the ocular harpsichord of louis-bertrand castel - Gewina

Castel's ocular harpsichord 27
... the great Newton, how could he have been so deeply wrong, and with that apparatus and that
fuss that impresses the wise and keeps the universe in admiration and as it were enslaved by its
brilliant spectrum of seven colours, no more, no less'.' ... I had challenged the prism and its
fantastic spectrum ... I regarded it with terror, as if it were a reef signaled by the wreckage of a
famous ship, followed by a thousand ships that had come to share its disaster, while collecting
its debris.'*
In his later book on Newtonian physics in general, Castel tried to give a theoretical
foundation to his critique of Newton's optics.** He presented some objections
against the emission theory of light (he had never discussed it before). For
instance, it could not explain how objects could have a blue colour; if the surface
of an object absorbs the larger and heavier red light-particles, it will surely also
absorb the smaller blue light-particles, instead of reflecting them. Referring to
Malcbranche's theoiy, in which light is a sequence of pulses, or a vibration, in a
special medium, the aether, Castel argued that both white and coloured light arc
vibrational phenomena. White light is a single, longitudinal vibration, and
coloured light results when a second transverse vibration is added to the
vibration of white light. In such a model, where coloured light is a modification
of white light, it is conceivable that the light that passes through a prism gets
coloured only at the edges; in the middle of the beam the transverse vibrations
annihilate each other, while they can survive at the edges.'*"
Castel's attempts actually to build an ocular harpsichord
In neither the "Nouvelles experiences" nor the Optique des couleurs did Castel
seriously discuss the actual construction of an ocular harpsichord. In his 1735
article he ended with an enormously elaborate extension of his earlier proof
more geometrico of the po.ssibility of colour music, building it into a complete
"logique du gout" with eight principal propositions and 73 theorems, and finally
lost himself in a delirious enumeration of all possible kinds of colour enjoyment.
Nevertheless, from some occasional remarks it becomes clear that Castel had
Ibid., pp. 373-374, 376-377: "le grand Newton, pourquoi & comment a-t-il pu s'y tromper
solemnellement. & avec cet appareil & ce fracas qui impose aux sages, & tient tout I'univers dans
I'admiration & prcsque dans I'esclavage de son spectre essentiellement brillant de 7 couleurs, ni plus
ni moins? ... Je me defiois du prisme & de son spectre fantastique ... Je le regardois avec terreur,
comme un ecueil signale par le naufrage d'un vaisseaux fameux, suivi de mille vaisseaux, qui
venoient a I'envi partager .son desastre, en recueillant ses debris."
39
Castel, Le vrai systeme, pp. 421-500.
40
Ibid., pp. 501-507. Malebranche had included the first version of his theory in the 1700 edition
of his De la recherche de la verite, but presented a slightly but significantly changed version in the
1712 edition, to make it compatible with Newton's prism experiments. See Hakfoort, Optica, pp. 60-
63. Castel apparently had read, or digested, only the 1700 version.