When these two beams became more distant there emerged out of the middle
of the purplish red, first a darker round Spot, and then out of the
middle of that Spot a brighter. And now the former Colours (purple,
blue, green, yellow, and purplish red) were become a Ring equal to the
first of the bright Rings mentioned in the four first Observations, and
the Rings about this Ring were grown equal to the Rings about that
respectively; the distance between the two beams of Light and the
Diameter of the white Ring (which was now become the third Ring) being
about 3 Inches.
The Colours of the Rings in the middle began now to grow very dilute,
and if the distance between the two Beams was increased half an Inch, or
an Inch more, they vanish'd whilst the white Ring, with one or two of
the Rings next it on either side, continued still visible. But if the
distance of the two beams of Light was still more increased, these also
vanished: For the Light which coming from several parts of the hole in
the Window fell upon the Speculum in several Angles of Incidence, made
Rings of several bignesses, which diluted and blotted out one another,
as I knew by intercepting some part of that Light. For if I intercepted
that part which was nearest to the Axis of the Speculum the Rings would
be less, if the other part which was remotest from it they would be
bigger.
_Obs._ 12. When the Colours of the Prism were cast successively on the
Speculum, that Ring which in the two last Observations was white, was of
the same bigness in all the Colours, but the Rings without it were
greater in the green than in the blue, and still greater in the yellow,
and greatest in the red. And, on the contrary, the Rings 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
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