey of
the heavens commenced. This series was not
closed until September 30,1802, with Sweep No.
1112.
Each object's description and position relative
to the nearest Flamsteed star was copied on
single sheets of Foolscap (the "Register Sheets").
A great deal of effort was made to obtain
accurate coordinates, and Herschel experimented
with various techniques, using pulleys, knotted
ropes, bells, and iron plates to tell him where he
was in the heavens. Eventually there was a total
of 2,508 new objects. Caroline also had a similar
set of sheets for Messier's nebulae and clusters.
She was very careful and accurate and made only
two or three positional mistakes of Γ in all of her
entries — and these she made in old age. A total
of seven "Sweep Books" was eventually
presented to the Royal Society.
THE 40- FOOT MONSTER
In 1785 Herschel petitioned the king for a large
instrument with a 30- or 40-foot focal length. The
king chose the 40-foot option and granted £2,000
toward its construction, and in Sep-
Appendix C 457