Lenses and Waves

44 CHAPTER 2
Adrien Auzout was published in Philosophical Transactions, in which he
described a method of determining the diameters of planets. 124 He had
devised – possibly with the help of Pierre Petit – and used – together with
Jean Picard – a grate of thin wires and a moveable reference frame inserted
in the focal plane of a telescope. In two letters, also published in Philosophical
Transactions, Towneley argued that Gascoigne had made and used a
micrometer much earlier. He described a pair of moveable fillets that could
be inserted into the focal plane. 125 He himself had used and improved the
device – probably since late 1665 – to make accurate observations. 126
The principle of the micrometer, however, had already been published in
1659; by Huygens in Systema Saturnium, his astronomical work in which he
presented his discoveries regarding the ring and the satellites of Saturn. In its
final section he explained the principle and described how to use it to make
measurements. He inserted a ring in the focal plane and then measured the
angular magnitude of the opening thus produced by timing the passage of a
star. Next, he inserted a cuneiform strip through a hole in the tube until it
just covered a planet. The angular diameter of the planet was determined by
taking out the strip and comparing its width at the point found with the
inner diameter of the ring. 127 It was not a real micrometer, but Huygens’
rather cumbersome method did produce reliable, accurate data. 128 It was a
convenient method for measuring the size of planets, Huygens said, as one
did not have to wait for a conjunction of the planet with the Moon or a
star. 129 Huygens had been acquainted with Auzout and Petit since 1660 and
had come to Paris in 1666 to give leadership to the Académie. His
explanation of the principle of the micrometer certainly inspired their work
on the micrometer, but the precise nature of Huygens’ contribution is hard
to determine. 130
The principle of the micrometer had another important application:
telescopic sights. By inserting crosshairs in the focal plane, a telescope could
reliably be aligned on a measuring arc. 131 With the telescopic sight the
accuracy of Brahe’s measurements could finally be improved. Several
programs of astronomical measurement now set off. In Paris, Picard and
other members of the Académie – completed in 1669 by Cassini – put into
use a new, well-equipped observatory. 132 Picard’s work on cartographic
124
OldCorr3, 293: “… prendre les diametres du soleil, de la lune et des planetes par une methode que nous
avons, Monsieur Picard et moy, que ie croy la meilleure de toutes celles que l’on a pratiquer Jusques a
present, ...”
125
McKeon, “Les débuts I”, 266-269.
126
McKeon, “Les débuts I”, 286. In Micrographia (1665) Hooke had suggested that a scale may be inserted
into the focal plane of telescopes. Hooke, Micrographia, 237.
127
OC21, 348-351.
128
Van Helden, Measure, 120-121.
129
OC21, 352-353.
130
McKeon, “Les débuts I”, 286; Van Helden, Measure, 118.
131
McKeon, “Renouvellement”, 122.
132
McKeon, “Renouvellement”, 126.