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 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