Lenses and Waves

220 CHAPTER 6
now open the second part of ‘Dioptrique’. In the adjusted plan, this part
would be a trimmed down version of Tractatus. Huygens remarked that he
had written his theory of lenses a long time ago and that some of its content
had been dealt with since by others. He probably had Barrow’s lectures in
mind. Matters like these he would sketch only briefly and then present the
most important theorems of Tractatus. The principal topic of the second part
of ‘Dioptrique’ was magnification, a topic that according to Huygens still
passed without proper treatment in existing literature. 30
Huygens’ dioptrical studies of the 1680s have been gathered by the
editors of the Oeuvres Complètes in a separate section in OC13, called
“Dioptrica. Pars Tertia”. For the large part it consists of an essay labeled ‘De
Telescopiis’. Dating it is hazardous, but it appears that the latest version is of
1692. Material with varying dates – ranging from the 1660s to the 1690s – is
collected and alternative versions of several parts can be found in
appendices. Huygens probably started assembling and elaborating this in
1684 or 1685. 31 In or after 1684, he made another outline for the second part
of ‘Dioptrique’, listing the order of subjects treated. 32 This outline
corresponds for the most part with ‘De Telescopiis’. The essay cannot have
been intended as a definite second part of ‘Dioptrique’, though. At several
places Huygens referred to theorems of Tractatus. If ‘De Telescopiis’ was
eventually intended to replace Tractatus entirely, Huygens still would have to
find a way to integrate these theorems.
At any rate, Huygens did not round off his new plans. In 1687 he
considered the state of his ‘Dioptrique’ once again. He still considered his
theory of waves and his theory of dioptrics as two parts of a single work. He
doubted, however, whether ‘Dioptrique’ was still an appropriate title. A note
gives a new title: ‘Optique. I partie.’ 33 In addition, the problem of languages
remained. He made a start with a Latin translation of the first part under the
title ‘Versio Diatribæ de Luce’ but got no further than some 10 pages. 34 At
the beginning of 1690, Traité de la Lumière was published as a autonomous
treatise. It did not, as we have seen, reveal that it had been intended as the
first part of a larger work. Still, it was not fully separated from the
‘Dioptrique’. At the time the Traité de la Lumière was being printed, Huygens
began a French translation of his dioptrics: “Beginning of the treatise on my
dioptrics in French that I planned to join with the treatise on light, …” 35 The
decision to treat them as separate treatises had been made just before. The
opening had at first read: “Beginning of my second part of the ‘Dioptrique’
30
OC13, 746-748.
31
OC13, 434-511. See notes 1 and 2 of pages 434-435 on the dating.
32
OC13, 750-752.
33
OC13, 754.
34
OC19, 458-470. He probably did not start translation before May 1687: OC9, 133.
35
OC13, 754; 755-770. “Commencement du Traitè de ma Dioptrique en François que j’avois dessein de
joindre au Traitè de la Lumière, …”