_Fig. 6._ Shews how the same Object or Circle, which was high at first, is to Appearance _depress'd_, as it passes through the _Vacuum_; by being refracted from the Perpendicular, in its Ingress into the Prism, and towards it, in its Egress into the common Air again.
_Fig. 7._ Is a Triangular Glass Prism, fitted to receive all sorts of Fluids, and when rightly apply'd to the Semi-circle of the next Figure, does exactly measure the refractive Power of all those Fluids. Where the vertical Angle G D H is 45 Degrees; and by consequence the half Angles C D H, C D G, C H G, are 22° 30′, and where all is to be so contriv'd, that the Rays within the Glass may be parallel to G H, and perpendicular to C D, and may fall on each side Plain of the Glass Prism in an Angle of 22° 30′ from their Perpendiculars; that so the Refractions at the Ingress and Egress may be equal, and the Computations easy.
_Fig. 8._ Is the Semicircle, with the Glass Prism full of its Liquor rightly apply'd thereto; and both Arms of the Index E D, F D, equally elevated above the horizontal Line A C. This shews the Proportion of the Sine of the Angle of Incidence to that of Refraction, in this Incidence of 22° 30′; which Proportion of Sines being the same in all other Angles, we hence learn that Proportion accurately and universally.
Figure 1. Is the Apparatus for Microscopes: Containing A C a Cylinder of Brass or Ivory; to which, near the Eye at K, the Microscope it self, or very small Sphere of Glass set Ivory, is apply'd; G H a small Slice of Ivory, and its _Muscovy_ Glass Circles, with the fine Objects upon them, inserted in their true Place; E F a Convex Glass, screwed into the former Cylinder, and at a due Distance casting Light on the Objects; with I L, the Handle of the Microscope.
_Fig. 2._ Is only one of the Slices of Ivory A B, like G H before-mentioned, set by it self; with the double Circles of _Muscovy_ Glass, and kept down by circular Wire; between which, on one of those Glasses, the small Objects are commonly plac'd.
_Fig. 3._ Is a Scheme to demonstrate how the double Microscope comes to magnify so much. Where G is the small Object; which, if there be Light sufficient, may by the small Microscope Glass E F, placed very near the Object, be cast into a larger Image H I: Which by the Means of the two Eye Glasses, are reduc'd into a Compass fit to enter into the Eye. And here by the way it is to be noted that die small Glasses, whereby single Microscope do magnify so much, and whereby the Magnitude is in Part increas'd in this double Microscope, is only a very small spherical Glass, or Segment of it, which does so suddenly reduce distant Rays to Parallelism, or nearly to it, that a small Object, which by its great Nearness could not be otherwise seen, is hereby made visible.