through violet, to be equivalent to the `natural' order of tones from C to B. Castel, however, believed that the color blue was analogous to C, and he modified Newton's distribution of the visible spectrum. Table 1 indicates the colors controlled by the harpsichord keyboard of Castel's clavecin oculaire.
Describing how one should play his instrument, Castel wrote:
Do you want blue? Put your finger on the first key to the left. Do you want the same only I degree lighter? Touch the 8th note. If you want it 2 degrees, or 3 degrees…, touch the 15th, or 22nd, or 29th, or the last to the right. If you want blue-green, touch the first black to the left. Do you want red, and which red? Crimson-red? That is the 4th black. You have only… to know your clavier and know that blue is C and red is G etc. This you can acquire with three days practice [9].
This description clearly implies an instrument of five octaves. In his Optique des couleurs, Castel proposed to implement his color system via harpsichords constructed with 12 octaves! He believed the limits of aural perception encompassed 12 octaves (from 16 to over 65,000 cycles
per second), and since colors were analogous to sounds, the arrangement of color tints should follow a similar pattern. Thus by mixing various amounts of white and black into each of the 12 pigments, 144 different colors would be obtained for the clavecin oculaire [10]. Castel had first attempted to use prisms for his instrument, but the colors obtained by refraction of light probably were not of sufficient luminosity. He abandoned this method. Later experiments were conducted with candles, mirrors and colored papers. Each key operated one of the 144 cylindrical candle covers, allowing light to shine through colored paper when the flame was exposed.
Telemann reported that Castel was encouraged by his friends to seek practical