Lenses and Waves

1653 - TRACTATUS 45
measurements resulted in the determination of the arc of the meridian,
published in Mesure de la Terre (1671). In London, Hooke and Wren devoted
themselves to carrying out the idea of telescopic sights. In 1669, Hooke
announced that he had established the motion of the earth by means of a
mural quadrant thus equipped. His claim met with great skepticism. In 1675,
Flamsteed was appointed Astronomer Royal at the London counterpart to
the Paris observatory. At the Royal Observatory, he erected a wealth of
precision instruments and set up a program of astronomical measurements,
eventually resulting in Historia coelestis brittanica (1725).
The usefulness of telescopic sights was not, however, beyond all doubt.
Hevelius, the most renowned astronomer in those days, was suspicious. He
believed that telescopic sights were unreliable and therefore preferred naked
eye views. 133 In 1672, a letter by Flamsteed was published in Philosophical
Transactions in which he defended the use of telescopes for astronomical
measurements. 134 He praised Hevelius for having improved Brahe’s
astronomical data, but doubted whether any further progress could be
possible as long as the latter refrained from using ‘glasses’. Hevelius took
offense at Flamsteed’s allegations, and responded in Machina coelestis pars prior
(1673) and in a letter that was published in part in Philosophical Transactions of
April 1674:
“For it is not only a question of seeing the stars somewhat more distinctly (…) but
whether the instruments point correctly in every direction, whether the telescopic sights
of the instrument can be accurately directed many times to any observations, and can
be reliably maintained; but I very much doubt whether this can be done with equal
precision every time.” 135
The argument went a bit out of hand when, later in 1674, Hooke interfered
with a vehement attack on Hevelius in Animadversions on the first part of the
machina coelestis. Deeply hurt, Hevelius sent Flamsteed a letter in which he
once more explained his doubts about the reliability of telescopic sights. The
dispute was settled only five years later after a visit to Gdansk by Halley. He
reported that Hevelius’ naked eye observations were indeed incredibly
accurate.
Hevelius had fought a lost battle – so it can be said with hindsight – but
he was right in his suspicions about the reliability of telescopic sights. He
knew from experience how difficult it is to align instruments reliably. Already
in 1668 – right after the announcement of the micrometer – he had written
to Oldenburg: “For many things seem most certain in theory, which in
practice often fall far enough from truth.” 136 He was astonished that Hooke
would claim great accuracy for his measurements on the basis of just single
observations. Hevelius knew that accuracy was gained by hard and systematic
work. Picard, Cassini and Flamsteed undertook such an arduous task, but
133
Flamsteed, Gresham lectures, 34-39 (Forbes’s introduction).
134
OldCorr9, 326-327.
135
OldCorr10, 520.
136
OldCorr4, 448.