[Letters to the Editor]

Abstract

IN Mr. Murphy's interesting letter in No. 26 of NATURE, April 28, 1870, he assumes that the number expressing the frequency of vibration producing a colour complementary to another, is the geometrical mean between the frequency of vibrations corresponding to that other, and its double. By this means he does not get colours complementary from sunlight. Thus red and bluish green (whose numbers are respectively 36˙4, 48˙3) are not complementary on his hypothesis; which would require the number for bluish green to be 51˙47. So for yellow and indigo, the numbers are 41˙4, 54˙7, but should be 41˙4, 58˙4. This he attributes to the impurity of the solar spectrum. There seems as much reason, however, for taking the harmonic mean instead of the geometric; and, on this supposition (the harmonic mean between two quantities being twice their product divided by their sum), the numbers would be red, 36˙4; bluish-green, 48˙5; yellow, 41˙4; indigo, 55˙2. The second and fourth, 48˙5, 55˙2, are not very different from 48˙3, 54˙7. Taking then a colour twice over in the spectrum and its intermediate complementary, the relation between the three would be that of a musical note, its fifth and its octave.