he progression of the even Numbers 0, 2, 4,
6, 8, &c. and from the second which reflected them, in that of the odd
Numbers 1, 3, 5, 7, &c. But these two Propositions will become much more
evident by the Observations in the following part of this Book.

THE

SECOND BOOK

OF

OPTICKS

_PART IV._

_Observations concerning the Reflexions and Colours of thick transparent
polish'd Plates._

There is no Glass or Speculum how well soever polished, but, besides the
Light which it refracts or reflects regularly, scatters every way
irregularly a faint Light, by means of which the polish'd Surface, when
illuminated in a dark room by a beam of the Sun's Light, may be easily
seen in all positions of the Eye. There are certain Phaenomena of this
scatter'd Light, which when I first observed them, seem'd very strange
and surprizing to me. My Observations were as follows.

_Obs._ 1. The Sun shining into my darken'd Chamber through a hole one
third of an Inch wide, I let the intromitted beam of Light fall
perpendicularly upon a Glass Speculum ground concave on one side and
convex on the other, to a Sphere of five Feet and eleven Inches Radius,
and Quick-silver'd over on the convex side. And holding a white opake
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, a