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Law of the Octave
There are numerous arbitrary arrangements setting forth the mutual relationships of the planets, the colors, and the musical notes. The most satisfactory system is that based upon the law of the octave. The sense of hearing has a much wider scope than that of sight, for whereas the ear can register from nine to eleven octaves of sound the eye is restricted to the cognition of but seven fundamental color tones, or one tone short of the octave. Red, when posited as the lowest color tone in the scale of chromatics, thus corresponds to do, the first note of the musical scale. Continuing the analogy, orange corresponds to re, yellow to mi, green to fa, blue to sol, indigo to la, and violet to si (ti). The eighth color tone necessary to complete the scale should be the higher octave of red, the first color tone. The accuracy of the above arrangement is attested by two striking facts: (1) the three fundamental notes of the musical scale--the first, the third, and the fifth--correspond with the three primary colors--red, yellow, and blue; (2) the seventh, and least perfect, note of the musical scale corresponds with purple, the least perfect tone of the color scale.
— Manly P. Hall; The Secret Teachings of all Ages