Chart, or a Quire of Paper at the center of the Spheres to which the
Speculum was ground, that is, at the distance of about five Feet and
eleven Inches from the Speculum, in such manner, that the beam of Light
might pass through a little hole made in the middle of the Chart to the
Speculum, and thence be reflected back to the same hole: I observed upon
the Chart four or five concentric Irises or Rings of Colours, like
Rain-bows, encompassing the hole much after the manner that those, which
in the fourth and following Observations of the first part of this Book
appear'd between the Object-glasses, encompassed the black Spot, but yet
larger and fainter than those. These Rings as they grew larger and
larger became diluter and fainter, so that the fifth was scarce visible.
Yet sometimes, when the Sun shone very clear, there appear'd faint
Lineaments of a sixth and seventh. If the distance of the Chart from the
Speculum was much greater or much less than that of six Feet, the Rings
became dilute and vanish'd. And if the distance of the Speculum from the
Window was much greater than that of six Feet, the reflected beam of
Light would be so broad at the distance of six Feet from the Speculum
where the Rings appear'd, as to obscure one or two of the innermost
Rings. And therefore I usually placed the Speculum at about six Feet
from the Window; so that its Focus might there fall in with the center
of its concavity at the Rings upon the Chart. And this Posture is always
to be understood in the following Observations where no other is
express'd.
_Obs._ 2. The Colours of these Rain-bows succeeded one another from the
center outwards, in the same form and order with those which were made
in the ninth Observation of the first Part of this Book by Light not
reflected, but transmitted through the two Object-glasses. For, first,
there was in their common center a white round Spot of faint Light,
something broader than the reflected beam of Light, which beam sometimes
fell upon the middle of the Spot, and sometimes by a little inclination
of the Speculum receded from the middle, and left the Spot white to the
center.
This white Spot was immediately encompassed with a dark grey or russet,
and that dark grey with the Colours of the first Iris; which Colours on
the inside next the dark grey were a little violet and indigo, and next
to that a blue, which on the outside grew pale, and then succeeded a
little greenish yellow, and after that a brighter yellow, and then on
the outward edge of the Iris a red which on the outside inclined to
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