1 The Birth of Science - MSRI

4.5 Naval Architecture. The Pharos 103
Arab historians, who visited it long after it had ceased to function, we
know very little of its technology. Yet some conjectures can be made on the
basis of its purpose and contemporary knowledge. First, we can imagine
that the reflector consisted, as it would today, of a parabolic mirror, all the
more so because the relevant theory arose precisely around the time of the
construction of the Pharos. 83
Even if it cannot be proved directly that scientists had a hand in designing
the Pharos, it cannot be a coincidence that the first reflector in
history appeared in Alexandria in the first half of the third century B.C.,
exactly when and where we first see scientists interested in conics and in
catoptrics — the latter being precisely the scientific theory created for designing
mirrors. (Note that, although a longstanding misattribution gives
Kepler the credit for first applying the classical theory of conics, there had
been several earlier applications, for example to cartography, as attested
by Ptolemy’s Geography.)
Because a light beam in a fixed direction is not very useful in guiding
ships, one can also suppose that the Pharos had a rotating light or reflector.
This would also explain the cylindrical shape of the light room, replicated
in every lighthouse ever known.
In the Middle Ages, catoptrics was lost, and with it the ability to build
lighthouses. At best attempts were made to keep some of the remaining
ancient ones in operation. Lighthouses started being built again in the page 143
twelfth century (Genoa got one in 1139), but these were vain attempts to
imitate the ancient pharoi. In the History of technology that we have often
cited we read:
It was, however, only in the closing years of the seventeenth century
that lighthouse-construction began in earnest, and on new and original
lines that were to lead to the modern types of structure. 84
The “new and original lines” consisted in the use of reflectors based on the
theory of conics. And the date when these structures reappeared might
be guessed approximately without any recourse to historical documents
83 The scarcity of sources does not allow one to document this statement as precisely as one
might desire. But we know that optics and the theory of conics were developed (particularly in
Euclid’s work) toward the end of the fourth century B.C. and that the theory of conics was applied
to mirrors. A burning mirror is nothing but a parabolic reflector “in reverse”, according to the
principle of reversibility of optical paths. Since the focal property of parabolas was applied to the
construction of burning mirrors around the middle of the third century (as we know from Diocles’
comments on Dositheus: see footnote 31 on page 55), and since Archimedes knew the principle
of reversibility of optical paths (see footnote 37 on page 56), the design of parabolic reflectors had
certainly become possible by the middle of the third century. Since the Pharos was built around
280, it is likely that parabolic reflectors in fact predate burning mirrors.
84 [Goodchild], p. 524.
Revision: 1.14 Date: 2002/10/24 04:25:47