An extremely fine scratch on a well-polished surface may be regarded as
having a concave, cylindrical, or at least a curved surface, capable of
reflecting light in all directions; this is evident, for it is visible
in all directions. Hence a single scratch or furrow in a surface may
produce colours by the interference of the rays reflected from its
opposite edges. Examine a spider’s thread in the sunshine, and it will
gleam with vivid colours. These may arise from a similar cause; or from
the thread itself, as spun by the animal, consisting of several threads
agglutinated together, and thus presenting, not a cylindrical, but a
furrowed surface.