An Explication of the Third Plate.

Figure 1. Is a Telescope, with two Convex Glasses, the one towards the Object and the Segments of a great Sphere, the other near the Eye, the Segments of a small Sphere ghi, and they are to be so placed that the distinct Base or Image may, by the Collection of the Rays, be in the common Focus of both the Glasses fed. By these two Glasses the parallel Rays, or those nearly so, as proceeding from the same Point of the Object ABC, (which is to be suppos'd considerably remote) are made to meet in the intermediate Image fed, at f, and e, and d; and again at the Bottom of the Eye, at r, and s, and t; but in an erect Position; and therefore so as to shew the Object inverted.

Fig.2. Is a Telescope with four Convex Glasses, the one towards the Object, and three nearer the Eye: Whose Images are made in the common Focus of two Glasses, as before. This is like the former; but only that two of the Eye Glasses serve merely to reinvert, or to erect the Image, that so it may be inverted at the Bottom of the Eye; and therefore may shew the Object in its true or erect Position.

Fig.3. Is a Telescope, with a Convex Object Glass, and a Concave Eye Glass; which last, by scattering the Rays, as if they came from a nearer Point, makes the Image inverted in the Bottom of the Eye, and therefore shews the Object in its true or erect Position. Only this takes in but a small Part of an Object, an so is less used than the two former Telescopes.

Fig.4. Is a Telescope with a Triangular Prism DB in its Axis; and that Prism's Gage FG for the Demonstration of the Refraction out of Vacuum into Air, and out of thinner Air into thicker; and both by the Means of an Object seen through the Prism, as well when the Air is condensed, as when it is exhausted. Where in the first Case the Object is seen higher, and in the other lower than in its natural Situation; as the two following Figures demonstrate.

Fig.5. Shews how the Object or Circle which was low at first, is to Appearance rais'd as it passes through condens'd Air; by being refracted towards the perpendicular, in its Ingress into a Glass Prism, and from it in its Egress into the common Air again.

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 GDH is 45 Degrees; and by consequence the half Angles CDH, CDG, CHG, are 22°30', and where all is to be so contriv'd, that the Rays within the Glass may be parallel to GH, and perpendicular to CD, 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 ED, FD, equally elevated above the horizontal Line AC. 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.