Lenses and Waves

1655-1672 - DE ABERRATIONE 59
technique used by mirror-makers. 28 It is
not known where Huygens got the idea.
Such tools for improving the
grinding process had been thought up
earlier, in particular by the most
prestigious lens makers. In Galileo’s
workshop – reigning until the 1640s – a
lathe was introduced that permitted
greater precision than was attained by
ordinary spectacle makers. 29 During the
1660s, the Campani brothers in Rome
became the undisputed masters of the
art. They used a range of machines of
their own design, producing lenses
unsurpassed until the eighteenth
century. The Huygens brothers kept a
close eye on developments like these and around 1665 references to a type of
lathe designed by Campani turned up in their writings. The quality of lenses
seems to have depended to some degree on the lens maker’s inventiveness to
convert laborious and unsteady handiwork into reliable tools.
Huygens had learned how to make lenses. He knew the limitations of the
art and of its products. Even the best lenses might suffer from flaws like
bubbles, irregular density, faults in shape, etcetera. In the end, the proof of
the pudding was in the eating. The quality of telescopes was determined by
trial, sometimes literally. Campani beat Divini early 1664 by a series of
carefully organized ‘paragoni’: open contests in which printed sheets at a
distance were read by means of the instruments of both competitors. 30
Figure 22 Beam to facilitate lens grinding.
Alternative configurations
Besides the quality of lenses, telescopes could be improved by configuring
lenses alternatively. Kepler could not have known that the configuration of
two convex lenses he discussed in Dioptrice had several advantages over the
Galilean one. The positive focus that made possible the micrometer has
already been discussed in the previous chapter. Scheiner, who used a
Keplerian telescope to project images of the sun, discovered by coincidence
that it had a wider field of view. There are indications that Fontana was the
first to put Kepler’s idea into practice, although Scheiner was the first to
mention using it. 31 Around 1640 Fontana was the first to challenge Galileo’s
dominance in the trade and he did so with Keplerian telescopes. Around that
time, the Galilean configuration was beginning to reach the limits to which
its power could be increased without the field of view becoming too small.
28
Beeckman, Journal, III, 232.
29
Bedini, “Makers”, 108-110; Bedini, “Lens making”, 688-691.
30
Bonelli, “Divini and Campani”, 21-25.
31
Van Helden, “Astronomical telescope”, 20-25. Compare Malet, “Kepler and the telescope”, 120.