Lenses and Waves

1677-1679 –WAVES OF LIGHT 167
subordinate to the speed of propagation of light. What distinguishes the
notion implicit in this analysis from Pardies is the idea that light spreads in all
directions. The flash of genius would then consist in the insight that Pardies’
premise, rays are normal to waves, still holds as it directly follows from the
fact that the wave is tangent to all wavelets. What Huygens had done was to reduce
waves to their speed of propagation.
Another question is what stimulated Huygens to resume the riddles of
light waves. It had, after all, been 5 years since he put aside the unsolved
questions raised by strange refraction. May an article in the Philosophical
Transactions of 25 June 1677 (6 July N.S.) have been the occasion for
Huygens’ study? The article, by Ole Rømer, contained a proof of the finitude
of the speed of light. 25 It was based on observed irregularities in the motions
of Jupiter's satellites. The idea had already been proposed by Cassini in 1675,
but he had withdrawn it shortly afterwards. 26 Cassini would remain one of
the main opponents of Rømer’s assertions. 27 Understandably, Huygens was
to welcome this observational confirmation of the main premise of his
understanding of light. In Traité de la Lumière he would repeat “the ingenious
demonstration of Mr. Romer.” 28 In his view, the wave theory and the finite
speed of light were necessary linked. 29 He wrote Rømer on 16 September
1677 to express his gratitude. 30 In the following months they exchanged
various letters on the subject.
Rømer’s article would not have affected Huygens’ conviction that the
speed of light is finite, but it is likely that reading it induced him to look at
the propagation of light anew. The centrality of the speed of propagation in
the notes we have just examined – from Fermat’s principle to the analysis of
caustics – makes this plausible. This would mean that this study took place
between the middle of July and 6 August 1677. 31 It would imply that he
moved on to the problem of strange refraction almost at once upon solving
the problem of caustics. In view of the content and appearance of the
manuscript material I find such a short span of time quite plausible.
25 OC8, 30n1. It was the translation of an article that had been published in French on 7 December 1676:
“A demonstration concerning the Motion of Light, communicated from Paris, in the Journal des Sçavans,
and here made English.” Apparently, Huygens had not seen the issue of Journal des Sçavans.
26 Sabra, Theories of Light, 205. Not all historians agree on this point; compare: Cohen, “First
determination”, 345-346; Van Helden, “Roemer’s speed of light”, 140n1 and Wroblewski, “De Mora
Luminis”, 629.
27 Other opponents to the view that the speed of light is finite were Hooke and Fontenelle. Wroblewski
argues that the controversy came to an end in 1729.
28 Traité, 7. “l’ingenieuse demonstration de M r Romer.”
29 In the ‘Projet’ he had made one reservation: “Light extends circularly and not in an instant, at least in
the bodies down here, because for the light of stars it is not without difficulty to say that it would not be
instantaneous.” Such a reservation was now needless, for Rømer’s argument was based on astronomical
observations. OC19, 742. “lumiere s’estend circulairement et non dans l’instant, au moins dans les corps
icy bas, car pour la lumiere des astres il n’est pas sans difficulté de dire qu’elle ne seroit pas instantanee.”
30 OC8, 30-31.
31 The editors of Oeuvres Complètes think that the study of caustics may have taken place as early as 1676.