hrough which
the Light enters into the dark Chamber, there is a Penumbra made in the
Circuit of the Spectrum Y, and that Penumbra remains in the rectilinear
Sides of the Spectrums PT and _pt_. I placed therefore at that hole a
Lens or Object-glass of a Telescope which might cast the Image of the
Sun distinctly on Y without any Penumbra at all, and found that the
Penumbra of the rectilinear Sides of the oblong Spectrums PT and _pt_
was also thereby taken away, so that those Sides appeared as distinctly
defined as did the Circumference of the first Image Y. Thus it happens
if the Glass of the Prisms be free from Veins, and their sides be
accurately plane and well polished without those numberless Waves or
Curles which usually arise from Sand-holes a little smoothed in
polishing with Putty. If the Glass be only well polished and free from
Veins, and the Sides not accurately plane, but a little Convex or
Concave, as it frequently happens; yet may the three Spectrums Y, PT and
_pt_ want Penumbras, but not in equal distances from the Prisms. Now
from this want of Penumbras, I knew more certainly that every one of the
Circles was refracted according to some most regular, uniform and
constant Law. For if there were any irregularity in the Refraction, the
right Lines AE and GL, which all the Circles in the Spectrum PT do
touch, could not by that Refraction be translated into the Lines _ae_
and _gl_ as distinct and straight as they were before, but there would
arise in those translated Lines some Penumbra or Crookedness or
Undulation, or other sensible Perturbation contrary to what is found by
Experience. Whatsoever Penumbra or Perturbation should be made in the
Circles by the cross Refraction of the second Prism, all that Penumbra
or Perturbation would be conspicuous in the right Lines _ae_ and _gl_
which touch those Circles. And therefore since there is no such Penumbra
or Perturbation in those right Lines, there must be none in the
Circles. Since the distance between those Tangents or breadth of the
Spectrum is not increased by the Refractions, the Diameters of the
Circles are not increased thereby. Since those Tangents continue to be
right Lines, every Circle which in the first Prism is more or less
refracted, is exactly in the same proportion more or less refracted in
the second. And seeing all these things continue to succeed after the
same manner when the Rays are again in a third Prism, and again in a
fourth refracted side