Newton and the Mystery of the Major Sixth 125
the same proportion that a string is, between the end and the middle, to sound the tones
in the eighth [octave], ” as he illustrates in figure 8.2. He regarded the color spectrum as
similar to the musical divisions of a string.
Though Newton presents the analogy as the result of careful experiment, consciously
constructed to avoid prior hypothesis, he asserts more generally that “ the analogy of nature
is to be observed ” with respect to the corresponding natures of the different human senses,
rather than between the natures of light and sound as such. 14 As Newton went to apply
this analogy to different optical phenomena, though apprehended by the same sense, he
obtained results whose divergence cannot be due to that single sense but to the nature of
light itself.
The crux of Newton ’ s analogy was that, as the upper note in an octave stands to the
lower (as d would to D an octave lower), so do the extremes of color, namely “ deep violet ”
and red, likewise represent an “ octave ” in color, within which the intermediate hues should
occupy the traditional seven scale degrees, thus interpreting the colors of the spectrum as
corresponding to musical notes spanning an octave. From this flows his assertion that, at
the appropriate points in the scale, the spectral colors orange and indigo should be inserted
at the very points in which the chosen mode (Newton took this to be Dorian) has the
semitones E – F and B – C. For those who came after, Newton ’ s musical analogy is the source
of the widely held opinion that orange and indigo are actually intrinsic in the spectrum,
despite the great difficulty (if not impossibility) of distinguishing indigo from blue, or
orange from yellow, in spectra. Thus the authority of Newton, even speaking far from his
primary expertise, carries unquestioned weight even to the present day. Yet in his Optical
Lectures (1670 – 1672) Newton had been rather diffident about the analogy and admitted
that “ I could not, however, so precisely observe and define this without being compelled
to admit that it could perhaps be constituted somewhat differently. ”^15
Here Newton acknowledges the difficulty of dividing the spectrum into seven “ more
prominent ” colors “ proportional to a string so divided that it would cause the individual
degrees of the octave to sound. ” That is, he admits that he imposed the seven colors by
analogy with the (Dorian) mode without being able to demonstrate that those specific
colors must necessarily be placed at those scale steps — hence his admission that the color
correspondence “ could perhaps be constituted somewhat differently. ”^16
Newton ’ s primary assumption is that color, like sound, admits of octave (2:1) ratios.
This assertion bears strongly on Newton ’ s theories about light. Though familiar with the
experiments of Francesco Maria Grimaldi that seemed to show wave effects in light ( figure
8.3 ), Newton argued that a light wave passing an obstacle should “ bend into the shadow, ”
which he felt had not been demonstrated even by what Grimaldi called “ interference. ”^17
Though he preferred a particulate description of light emission, Newton never presented
his preference as more than an hypothesis; while denying that light itself was a wave, he
put forward his idiosyncratic (and rather puzzling) “ fits of easy transmission and