f those Rays which are reflected more obliquely than they enter, must
be greater after Reflexion than before, by the 15th and 20th
Propositions. And thence it may happen that the Rays at their return to
the first Surface, may in certain Obliquities be in Fits of easy
Reflexion, and return back to the Quick-silver, and in other
intermediate Obliquities be again in Fits of easy Transmission, and so
go out to the Chart, and paint on it the Rings of Colours about the
white Spot. And because the Intervals of the Fits at equal obliquities
are greater and fewer in the less refrangible Rays, and less and more
numerous in the more refrangible, therefore the less refrangible at
equal obliquities shall make fewer Rings than the more refrangible, and
the Rings made by those shall be larger than the like number of Rings
made by these; that is, the red Rings shall be larger than the yellow,
the yellow than the green, the green than the blue, and the blue than
the violet, as they were really found to be in the fifth Observation.
And therefore the first Ring of all Colours encompassing the white Spot
of Light shall be red without any violet within, and yellow, and green,
and blue in the middle, as it was found in the second Observation; and
these Colours in the second Ring, and those that follow, shall be more
expanded, till they spread into one another, and blend one another by
interfering.

These seem to be the reasons of these Rings in general; and this put me
upon observing the thickness of the Glass, and considering whether the
dimensions and proportions of the Rings may be truly derived from it by
computation.

_Obs._ 8. I measured therefore the thickness of this concavo-convex
Plate of Glass, and found it every where 1/4 of an Inch precisely. Now,
by the sixth Observation of the first Part of this Book, a thin Plate of
Air transmits the brightest Light of the first Ring, that is, the bright
yellow, when its thickness is the 1/89000th part of an Inch; and by the
tenth Observation of the same Part, a thin Plate of Glass transmits the
same Light of the same Ring, when its thickness is less in proportion of
the Sine of Refraction to the Sine of Incidence, that is, when its
thickness is the 11/1513000th or 1/137545th part of an Inch, supposing
the Sines are as 11 to 17. And if this thickness be doubled, it
transmits the same bright Light of the second Ring; if tripled, it
transmits that of the third, and so on; the bright yellow Lig