Lenses and Waves

160 CHAPTER 5
indirect empirical confirmation by means of an experimental test of an
hypothesis derived from it. The test succeeded, giving rise to a second
“EUPHKA” on 6 August 1679. This event underscores the remarkable fact
that until this late point, Huygens had largely proceeded by rational analysis
without much thought for the empirical foundation of his ideas.
This chapter discusses the development of Huygens’ wave theory of light
from 1677 until the events of 1679. It is marked by a twofold “EUPHKA”
written out in Huygens’ notebook, and that may be said to signify the
context of discovery and of justification respectively. Unlike the “EUPHKA”
that had hailed Huygens design for a perfect telescope in 1669, these two
would stand the test of time. Traité de la Lumière of 1690 contains the larger
part of the text he read at the Académie in 1679. It remained unchanged
until 1689, when Huygens made some corrections and additions as he finally
prepared his theory for publication. 1
Times were still eventful for Huygens. After his return, upon his
recovery, to Paris in June 1671, years of productivity followed. His encounter
with Newton and his reflective telescope has been discussed in chapter 3, the
‘Projet’ and the first attack on strange refraction in the previous chapter. A
discussion of Alhacen’s problem with de Sluse had started early 1671 and
reached its peak late 1672. 2 On 9 August 1673 he wrote a letter for Colbert,
in which he summarized the state of the art in contemporary dioptrics. He
explained the value of dioptrical theory: with “… the rules of refraction …,
one could predict in advance the effect of telescopes”. 3 He described the
most powerful telescopes available and the problems with grinding good
lenses, especially in Paris. A design for grinding non-spherical lenses by
Smethwick had caught his attention in 1671. In 1675, he discussed it, without
becoming fully convinced of the validity of the method. 4 In addition, he was
engaged in various activities, like trials of his pendulum clock at sea and the
invention of the spring balance and its subsequent priority disputes, early
1675, with Hooke and Hautefeuille. Several papers in Journal des Sçavans and
Philosophical Transactions appeared and in 1673 Huygens published his master
piece Horologium Oscillatorium. He dedicated it to Louis XIV, in spite of the
fact that his patron had invaded his fatherland the previous year and
occasioning the Republic’s ‘disaster year’ and the lynching of his
mathematics soul mate Johan de Witt .
1
The text in which these changes are made is preserved in two manuscript copies. OC19,
“Avertissement”, 383.
2
The problem is, to find the point on a spherical mirror were a light ray is reflected when the position of
the light source and the eye of the observer are given. In June 1669, Huygens sent his initial solution to
Oldenburg, who began sending de Sluse’s work to Huygens in August 1670. Extracts of ensuing letters
were printed in Philosophical Transactions of October and November 1673 after the discussion had
ended in January. It is primarily a mathematical problem and less relevant for my account of Huygens’
optics, so I will not discuss it. For a detailed account of the problem and its solution: Bruins, “Problema”.
3
OC7, 350-351. “… les regles de refraction …, l’on pouvoit predire par avance l’effect des lunettes
d’approche” It is not clear on what occasion he wrote this.
4
OC7, 111 (October 1671); 117 (November 1671); 487 (July 1675); 511-513 (October 1675)