An Explication of the Fifth PLATE.
Figure 1. Is one of Sir _Isaac Newton_'s Experiments, to shew the
different Refrangibility of the Rays of Light, of the different Colours,
Red, Orange, Yellow, Green, Blue, Indigo, Violet. Where D E is a
Parallelogram of Pastboard, having the one half D G blue, and the other
half F E red; both strongly illuminated by the same Candle: and having
black Silk wrapped several times round it. M N is a Lens or double Convex
Glass interpos'd, which gathers upon white Paper the blue Rays sooner at
_h i_ than the Red at H I: As appears by the Distinctness of the Colours
and of the Silk at those and only those Distances. Where also at somewhat
above 12 Feet from the Colours to the Images, the Distance between _h i_
and H I is no less than an Inch and half.
_Fig. 2._ Is another of Sir _Isaac_'s Experiments to the same Purpose:
Where X Y represents the Sun: E G, a Window, with a small round Hole at F:
within which is a Triangular Glass Prism A B C, by which the Rays of the
Sun are differently refracted upon a white Wall or Paper M N; and become
an Oblong Image P T; the Violet seen at P as most refracted; and the Red
at T, as least refracted: And the intermediate Colours seen in
intermediate Places, according to the different Degrees of their
Refraction.
_Fig. 3._ Is another of his Experiments, to shew that White is a Mixture
of all Sorts of colour'd Rays; where D C is a Hole in the Window, which
admits the Sun's Rays. E F G a Prism, casting its oblong colour'd Image
upon a Lens, or double Convex Glass; which collects all those Rays into
its Focus. In which Case, the Point of Concourse exhibits a perfect White
Colour; tho' upon their Separation again, the oblong colour'd Image
appears again, only in an inverted Position: as the crossing of the Rays
in the Focus must of Necessity occasion.
_Fig. 4._ Is the last Experiment improv'd; by shewing that the White Light
made by the Mixture of all the Colours is but imperfectly so, when any of
the several Colours are intercepted in their Passage to their Focus, or
Place of Mixture.
_Fig. 5._ Is the _Experimentum Crucis_, or determining Experiment. Where
B F is the Hole that lets in a large Ray of Light: whose middle Part,
after it has pass'd through the Prism A B C, is let through a lesser Hole
at G, and forms an oblong colour'd Image at _d e_: where another small
Hole lets thro' one Colour only; which passing through the Second Prism
_a b c_ it is refracted again, and cast upon N M. And here it is most
remarkable, that the two Holes and second Prism are kept immoveable; and
so the Rays G _g_ fall upon the second Prism in the very same Angle,
whatever Colour they are of, and that by the Motion of the first Prism,
all the Colours may successfully pass through the same Holes. Yet is the
Refraction by the second Prism never then able to produce any Variety of
Colours; but exhibits the Image always of that Colour alone, which falls
upon it before the second Refraction.
_Fig. 6._ Is a Figure for the Explication of the several Refractions and
Reflections of Light, which cause the _Phænomena_ of the Rainbow. Thus if
the greatest Crowd of Rays enter in Parallel to B Q along or near to A N,
the round Drop of Water L B G Q will refract Part of those Rays to F,
whence Part of them will be reflected to G: And going there out of the
Drop, will be thereby refracted to R, which double Refraction will so
separate the several Colours, and make them go out in Angles so sensibly
different, that as the Eye is placed a little higher or lower, it will see
a different Colour; and that in Angles as A X R, of about 41 Degrees; and
this is the Case of the primary Rainbow, which appears in about that Angle
from the Axis B Q, or its Parallel A X. Thus also, if the same Line A N be
now suppos'd to represent another Drop, and that some of the Rays at G are
reflected a second time, and so pass out at H, and are there refracted to
S; here will be a weaker Impression, but a like Refraction and Separation
of the Colours as before; and the Eye placed a little higher or lower will
also see different Colours, tho' in a contrary Order to the former; and
that in an Angle, as A Y S, of about 52 Degrees and a half; which is the
Case of the secondary Rainbow.
