ngs within that
white Circle were less in the green than in the blue, and still less in
the yellow, and least in the red. For the Angles of Reflexion of those
Rays which made this Ring, being equal to their Angles of Incidence, the
Fits of every reflected Ray within the Glass after Reflexion are equal
in length and number to the Fits of the same Ray within the Glass before
its Incidence on the reflecting Surface. And therefore since all the
Rays of all sorts at their entrance into the Glass were in a Fit of
Transmission, they were also in a Fit of Transmission at their returning
to the same Surface after Reflexion; and by consequence were
transmitted, and went out to the white Ring on the Chart. This is the
reason why that Ring was of the same bigness in all the Colours, and why
in a mixture of all it appears white. But in Rays which are reflected in
other Angles, the Intervals of the Fits of the least refrangible being
greatest, make the Rings of their Colour in their progress from this
white Ring, either outwards or inwards, increase or decrease by the
greatest steps; so that the Rings of this Colour without are greatest,
and within least. And this is the reason why in the last Observation,
when the Speculum was illuminated with white Light, the exterior Rings
made by all Colours appeared red without and blue within, and the
interior blue without and red within.

These are the Phaenomena of thick convexo-concave Plates of Glass, which
are every where of the same thickness. There are yet other Phaenomena
when these Plates are a little thicker on one side than on the other,
and others when the Plates are more or less concave than convex, or
plano-convex, or double-convex. For in all these cases the Plates make
Rings of Colours, but after various manners; all which, so far as I have
yet observed, follow from the Propositions in the end of the third part
of this Book, and so conspire to confirm the truth of those
Propositions. But the Phaenomena are too various, and the Calculations
whereby they follow from those Propositions too intricate to be here
prosecuted. I content my self with having prosecuted this kind of
Phaenomena so far as to discover their Cause, and by discovering it to
ratify the Propositions in the third Part of this Book.

_Obs._ 13. As Light reflected by a Lens quick-silver'd on the backside
makes the Rings of Colours above described, so it ought to make the like
Rings of Colours in passing through a d