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.