Lenses and Waves

64 CHAPTER 3
that the power of a telescope could better be determined by calculation than
using the ordinary ways of comparison. He referred to a theorem in
“Dioptricis nostris”: the magnification is equal to the proportion of the focal
distances of objective and ocular. 49
Figure 24 Huygens’ eyepiece. (see also the diagram in Figure 25)
The year 1662 marks a turn in Huygens’ dioptrics. He invented something
new and then turned to dioptrical theory again. The invention was a
particular configuration of three lenses in a compound ocular (Figure 24).
Nowadays called ‘Huygens’ eyepiece’, it had considerable advantages over
earlier solutions: it produced a large field of view and images that suffered
relatively little from aberrations. 50 Huygens had developed the eyepiece after
his trip to Paris and London in 1660-1, where he had talked much on
telescopes and related matters. In Paris he had seen the artisan Menard and
the ingeneer Pierre Petit, who had the best collection of telescopes in Paris.
In London he saw telescopes with compound eyepieces made by the
telescope makers Paul Neile and Richard Reeve. 51
In 1662, Huygens made his first telescopes with field lenses. Later that
year, he found out what configuration of lenses produced bright images and
a wide field. On 5 October he wrote to his brother Lodewijk in Paris:
“As for oculars, you will see that I have found something new that causes that
distinctness in daytime telescopes [i.e. terrestrial], and the same thing in the very long
ones, while giving them at the same time a wide opening.” 52
Huygens’ design quickly became known and was adopted widely. How
Huygens had found the precise configuration is unknown, yet everything
points at it being a matter of trial-and-error inspired by the examples he had
seen. 53
After the invention, however, Huygens did something others like Wiesel
and Reeve did not do. He set out to understand how it worked by analyzing
the dioptrical properties of his eyepiece. Huygens described its configuration
in a proposition inserted in the third part of Tractatus. 54
49
OC15, 230-233.
50
Van Helden, “Compound eyepieces”, 33; Van Helden, “Huygens and the astronomers”, 158.
51
OC22, 568-576.
52
OC4, 242-3: “car pour les oculaires vous voyez bien que j’y ay trouvè quelque chose de nouveau, qui
cause cette nettetè dans les lunettes du jour, et de mesme dans les plus longues, leur donnant en mesme
temps une grande ouverture.”
53
Van Helden, “Compound eyepieces”, 33.
54
OC13, 252-259. The text in Oeuvres Complètes is probably from 1666. The notes contain some previous
phrasing, probably from 1662. OC13, 252n1