iments, as often as I would have the Refractions
on both sides the Prism to be equal to one another, I noted the Place
where the Image of the Sun formed by the refracted Light stood still
between its two contrary Motions, in the common Period of its Progress
and Regress; and when the Image fell upon that Place, I made fast the
Prism. And in this Posture, as the most convenient, it is to be
understood that all the Prisms are placed in the following Experiments,
unless where some other Posture is described. The Prism therefore being
placed in this Posture, I let the refracted Light fall perpendicularly
upon a Sheet of white Paper at the opposite Wall of the Chamber, and
observed the Figure and Dimensions of the Solar Image formed on the
Paper by that Light. This Image was Oblong and not Oval, but terminated
with two Rectilinear and Parallel Sides, and two Semicircular Ends. On
its Sides it was bounded pretty distinctly, but on its Ends very
confusedly and indistinctly, the Light there decaying and vanishing by
degrees. The Breadth of this Image answered to the Sun's Diameter, and
was about two Inches and the eighth Part of an Inch, including the
Penumbra. For the Image was eighteen Feet and an half distant from the
Prism, and at this distance that Breadth, if diminished by the Diameter
of the Hole in the Window-shut, that is by a quarter of an Inch,
subtended an Angle at the Prism of about half a Degree, which is the
Sun's apparent Diameter. But the Length of the Image was about ten
Inches and a quarter, and the Length of the Rectilinear Sides about
eight Inches; and the refracting Angle of the Prism, whereby so great a
Length was made, was 64 degrees. With a less Angle the Length of the
Image was less, the Breadth remaining the same. If the Prism was turned
about its Axis that way which made the Rays emerge more obliquely out of
the second refracting Surface of the Prism, the Image soon became an
Inch or two longer, or more; and if the Prism was turned about the
contrary way, so as to make the Rays fall more obliquely on the first
refracting Surface, the Image soon became an Inch or two shorter. And
therefore in trying this Experiment, I was as curious as I could be in
placing the Prism by the above-mention'd Rule exactly in such a Posture,
that the Refractions of the Rays at their Emergence out of the Prism
might be equal to that at their Incidence on it. This Prism had some
Veins running along within the Glass from one e