224 Chapter 14
Figure 14.5
(a) Helmholtz ’ s plate from his Handbook of Physiological Optics (Part II, 1860) showing the solar spectrum
with the more prominent Frauenhofer lines indicated in capital letters (to the left of the corresponding dark lines)
and a numerical scale (to the right) showing the correspondence between musical intervals of a semitone (labeled
by successive numbers) and the spectral colors. The Frauenhofer line C roughly corresponds to red; E , green;
F , “ cyan-blue ” ; H – L , violet. (b) Isaac Newton, “ An Hypothesis Explaining the Properties of Light ... ” (1675 – 76),
showing his comparison between the musical scale and the spectral colors; he introduced indigo and orange in
order to fill out the analogy between a complete spectrum and the seven diatonic notes in an octave.AB