made as
dark as can be, lest any Foreign Light mingle it self with the Light of
the Spectrum _pt_, and render it compound; especially if we would try
Experiments in the more simple Light next the Side _gm_ of the Spectrum;
which being fainter, will have a less proportion to the Foreign Light;
and so by the mixture of that Light be more troubled, and made more
compound. The Lens also ought to be good, such as may serve for optical
Uses, and the Prism ought to have a large Angle, suppose of 65 or 70
Degrees, and to be well wrought, being made of Glass free from Bubbles
and Veins, with its Sides not a little convex or concave, as usually
happens, but truly plane, and its Polish elaborate, as in working
Optick-glasses, and not such as is usually wrought with Putty, whereby
the edges of the Sand-holes being worn away, there are left all over the
Glass a numberless Company of very little convex polite Risings like
Waves. The edges also of the Prism and Lens, so far as they may make any
irregular Refraction, must be covered with a black Paper glewed on. And
all the Light of the Sun's Beam let into the Chamber, which is useless
and unprofitable to the Experiment, ought to be intercepted with black
Paper, or other black Obstacles. For otherwise the useless Light being
reflected every way in the Chamber, will mix with the oblong Spectrum,
and help to disturb it. In trying these Things, so much diligence is not
altogether necessary, but it will promote the Success of the
Experiments, and by a very scrupulous Examiner of Things deserves to be
apply'd. It's difficult to get Glass Prisms fit for this Purpose, and
therefore I used sometimes prismatick Vessels made with pieces of broken
Looking-glasses, and filled with Rain Water. And to increase the
Refraction, I sometimes impregnated the Water strongly with _Saccharum
Saturni_.

_PROP._ V. THEOR. IV.

_Homogeneal Light is refracted regularly without any Dilatation
splitting or shattering of the Rays, and the confused Vision of Objects
seen through refracting Bodies by heterogeneal Light arises from the
different Refrangibility of several sorts of Rays._

The first Part of this Proposition has been already sufficiently proved
in the fifth Experiment, and will farther appear by the Experiments
which follow.

_Exper._ 12. In the middle of a black Paper I made a round Hole about a
fifth or sixth Part of an Inch in diameter. Upon this Paper I caused the
Spectrum of homogeneal Ligh