The iridescent colours sometimes seen on a soap bubble, as in the
illustration, may also be seen in very fine sections of crystals, in
glass blown into extremely fine bulbs, on the wings of dragon-flies and
the surface of oily water. The different colours correspond to different
thicknesses of the surface. Part of the light which strikes these thin
coatings is reflected from the upper surface, but another part of the
light penetrates the transparent coating and is reflected from the lower
surface. It is the mixture of these two reflected rays, their
"interference" as it is called, which produces the colours observed. The
"black spots" on a soap bubble are the places where the soapy film is
thinnest. At the black spots the thickness of the bubble is about the
three-millionth part of an inch. If the whole bubble were as thin as
this it would be completely invisible.]
§ 3