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
continuity suffices. Smite with an axe the black, transparent
ice--black, because it is pure and of great depth--under the moraine
of a glacier; you readily produce in the interior flaws which no air
can reach, and from these flaws the colours of thin plates sometimes
break like fire.
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