twentieth Part of an Inch broad, or narrower; the Light of the Image
_pt_ will be as simple as before, or simpler, and the Image will become
much broader, and therefore more fit to have Experiments try'd in its
Light than before.
Instead of this Parallelogram Hole may be substituted a triangular one
of equal Sides, whose Base, for instance, is about the tenth Part of an
Inch, and its Height an Inch or more. For by this means, if the Axis of
the Prism be parallel to the Perpendicular of the Triangle, the Image
_pt_ [in _Fig._ 25.] will now be form'd of equicrural Triangles _ag_,
_bh_, _ci_, _dk_, _el_, _fm_, &c. and innumerable other intermediate
ones answering to the triangular Hole in Shape and Bigness, and lying
one after another in a continual Series between two Parallel Lines _af_
and _gm_. These Triangles are a little intermingled at their Bases, but
not at their Vertices; and therefore the Light on the brighter Side _af_
of the Image, where the Bases of the Triangles are, is a little
compounded, but on the darker Side _gm_ is altogether uncompounded, and
in all Places between the Sides the Composition is proportional to the
distances of the Places from that obscurer Side _gm_. And having a
Spectrum _pt_ of such a Composition, we may try Experiments either in
its stronger and less simple Light near the Side _af_, or in its weaker
and simpler Light near the other Side _gm_, as it shall seem most
convenient.
[Illustration: FIG. 25.]
But in making Experiments of this kind, the Chamber ought to be 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
41