Lenses and Waves

62 CHAPTER 3
Keplerian telescope is set equal to the doubled radius of its objective lens,
which - correctly - implies that the focal distance of the objective is twice the
radius of its convex side. Another table prescribed the size of the aperture
for a given objective, in each case 1
80
th of the tube’s length. Bolantio
explained that the objective should be partially covered by a ring so that no
light fell on the interior of the tube, which apparently implied the ratio used
in the table. 40 Whatever the dioptrical understanding implicit in Bolantio’s
manuscript, it was presented in a procedural, how-to style that did not
require further theoretical knowledge. 41
Some telescope makers published their own observations, to promote
their products. 42 They did not publish the secrets of their art, as their
revenues depended on them. Information on the manufacture of lenses and
telescopes could be found in books that were mostly written by scholars.
Examples are Telescopium (1618) by Girolamo Sirtori, Selenographia (1647) by
Hevelius and La Dioptrique oculaire (1672) by Cherubin d’Orleans. In 1685,
Huygens wrote a treatise on lens grinding in Dutch, Memorien aangaende het
slijpen van glasen tot verrekijckers, published posthumously in Latin in the
Opuscula posthuma (1703). 43 Memorien was the elaboration of notes like the one
cited above. Huygens described the process of lens making as a set of
directives, procedures, tips and tricks. No attempt is made to explain why
things work as they work: for example a geometrical account of the grinding
device is absent. Memorien supplied the kind of experiential knowledge also
found in Bolantio’s manuscript: a description of skills Huygens had acquired
through long-time practice.
To what extent a telescope maker like Campani understood the dioptrics
implicit in tables like those in Bolantio’s manuscript cannot be determined.
First rank, specialized telescope makers like Divini and Campani had
received some formal education, so they may have been able to read and
study a book like Dioptrice. It remains to be seen whether a question like this
is relevant at all. I doubt whether dioptrical knowledge would have been of
any use in the design and manufacture of telescopes. They knew very well
the effect of diverse types of lenses, but this probably was experiential
knowledge. Innovative craftsmen like Wiesel were able to find new
configurations with improved properties. These are likely to have been the
product of trial and error. It has been said that Kepler’s configuration was
the only contribution from the theory of dioptrics to the improvement of the
telescope. 44 Still, its advantages had to be discovered in practice. The
40
Bedini & Bennet, “Treatise”, 117.
41
Willach discusses dioptrical theory emerging from the correspondence of Rheita en Wiesel which
suggests similar lines. Willach, “Development of telescope optics”, 390-394.
42
For example: Fontana’s Novae coelestium (1646) and Campani, Lettere di Giuseppe Campani intorne all'ombre
delle Stelle Medicee (1665).
43
OC21, 252-290.
44
Van Helden, “The telescope in the 17th century”, 44-49.