A favourable wind arises and dissipates the vapours in the very
direction where some important phenomenon will manifest itself, and is
to last only a few seconds. The astronomer, exposed to all the
transitions of weather, (it is one of the conditions of accuracy,) the
body painfully bent, directs the telescope of a great graduated circle
in haste upon the star that he impatiently awaits. His lines for
measuring are a spider's threads. If in looking he makes a mistake of
half the thickness of one of these threads, the observation is good for
nothing; judge what his uneasiness must be; at the critical moment, a
puff of wind occasioning a vibration in the artificial light adapted to
his telescope, the threads become almost invisible; the star itself,
whose rays reach the eye through atmospheric strata of various density,
temperature, and refrangibility, will appear to oscillate so much as to
render the true position of it almost unassignable; at the very moment
when extremely good definition of the object becomes indispensable to
insure correctness of measures, all becomes confused, either because the
eye-piece gets steamed with vapour, or that the vicinity of the very
cold metal occasions an abundant secretion of tears in the eye applied
to the telescope; the poor observer is then exposed to the alternative
of abandoning to some other more fortunate person than himself, the
ascertaining a phenomenon that will not recur during his lifetime, or
introducing into the science results of problematical correctness.
Finally, to complete the observation, he must read off the microscopical
divisions of the graduated circle, and for what opticians call _indolent
vision_ (the only sort that the ancients ever required) must substitute
_strained vision_, which in a few years brings on blindness.[6]

When he has scarcely escaped from this physical and moral torture, and
the astronomer wishes to know what degree of utility is deducible from