of the film is considerably
darker than the rest of the spectrum; around this is a bright ring of
white, succeeded by constantly increasing concentric rings of different
colors apparently repeating themselves. Lord Rayleigh also obtained the
same results with a film of a solution of soap and glycerine, but in this
case the dark portion was observed at the top of the spectrum, the other
colors arranging themselves in order in the soap film thinned by the force
of gravitation, thus showing that the colors vary according to the
thickness of the film. Another form of the experiment called forth a
considerable amount of applause from the audience. Lord Rayleigh caused a
gentle stream of air to play obliquely upon a soap film, so that the part
struck was moved forward and the whole film rotated. Then with the
alteration of the force of the current of air, which of course regulated
the centrifugal force, alternating thicknesses of film were obtained,
causing a varying display of beautiful colors and combinations of colors.
This last experiment also tended to prove that the bands of color are not
arranged in a certain order, but vary according to the thickness of the
film, a conclusion arrived at by Brewster, who observed that if a film
reflecting certain colors be carefully inverted so as not to disturb the
gravity, the colors reflected are also inverted. Lord Rayleigh explained
the phenomenon by referring to Young's wave theory of light. He regarded
the film as having two surfaces from which light is reflected, an anterior
exterior surface and a posterior interior surface. If a ray of light be
thrown upon the film, a part of the light is reflected from the first
surface, but the greater part is transmitted, and some of this is reflected
from the second surface, passes back through the film, and is combined with
the light reflected from the first surface. If then the light reflected
from the second surface be in the same state of vibration as that reflected
from the first surface, the effect of their combination will be to increase
the amount of light reflected from the first surface, but if otherwise, the
effect will be a partial neutralization of the light reflected from the
first surface. That is to say, if the retardation of the light which is
reflected from the second surface, owing to its twice traversing the
thickness of the film, be equivalent to a wave length of the vibration of
the light, it will increase the intensity