going in and out at the first Surface be in Fits of easy Transmission,
and the Intervals and Numbers of those Fits between the first and second
Surface, before and after Reflexion, be equal, the distances of the Fits
of easy Transmission from either Surface, must be in the same
progression after Reflexion as before; that is, from the first Surface
which transmitted them in the 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 Ph nomena 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
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