Lenses and Waves

1677-1679 –WAVES OF LIGHT 195
proof. 107 He set great store by his experimental refutation of Descartes’
theory. As an experimental philosopher he proceeded by minutely recording
observations and experiments of colors in order to infer their causes. His
observations were largely qualitative. As regards the colors in thin films,
Hooke admitted that he had not been able to “… determine the greatest or
least thickness requisite for these effects, …” 108 He suggested that the colors
in thin films were periodical in some way, without attempting to state this in
more exact terms. Upon reading Micrographia both Huygens and Newton
readily determined the thickness of the film and the kind of periodicity
involved. 109 In this way, Micrographia is typical of the experimental philosophy
in which observations and explanations were qualitative and theories never
rose to an exact level.
5.2.2 ‘RAISONS DE MECHANIQUE’
From the perspective of Traité de la Lumière, Pardies’ theory of waves met the
standards of proper ‘raisons de mechanique’, even if it could be improved
somewhat. In the first place, it employed mechanistic concepts Huygens
accepted. It was based on the idea that light consists of motion without
transport of matter, and crystallized in a conception of waves produced by
successive collisions of ethereal particles. The basic corpuscular entity was
the particle and the basic motion was impact, instead of some kind of body
whose exact motions were obscure. The combination of these constituted an
action governed by established mathematical laws. Waves produced by
impacts of ethereal particles were, in other words, proper ‘raisons de
mechanique’.
Secondly, the form of Pardies’ theory met Huygens’ demands. It was cast
in the form of a geometrical construction. In this way, the mechanistic
consideration of refracting waves reduced to geometrical manipulation on
the basis of some mathematical premises. As we have seen in section 4.2.2,
this was not wholly unproblematic in Pardies’ explanation of refraction. The
curve resulting from the construction had an ambiguous meaning in terms of
waves. In Ango’s rendition Pardies’ waves remain entities whose existence is
presupposed in the derivation of the sine law.
In his own theory, Huygens rigorously defined waves as an effect of
colliding ethereal particles. At all times, a wave is the resultant of the way this
action has spread indifferently through a sea of disconnected particles.
Waves are reduced to the one property that was central to his understanding
of motion: velocity. The idea that each part of a wave is the source of a new
wave, combined with the assumption that visible light is produced only
where secondary waves coincide, enabled Huygens to consider speeds of
propagation only. In his ‘principal foundation’ this was cast in mathematical
form, thus reducing the consideration of wave propagation to geometrical
107 Hooke, Micrographia, 57.
108 Hooke, Micrographia, 67.
109 Westfall, “Rings”, 64-65.