orrect
the imperfections of instruments by the manner of their use.

The manufacture of astronomical circles was brought to a very refined
state of excellence early in the nineteenth century by Reichenbach at
Munich, and after 1818 by Repsold at Hamburg. Bessel states[342] that
the "reading-off" on an instrument of the kind by the latter artist was
accurate to about 1/80th of a human hair. Meanwhile the traditional
reputation of the English school was fully sustained; and Sir George
Airy did not hesitate to express his opinion that the new method of
graduating circles, published by Troughton in 1809,[343] was the
"greatest improvement ever made in the art of instrument-making."[344]
But a more secure road to improvement than that of mere mechanical
exactness was pointed out by Bessel. His introduction of a regular
theory of instrumental errors might almost be said to have created a new
art of observation. Every instrument, he declared in memorable
words,[345] must be twice made--once by the artist, and again by the
observer. Knowledge is power. Defects that are ascertained and can be
allowed for are as good as non-existent. Thus the truism that the best
instrument is worthless in the hands of a careless or clumsy observer,
became supplemented by the converse maxim, that defective appliances
may, through skilful use, be made to yield valuable results. The
Koenigsberg observations--of which the first instalment was published in
1815--set the example of regular "reduction" for instrumental errors.
Since then, it has become an elementary part of an astronomer's duty to
study the _idiosyncrasy_ of each one of the mechanical contrivances at
his disposal, in order that its inevitable, but now certified deviations
from ideal accuracy may be included amongst the numerous corrections by
which the pure essence of even approximate truth is distilled from the
rude impressions of sense.

Nor is this enough; for the casual circumstances attending each
observation have to be taken into account with no less care than the
inherent or _constitutional_ peculiarities of the instrument with which
it is made. There is no "once for all" in astronomy. Vigilance can never
sleep; patience can never tire. Variable as well as constant sources of
error must be anxiously heeded; one infinitesimal inaccuracy must be
weighed against another; all the forces and vicissitudes of
nature--frosts, dews, winds, the interchanges of heat, the disturbing
effec