differ in nothing but their thickness, the Diameters of the
Rings are reciprocally in a subduplicate Proportion of the thicknesses
of the Plates. And this shews sufficiently that the Rings depend on both
the Surfaces of the Glass. They depend on the convex Surface, because
they are more luminous when that Surface is quick-silver'd over than
when it is without Quick-silver. They depend also upon the concave
Surface, because without that Surface a Speculum makes them not. They
depend on both Surfaces, and on the distances between them, because
their bigness is varied by varying only that distance. And this
dependence is of the same kind with that which the Colours of thin
Plates have on the distance of the Surfaces of those Plates, because the
bigness of the Rings, and their Proportion to one another, and the
variation of their bigness arising from the variation of the thickness
of the Glass, and the Orders of their Colours, is such as ought to
result from the Propositions in the end of the third Part of this Book,
derived from the Phaenomena of the Colours of thin Plates set down in the
first Part.

There are yet other Phaenomena of these Rings of Colours, but such as
follow from the same Propositions, and therefore confirm both the Truth
of those Propositions, and the Analogy between these Rings and the Rings
of Colours made by very thin Plates. I shall subjoin some of them.

_Obs._ 10. When the beam of the Sun's Light was reflected back from the
Speculum not directly to the hole in the Window, but to a place a little
distant from it, the common center of that Spot, and of all the Rings of
Colours fell in the middle way between the beam of the incident Light,
and the beam of the reflected Light, and by consequence in the center of
the spherical concavity of the Speculum, whenever the Chart on which the
Rings of Colours fell was placed at that center. And as the beam of
reflected Light by inclining the Speculum receded more and more from the
beam of incident Light and from the common center of the colour'd Rings
between them, those Rings grew bigger and bigger, and so also did the
white round Spot, and new Rings of Colours emerged successively out of
their common center, and the white Spot became a white Ring
encompassing them; and the incident and reflected beams of Light always
fell upon the opposite parts of this white Ring, illuminating its
Perimeter like two mock Suns in the opposite parts of an Iris. So then
the D