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 _g h i_, 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 _f e d_. By these two Glasses the
parallel Rays, or those nearly so, as proceeding from the same Point of
the Object A B C, (which is to be suppos'd considerably remote) are made
to meet in the intermediate Image _f e d_, 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 D B in its Axis; and that
Prism's Gage F G 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 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.
[[Plate IIII. ― Sutton Nicholls sculp:]]