_Fig. 7._ Are the two Rainbows themselves, r presented as they appear in
Nature. Where A E B F represents the Air full of spherical Drops of Rain,
in such Parts as the Angles E O P, F O P are about 41 Degrees from the
Axis O P, which Axis is the Line from the Sun's Center, through the Eye of
the Spectator, to the Center of the Rainbow: And where C G D H represents
the same Air, full of the like Drops, in such Parts where the Angles
G O P, H O P are about 52 Degr. and a half. Where also the Rays S E, S F,
S G, S H, coming from the Sun's Center, are represented as parallel, by
reason of its vast Distance. These Rays, when they fall upon the higher
Quadrant of the Drop, as at S E, S F, come to the Eye at O in about an
Angle of 41 Degrees, after two Refractions, and one Reflection; and so
cause the primary Rainbow: the Red is without, by the least refrangible
Rays at F: and the blue within, by the more refrangible Rays at E. But
when they fall upon the lower Quadrant of the Drop, as at S G, S H, they
come to the same Eye at O, but in an Angle of about 52 Degrees and a half,
after two Refractions, and two Reflections, and so cause the secondary
Rainbow. Which is Blue without, by the more refrangible Rays at H; and Red
within by the least at G. Where note, that because the Angles F O P,
E O P, as well as those H O P, G O P, are ever the same, the same Colours
must still be circular, or appear in the Surface of a right Cone, whose
Axis is O P, and whose Sides are the Lines turned round thereon, as O E
O F, and O G O H.
[[Hydrostaticks Plate I. ― I. Senex sculp.^t]]
12
HYDROSTATICKS.
An Explication of the First PLATE.
Figure 1. Is a Balance, to weigh Water in its own Element, and in the Air;
and to prove that its Weight is the very same in the former Case as in the
latter. For when the Glass Bottle F is exhausted of Air, it will indeed
require much more Weight to counterpoise it in the Air, than in the Water;
by Reason of the much greater Weight of the Water thrust out by it, than
of the Air; yet when upon the Admission of Water within, you weigh it
again in the Air, and then in the Water, the additional Counterpoise now
necessary is the very same; and shews that the real Weight of the Water
admitted, is the same in both Elements. This Figure does also shew how
Trials may be made to shew the respective Weight of those Bodies in Fluids
that sink in them.
_Fig. 2._ Is an inverted Syphon, to shew why Fluids ever press according
to perpendicular Altitude, and not according to Quantity of Matter: As the
small Quantity of Water in the smaller Tube is a Balance for the great
Quantity in the greater, and stands upon the same Level C D E G; because
in all possible Motions and Vibrations of the Fluid, the Velocity in the
smaller must, by the Make of the Syphon, compensate the Quantity in the
larger; the one ascending or descending as far as B D, while the other
ascends only as far as E H, and so the Force is equal on both Sides, as is
the known Case in the Stiliard also.
_Fig. 3._ Is to shew the same equal perpendicular Height or Level in a
common Syphon inverted.
_Fig. 4._ Is a Number of hollow Tubes, of all Shapes and Directions, to
shew that if their lower Orifices be put under tinged Water, and Oil be
poured on the Surface of that Water, from G H to E F, the tinged Water
will equally be pressed upwards through all the Tubes, according to all
Directions; and will stand upon a common Level; tho' somewhat under the
Surface of the Oil, because Oil is lighter than Water.
_Fig. 5._ Is for the same Experiment with Water on the Surface of
Quicksilver; into which Quicksilver a hollow Tube is inserted before the
pourings in of the Water. For the Water will press upon the Quicksilver,
and raise it in the small Tube, till it bears the same Proportion to the
Height of the Water, that the Specifick Gravity of Water bears to that of
Quicksilver, or about a fourteenth Part so high. Which, by the by, is one
ready Way also of finding the Specifick Gravity of Quicksilver to Water,
by measuring their several Altitudes.
_Fig. 6._ Is to shew how Water in a very small Tube may elevate
Quicksilver it self, when it is thrust more below the Surface of the
Water, than the Difference of their Specifick Gravity requires; and that
it will rise or fall as you thrust it lower, or raise it higher; and will
at last fall out at the Bottom, if you raise it too high.
_Fig. 7._ Is to shew that Fluids of different Specifick Gravities, as
Water A B, and Oil A C, will stand at unequal perpendicular Altitudes, in
Proportion to their Quantities, and Difference of Specifick Gravities.
_Fig. 8._ Is a Part of a Compound Balance, to be joined to that of
_Fig. 1._ for the weighing of Levity, or of the Power of Ascent in a Body,
as F, lighter than the Fluid wherein it is; and will shew that that Levity
is the Difference of the Weight of that Body, and of an equal Bulk of the
Fluid: Which is also the respective Gravity of those Bodies which are
heavier than their Fluids, as may be tried by the same Balance of
_Fig. 1._ alone.
[[Plate II. ― I. Senex sculp.^t]]