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74

Now we place a little water against a
part of that surface. There are now three
surfaces, one of glass-air, a second of glass-
water, a third of air-water. There is some
thing that is, in the same sense, between the
glass-air surface and the glass-water
surface; and this must necessarily also
be between either of those and the water-air
surface. It is the line where glass
air and water meet. Now let us put
a piece of putty over a portion
of that line. There are now three additional
surfaces, the putty-glass surface, the putty-air
surface, and the putty-water surface, and
there are three additional lines, the