Lens I placed a
Prism, by which the trajected Light might be refracted either upwards or
sideways, and thereby the round Image, which the Lens alone did cast
upon the Paper might be drawn out into a long one with Parallel Sides,
as in the third Experiment. This oblong Image I let fall upon another
Paper at about the same distance from the Prism as before, moving the
Paper either towards the Prism or from it, until I found the just
distance where the Rectilinear Sides of the Image became most distinct.
For in this Case, the Circular Images of the Hole, which compose that
Image after the same manner that the Circles _ag_, _bh_, _ci_, &c. do
the Figure _pt_ [in _Fig._ 23.] were terminated most distinctly without
any Penumbra, and therefore extended into one another the least that
they could, and by consequence the Mixture of the heterogeneous Rays was
now the least of all. By this means I used to form an oblong Image (such
as is _pt_) [in _Fig._ 23, and 24.] of Circular Images of the Hole,
(such as are _ag_, _bh_, _ci_, &c.) and by using a greater or less Hole
in the Window-shut, I made the Circular Images _ag_, _bh_, _ci_, &c. of
which it was formed, to become greater or less at pleasure, and thereby
the Mixture of the Rays in the Image _pt_ to be as much, or as little as
I desired.

[Illustration: FIG. 24.]

_Illustration._ In the twenty-fourth Figure, F represents the Circular
Hole in the Window-shut, MN the Lens, whereby the Image or Species of
that Hole is cast distinctly upon a Paper at J, ABC the Prism, whereby
the Rays are at their emerging out of the Lens refracted from J towards
another Paper at _pt_, and the round Image at J is turned into an oblong
Image _pt_ falling on that other Paper. This Image _pt_ consists of
Circles placed one after another in a Rectilinear Order, as was
sufficiently explained in the fifth Experiment; and these Circles are
equal to the Circle J, and consequently answer in magnitude to the Hole
F; and therefore by diminishing that Hole they may be at pleasure
diminished, whilst their Centers remain in their Places. By this means I
made the Breadth of the Image _pt_ to be forty times, and sometimes
sixty or seventy times less than its Length. As for instance, if the
Breadth of the Hole F be one tenth of an Inch, and MF the distance of
the Lens from the Hole be 12 Feet; and if _p_B or _p_M the distance of
the Image _pt_ from the Prism or Lens be 10 Feet, and the refracting
Angle of the P