Lenses and Waves

1653 - TRACTATUS 49
images of extended objects, Molyneux displayed a better understanding of
the focusing of rays. In a section on “… the Representation of outward
Objects in a Dark Chamber; a Convex-Glass”, he described how the image is
formed by the focusing of pencils of rays originating in the points of an
object. 156 He then remarked that “… tho all the Rays from each point are not
united in an answerable Point in the Image, yet there are a sufficient quantity
of them to render the Representation very perfect.” 157 Rather than
mathematically precise, this was a practical definition of focus. It explained
why in practice images may appear sharp. Besides all the objections that can
be raised against Molyneux’ theory from a mathematical point of view we
should bear in mind that Dioptrica Nova was the first published dioptrical
account of telescopes, since Dioptrice. It was up-to-date with developments in
telescope making and was intended to be useful for practice.
Before coming to a conclusion of this chapter, we go back in time and
cross back over the Channel. Flamsteed’s ally in the debate over telescopic
sights, Picard, also saw the importance of theory. In a letter to Hevelius, he
had briefly explained the working of the telescopic sight in dioptrical terms. 158
Somewhat earlier – probably in 1668 – he had pointed out the need for such
an analysis: “[optical devices] can also be subject to certain refractions that
should be known well.” 159 In Mesure de la Terre, he had briefly discussed
matters of aligning and rectifying telescopic sights in these terms. Picard was
known for his interest and ability in matters dioptrical. At the Académie, he
frequently discoursed of dioptrical theory. 160 In this, the telescope stood
central:
“What we have just explained about the construction of telescopes, concerns only its
use in instruments made for observation, …” 161
Picard never published his dioptrics, but a collections of papers he had read
at the Académie was published posthumously in 1693 under the title
‘Fragmens de Dioptrique’. 162 Picard had a major advantage over Flamsteed.
He was acquainted with one of the most knowledgeable men in dioptrics:
Huygens. Besides his learning, in 1666 Huygens had brought a copy of the
manuscript of Tractatus to Paris. 163 At the Académie, Huygens had also
discoursed on dioptrics. “Fragmens de Dioptrique” make it clear that Picard
must have been among Huygens’ most attentive listeners. They are for the
156 Molyneux, Dioptrica nova, 36-38.
157 Molyneux, Dioptrica nova, 38.
158 Picolet, “Correspondence”, 38-39.
159 “… peuuent aussi estre sujets a certaines refractions qu’il faut bien connoistre.” Quoted in McKeon,
“Renouvellement”, 126-128. It is found in: A. Ac. Sc., Registres, t. 3, fol 156 r o - 164 v o spéc. 157 v o.
160 Blay, ”Travaux de Picard”, 329-332. Blay cites several references.
161 Blay, “Travaux de Picard” 343. “Ce que nous venons d’expliquer touchant la construction des lunettes
d’approche, n’est que par rapport à l’usage que l’on en fait dans les instruments qui servent à l’observer,
…”
162 Divers Ouvrages de Mathematique et de Physique, par Messieurs de l’Academie Royale des Sciences (1693), 375-412.
163 OC13, “Avertissement”, 7.