An Explication of the Second PLATE.
Figure 1. Shews that an Object, as K, seen through a plain Glass, whose
Sides A B, C D, are parallel, by the Eye at G, appears out of its true
Place; and this so much the more as the Glass is thicker: While at the
same time the two Surfaces do exactly balance each other's Refraction, and
make the two Rays H K, G F exactly parallel.
_Fig. 2._ Exhibits a plain Method of measuring the Refraction of Fluids at
all Angles, and of proving thereby that it is always in one fixed
Proportion of the Sines, as the next Figure will explain it. For if the
moveable Rule K C L, with its measuring Circle A B D E fix'd by the Prop
E, to a heavy Pedestal F G, in a large Glass A H I D, be so far immers'd
in the Fluid, that the Center C may be in the Surface of the Fluid, and
one of its Legs C L be so far bent from a rectilinear Position, that the
Refraction of the Fluid can just make it appear as if it were in a strait
Line, the Angle B C K, or its equal M C E, is the Angle of Incidence: And
L C E the Angle of Refraction: And L C M the Difference, or the refracted
Angle.
_Fig. 3._ Is for the Illustration of the former Proposition, and shews the
Sines afore-mentioned; as A D or G N (for they are suppos'd equal, and the
Line A C N one strait Line,) is the Sine of the Angle of Incidence, and
F E the Sine of the Angle of Refraction, which Sines do in the same Fluid
at all Angles bear one and the same Proportion to each other; till at
last, if the Refraction be out of a thick Medium into a thin one, and
makes the second Sine equal to the Radius, that Ray cannot emerge at all,
but will be reflected back by the Surface into the same Medium whence it
came, along the Line C R.
_Fig. 4._ Is a Bason of Water, or other Fluid; to shew the common
Experiment of Refraction; where a Shilling, or other Object at A, (which
is so plac'd that it cannot be seen by the Eye at O, the Side of the Bason
C interposing) is readily seen there, as soon as the Water or other Fluid
is put in to the same Bason, and appears to be remov'd to the Point B.
_Fig. 5._ Is the Alteration of a round white Object D, as seen through a
Triangular Glass Prism A B C, by the Eye at G, where the double Refraction
of the Glass at E and F makes the Object appear at _d_; and that as an
oblong colour'd Image; wherein the upper Part is made by the violet Rays,
which are most refrangible; and the lower by the red Rays, which are least
so; and the intermediate Parts by those that are refrangible in a mean
Degree; after the Order of the Colours of the Rainbow.
_Fig. 6._ Shews the Nature of a multiplying Glass A D, and its Plains A B,
B C, C D, _&c._ and the Reason why the different Refraction of every
oblique Plain, as A B, C D, _&c._ exhibits the same Object K as a
different Object k, k, _&c._ according to the Number of the oblique
Plains: While the direct Plain B C shews it still in its own Place: And
while the Convolution of the Glass on the Axis K L removes all the oblique
Images, but does not remove the direct one, on Account of the Change of
the Position of those oblique Plains, and of the unchanged Position of the
direct Plain.
_Fig. 7._ Shews the Effect of the Lens, or double Convex Glass, in
gathering parallel Rays, as G L, H M, A B, I N, K O, _&c._ towards a
Point, as D; because, as in the Case of the Prism above, the Refraction
_to_ the perpendicular in the Entrance, and _from_ it in the Exit of those
Rays, do still, by the different Position of that Perpendicular, conspire
to unite the same Rays.
_Fig. 8._ Shews the contrary Effect of the double Concave Glass, in
scattering the parallel Rays; and that exactly on the like Account; and so
this needs no new Explication.
_Fig. 9._ Shews the Reason why a Lens, or double Convex, shews a near
Object at Q, as more remote at _q_, because it refracts it so that the
Rays from the same Point meet more backward than before: And why it shews
the same Object larger also: Which must needs be, because every Point in
the Object appearing so much more backward, and yet in the same apparent
Angle, its Length and Breadth must every where be proportionably enlarg'd.
_Fig. 10._ Shews how such a Lens inverts Objects, as A, B, _b a_, which it
does on Account of the Intersection of the Rays from each Point, in or
near the Lens it self: Which necessarily infers such an Alteration: just
as the Images of all Objects are in the Eye in an inverted Position, on
the like Account.
_Fig. 11._ Shews how a Lens does so refract the Rays from every Point of
an Object, that is in its Focus C, and B, and A, that the Rays from each
of those Points do become parallel afterward; and also how parallel Rays
of different Positions are gather'd in that Focus.
_Fig. 12._ Is the Nature of direct Vision by the Eye, in some Conformity
to the 10th Figure: only in this Case the Crystalline Humour is the Lens.
_Fig. 13._ Is the Case of a Concavo-convex Glass, with its parallel
Surfaces, as in _Fig. 1_.
[[Plate III. ― Sutton Nicholls sculp:]]