of the light reflected from the
first surface. If, however, the retardation be only equivalent to half a
wave length, the intensity of the light will be decreased. Thus, then, with
a ray of monochromatic light it will be seen that the effect of difference
in the thickness of the film will be to alter the intensity of the
reflected ray, but with a white light composed of several colors the result
will be more complicated. As each color has a different wave length in
vibration, it will be seen that each color will act independently of the
others, and a certain thickness of film which, upon the combination of the
two reflected rays, will cause one particular color to be intensified, will
at the same time cause the other colors to be more or less obscured.

Thus as the thickness of the film is altered different colors preponderate,
causing the appearance of rings or bands, according to the nature of the
experiment. The dark appearance on the screen corresponding to the thinnest
part of the film is probably due to refraction of the ray of light
reflected from the second surface, consequent in its passing from a rare
into a denser medium, and again from the denser medium into the rare, which
refraction Lord Rayleigh considers to effect a retardation equivalent to
half a wave length. Lord Rayleigh supported this theory of the formation of
Newton's rings by several interesting experiments. A beam of light was
intercepted by two of Nicol's prisms, one of which acted as a polarizer and
the other as an analyzer of the light, so that no light was able to pass
through both on to the screen. Between the two prisms a double refractive
lens was now placed, in this case a double concave lens of selenite, when
the same series of concentric rings observed with the film of air was
obtained on the screen, only much more intense, while a wedge of selenite
gave the bands of color in the same order as with the soap bubble.

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 b