Now, that I might avoid all the Objections of this kind, by exhibiting an
Experiment that might by ocular proof convince those whom other reasons
would not prevail with, I provided me a _Prismatical Glass_, made hollow,
just in the form of a Wedge, such as is represented in the tenth _Figure_
of the sixth _Scheme_. The two _parallelogram_ sides ABCD, ABEF, which met
at a point, were made of the clearest Looking-glass plates well ground and
polish'd that I could get; these were joyn'd with hard cement to the
_triangular_ sides, BCE, ADF, which were of Wood; the _Parallelogram_ base
BCEF, likewise was of Wood joyn'd on to the rest with hard cement, and the
whole _Prismatical_ Box was exactly stopt every where, but onely a little
hole near the base was left, whereby the Vessel could be fill'd with any
liquor, or emptied again at pleasure.

One of these Boxes (for I had two of them) I fill'd with a pretty deep
tincture of _Aloes_, drawn onely with fair Water, and then stopt the hole
with a piece of Wax, then, by holding this Wedge against the Light, and
looking through it, it was obvious enough to see the tincture of the liquor
near the edge of the Wedge where it was but very thin, to be a pale but
well colour'd Yellow, and further and further from the edge, as the liquor
grew thicker and thicker, this tincture appear'd deeper and deeper, so that
near the blunt end, which was seven Inches from the edge and three Inches
and an half thick; it was of a deep and well colour'd Red. Now, the clearer
and purer this tincture be, the more lovely will the deep Scarlet be, and
the fouler the tincture be, the more dirty will the Red appear; so that
some dirty tinctures have afforded their deepest Red much of the colour of
burnt Oker or _Spanish_ brown; others as lovely a colour as _Vermilion_,
and some much brighter; but several others, according as the tinctures were
worse or more foul, exhibited various kinds of Reds, of very differing
degrees.