surface, and is again in part reflected. The waves from the second
surface thus turn back and hotly pursue the waves from the first
surface. And, if the thickness of the film be such as to cause the
necessary retardation, the two systems of waves interfere with each
other, producing augmented or diminished light, as the case may be.

But, inasmuch as the waves of light are of different lengths, it is
plain that, to produce extinction in the case of the longer waves, a
greater thickness of film is necessary than in the case of the shorter
ones. Different colours, therefore, must appear at different
thicknesses of the film.

Take with you a little bottle of spirit of turpentine, and pour it
into one of your country ponds. You will then see the glowing of those
colours over the surface of the water. On a small scale we produce
them thus: A common tea-tray is filled with water, beneath the surface
of which dips the end of a pipette. A beam of light falls upon the
water, and is reflected by it to the screen. Spirit of turpentine is
poured into the pipette; it descends, issues from the end in minute
drops, which rise in succession to the surface. On reaching it, each
drop spreads suddenly out as a film, and glowing colours immediately
flash forth upon the screen. The colours change as the thickness of
the film changes by evaporation. They are also arranged in zones, in
consequence of the gradual diminution of thickness from the centre
outwards.

Any film whatever will produce these colours. The film of air between
two plates of glass squeezed together, exhibits, as shown by Hooke,
rich fringes of colour. A particularly fine example of these fringes
is now before you. Nor is even air necessary; the rupture of optical