Lenses and Waves

60 CHAPTER 3
The inversion of the picture a Keplerian telescope produced could be
overcome by adding a third lens. Given the quality of the earliest lenses it
was not advisable to ‘multiply’ glasses, but by the 1640s multi-lens telescopes
were beginning to become acceptable. With the increase of length and
magnification, however, the field of a Keplerian telescope also became
narrow. For example, the 23-foot telescope that Huygens used in his
observations of Saturn had a field of only 17'. It could not display the entire
Moon at once. The limited field of view could be overcome by adding a field
lens. The Augsburg telescope maker Johann Wiesel was probably the first to
make telescopes with such compound oculars. 32 In a letter of 17 December
1649, Wiesel described a four-lens telescope. It had an eyepiece consisting of
two plano-convex lenses fitted in a small tube. The eyepiece tube was
inserted in a composition of tubes which held an objective lens at the far side
and a plano-convex ocular which acted as a field lens. Wiesel added:
“Sir you may bee assured this is y. e first starrie tubus wch I have made of this manner &
so good yt it goes farre beyond all others wherof my selfe also doe not little rejoyce.” 33
The fame of Wiesel’s telescopes spread quickly and throughout Europe
telescope makers tried to equal them. On a visit to his relative Edelheer in
Antwerpen on New Year’s eve 1652, Huygens saw a Wiesel telescope and
was very impressed. It was a four-lens telescope, probably comparable to one
Wiesel described in 1649. Towards the end of 1654 Huygens acquired two
letters written by Wiesel - one to his cousin Vogelaer - describing the
construction and use of several optical instruments. 34 In the first letter a sixlens
telescope was described, which could be used for both terrestrial and
celestial purposes. Wiesel was an artisan, a very good one with an unmatched
understanding of lenses and their configurations. His was another kind of
understanding than the dioptrical theory Huygens developed in 1653. This
kind of experiential knowledge Huygens acquired the following years in his
practical dioptrics. Then, some ten years later, in an artisanal manner
Huygens made his own compound eyepiece with excellent dioptrical
properties.
Experiential knowledge
Telescope makers had a great deal of knowledge of dioptrics, as witnessed by
the fruits of their labor that are preserved. Like the process of production,
the thinking behind these products is more difficult to retrieve. It is barely
documented as craftsmen in general were reluctant to reveal the secrets of
their trade. There is reason to believe that their knowledge of dioptrics was
of a different kind than that of mathematicians. That much we can infer
from what little material that has been preserved. Lens makers knew very
32
Van Helden, “Compound”, 27-29; Keil, “Technology transfer”, 272-273. They are first mentioned in
Rheita’s Oculus Enoch et Eliae (1645), who referred his readers to Wiesel. For the relationship between
Rheita and Wiesel see Keill, Augustanus Opticus, 66-77.
33
Van Helden, “Compound eyepieces”, 34. The entire letter is reproduced on 34-35.
34
OC1, 308-311.