Lenses and Waves

1677-1679 –WAVES OF LIGHT 205
some thinking on refraction, in particular on Descartes’ account of it. He
threw doubts of the validity of Descartes’ proof. It was unclear to him what
kind of impact Descartes had in mind when comparing light rays with balls
struck into water. And, according to him, contrary conclusions could be
derived from Descartes’ assumptions. Rømer himself had read a paper to the
Académie in which he rejected the assumption that the speed of light is
larger in denser mediums and derived the sine law in a way similar to
Fermat’s derivation. 138 Huygens’ letter had made him consider strange
refraction and he had promising ideas, he said. But without a piece of crystal
and precise data he could not pursue his thinking in a satisfactory manner. 139
5.3.1 DANISH OBJECTIONS
Rømer had to wait for one and a half year before hearing the details of
Huygens’ explanation. And when the time came, in the summer of 1679, he
raised serious objections. Huygens was to recall what happened when he sent
him a copy of Traité de la Lumière in 1690. 140 From this letter it is also clear
that some notes Huygens wrote in July and August 1679 were directed at
Rømer’s objections. From all this we can infer that Rømer had advanced the
theory of Bartholinus – his father in law – as a viable alternative to Huygens’
ellipse construction. 141 Bartholinus had argued that strange refraction is
governed by an ‘oblique perpendicular’ that is parallel to the edge of the
crystal. Evidence for this he found in the fact that the unrefracted oblique
ray is parallel to the edge of the crystal. Therefore, so Bartholinus had
concluded, the unrefracted oblique ray must have a function similar to the
perpendicular ray in ordinary refraction. According to Bartholinus’ ‘oblique’
sine law, the sines of incident and strangely refracted rays are in constant
proportion when measured with respect to the unrefracted oblique ray.
Bartholinus had suggested that pores in the crystal could explain the
unrefracted passage of the ray parallel to the edge of the crystal.
According to the registers of the Académie, Huygens read from his
‘Dioptrique’ on 1 July 1679. 142 This may well have been the session at which
Rømer pointed out that Bartholinus’ explanation was equally plausible and
had not been refuted by Huygens’ explanation. On 3 July, Huygens in his
turn could refute a central assumption of Bartholinus’ explanation:
“Observation made on 3 July 1679. which proves manifestly that it is not the ray
parallel to the sides of the crystal that passes without refraction as I thought until
now.” 143
138
Cohen, “Roemer”, 344.
139
OC8, 45-46.
140
OC9, 489. No direct evidence from the late 1670s of Rømer’s objections is available.
141
As Ziggelaar also assumes; “How”, 185.
142
OC19, 440n2.
143
OC19, 440. “Observation faite le 3 juillet 1679. qui prouve manifestement que ce n’est pas le rayon
parallele aux costez du cristal qui passe sans refraction comme j’avois creu jusqu’icy.”