Dr. Hooke had succeeded in splitting a mineral substance called mica
into films of such extreme thinness as to give brilliant colours. One
plate, for example, gave a yellow colour, another a blue colour, and the
two together a deep purple, but as plates which produced this colour
were always less than the twelve-thousandth part of an inch thick it was
quite impracticable, by any contrivance yet discovered, to measure their
thickness, and determine the law according to which the colours varied
with the thickness of the film. Newton surmounted this difficulty by
laying a double convex lens, the radius of the curvature of each side of
which was fifty feet, upon the flat surface of a plano-convex
object-glass, and in the way he obtained a plate of air, or of space,
varying from the thinnest possible edge at the centre of the
object-glass where it touched the plane surface to a considerable
thickness at the circumference of the lens. When the light was allowed
to fall upon the object-glass, every different thickness of the plate of
air between the object-glasses gave different colours, so that the point
where the two object-glasses touched one another was the centre of a
number of concentric coloured rings. Now, as the curvature of the
object-glass was known, it was easy to calculate the thickness of the
plate of air at which any particular colour appeared, and thus to
determine the law of the phenomena.

By accurate measurements Newton found that the thickness of air at which
the most luminous parts of the first rings were produced were, in parts
of an inch, as 1, 3, 5, 7, 9, and 11 to 178,000.

If the medium or the substance of the thin plate is water, as in the
case of the soap-bubble, which produces beautiful colours according to
its different degrees of thinness, the thicknesses at which the most