[Illustration: FIG. 2.]

_Obs._ 5. To determine the interval of the Glasses, or thickness of the
interjacent Air, by which each Colour was produced, I measured the
Diameters of the first six Rings at the most lucid part of their Orbits,
and squaring them, I found their Squares to be in the arithmetical
Progression of the odd Numbers, 1, 3, 5, 7, 9, 11. And since one of
these Glasses was plane, and the other spherical, their Intervals at