But perhaps the most striking proof of the dependence of the colors upon
the thickness of the film was shown by the reflection of a beam of light
from a piece of mica composed of twenty-four very attenuated plates
overlapping each other. With each layer a marked gradation in color was
visible.

The remainder of the lecture was devoted to an explanation of the
determination of the chromatic relations of the colors of the spectrum.
Lord Rayleigh at this point made a rather startling statement that any
color can be produced by two other colors. As an example of such a
formation, a ray of white light was passed separately through a solution of
yellow chromate of potash and an alkaline litmus solution, throwing
respectively a yellow and violet-blue color upon the screen. When the ray
was made to pass through the two solutions successively, an orange-yellow
color was obtained upon the screen, which color Lord Rayleigh asserted to
be made up of red and green rays. To prove this, the ray of white light was
decomposed by means of a prism, and the decomposed rays passed through the
two solutions. The one solution was found to exclude all the yellow and
orange rays from the spectrum, while the other excluded all the blue and
violet rays, so that when the ray had passed through both solutions, only
the red and green rays were left. If, instead of allowing the decomposed
ray of light to pass through a slit, and thus obtain definite bands in the
spectrum, the ray was passed through a circular hole, the red and green
colors overlapped each other on the screen, forming by their combination
the identical orange-yellow color obtained with the primary white light. It
was then stated that if three definite positions be taken in a spectrum in
the red, green, and violet bands respectively, and these positions be
represented by the corners of an equilateral triangle (Clerk Maxwell's
triangle), it has been mathematically determined in what position within