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Chambers, Ephraim, 1680 (ca.)-1740 / Cyclopædia, or, An universal dictionary of arts and sciences : containing the definitions of the terms, and accounts of the things signify'd thereby, in the several arts, both liberal and mechanical, and the several sciences, human and divine : the figures, kinds, properties, productions, preparations, and uses, of things natural and artificial : the rise, progress, and state of things ecclesiastical, civil, military, and commercial : with the several systems, sects, opinions, &c : among philosophers, divines, mathematicians, physicians, antiquaries, criticks, &c : the whole intended as a course of antient and modern learning

Priority - pro-prefect,   pp. 879-898 PDF (19.3 MB)

Page 880

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of taking Provifions for the Support of the Garrifon of his
Chifle, upon paying for them within 40 Days. See Stat.
I Car. a. cap. 34.
PRISM, in Geometry, an oblong Solid or Body, whofe
Planes are all recfilinear and regular, and the oppofite ones
equal. See SOLID, WC.
It is thus call'd from the Greek ;re-t, fomethinga'wn,
or cut off
The 'Prifm is generated by the Motion of a reffilinear
Figure, as ACB (Tab. GEOMETRY, fig. 16.) defcending
always parallel to itfelf, along the right Line A E.
If the Defcribent be a Triangle, the Body is faid to be
a triangular Prifim; if fquare, a quadrangular one, ec.
From the Genefis of the Prifim, 'tis evident it has two
equal and oppofite Bafes; that it is terminated by as many
Parallelograms as the Bafe confifis of Sides; and that all
the Seafions of a Prifim parallel to its Bafe, are equal.
Every triangular 'Prifim may be divided into three equal
Pyramids. See PYRAMID.
To meaflre the Surface and Solidity of a PRISM.
Find the Area of the Bafe, e. gr. A B C (fee TRIANGLE)
and multiply it by E; find the Areas of the Planes or Pa-
rallelograms, that include or circumscribe it, and add their
Sum  to the former Produ&. The Sum is the whole Sur-
face of the Prifia.
Multiply then, the Bafe BAC, by the Altitude CD;
the Product is the Solidity of the Cube A B C D E F. See
All Trifins are in a Ratio compounded of their Bafes and
Altitudes: If then their Bafes be equal, they are to each
other as their Heights; and vice verfia Similar Prifims, -c.
are in a triplicate Ratio of their homologous Sides, as alfo
of their Altitudes.
PRISM, in Dioptricks, is a Glafs in form of a triangular
Prifin, much ufed in Experiments about the Nature of
Light and Colours. See LidnT, E&c.
The Phenomena and Ufe of the Prifim, arife from its
Separating the Rays of Light in their Paffage thro' it. See
The more general of thefe Phenomena are as follow:
For, to enumerate all, would be endlefs; and even thefe
are fufficient to demonfirare, that Colours don't either con-
fift in the Contorfion of the Globules of Light, as fDes Car-
tes imagined; nor in the Obliquity of the Pulfes of the
.lEtherial Matter, as Hook fancied ; nor in the Conflipation
of Light, and its greater or lefs Concitation, as Dr. Barrow
conjeaured: but that they are original and unchangeable
Properties of Light itfelf.
Phge'nomena of the PRISM.
IQ. The Sun's Rays tranfmitted thro' a Prifm to an op-
pofite Wall, projea an Image like the Rainbow, of va-
rious vivid Colours ; the chief whereof are red, yellow,
green, blue, and violet. See RAINBOW.
The reafon is, that the various colour'd Rays, which
were before mixed and blended together, are now, in virtue
of their different Refrangibilities, feparated by Refraction,
in paffing thro' the  Prifi, and thrown, each Colour, by
For the blue Rays, e. gr. represented by the dotted
Lines, (Tab. OPTICKS, fig. 50.) beginning to be fepa-
rated from the rell in the Sideca, of the Prifm abc, by
the firfi Refradfion in dd; are again Separated further in
the other face of the Prifm b c, by a fecond Refraffion,
the fame way, in e e. Whereas in a plain Glafs, or even in
a 'Prifm, in a different Pofition; the blue Rays feparated
by the firfl Refrafaion in the fir{t Surface, are again mixed
by the fecond Refraffion at the other Surface, which is
made a contrary way. See REFRACTION.
a0. The Image thus projecled, is not round; but when
the Angle of the Prifin is 60 or 65 Deg. about five times
as long as broad.
Becaufe fome of the Rays are refracled more than o-
thers, and therefore exhibit feveral Images of the Sun,
firetch'd out in length, as if it were but one.
3Q. Thofe Rays which exhibit the yellow Colour, fwerve
more from the recailinear Courfe, than thofe which exhibit
the Red; and the Green more than the Yellow; and the
Violet mofi of all.
40. If the Prifm thro' which the Rays are tranfmitted,
be turn'd about its Axis ; fo as the red, yellow, green, Edc.
Rays be receiv'd in order, on another Grifm, about twelve
Foot diflant from the former, thro a little hole, and thence
projeled further; the yellow, red, Zc. Rays, rho' they
fall in the fame manner on the fecond Prifm, yet will not
be projecaed on the fame place as the red, but will be de-
fle &ed further, that way towards which the Refraclion is.
And if in lieu of the fecond Prifw, they be receiv'd
on a Lens a little gibbous i the yellow, green, ek. Rays,
will be colleaed, each in its order, into a neater Nocus
than the red ones: The reafon of which two laif Phanomena
is, that the yellow Rays are refraced more than the red
ones, the green ones more than the yellow ones, and the
violet ones mofd of all.
59 The Colours of colour'd Rays well feparated, can
neither be deflroy'd, nor in any manner alter'd by repeated
Refraftions thro a number of Prizfms; nor by paiffing
thro' an illumined Space, nor by their mutual Decu ations,
nor by the Neighbourhood of the Shade, nor by being re-
fleced from any natural Bodies.
Becaufe their Colours are not Modifications arifing from
Refraffion, but original and immutable Properties there.
of. See COLOUR.
60. All colour'd Rays colleded together in any manner,
either by feveral Prifms, or a convex Lens, or concave
Speculum, form Whitenefs; but being again Separated af-
ter Decuflation, each exhibits its proper Colour.  See
Becaufe, as the Ray was white e'er its parts were fepa-
rated by Refraffion i fo thofe parts being re-mix'd, it re-
covers its Whitenefs ; and the colour'd Rays when they
meet, don't defiroy one another, but only intermix.
Hence Dufls,or Pouders,red,yellow, green, blue,violet,$'c.
mix'd in a certain proportion, become grey i or of the Colour
arifing from a Mixture of black and white ; and would be
perfectly white, but that fome of the Rays are abforb'd.
Thus if a Circle of Paper be fmenar'd with all thefe Co.
lours a-part, in a certain proportion, and turn'd fwiftly a.
bout its Centre, fo that the Species of the feveral Colours
may be confounded in the Eye by the Velocity of the Mo-
tion; the feveral Colours will difappear, and the whole be
feen of one uniform Colour, between black and white.
70. If the Sun's Rays firike very obliquely on the inner
Superficies of a Prifm  i the Rays reflected will be violet;
thofe tranfmitted, red.
8Q. If there be two Prizfis, the one full of a red Liquor,
the other of a blue one; the two join'd together will be
opake: tho' if both be fill'd either with a blue or a red
Liquor, they will, together, be tranfparent. For the one
trani miritng none nut blue, the other none but re
the two together will tranfmit none at all. See Bi
9Q. All natural Bodies, especially white ones, vie
a Prifim held to the Eye; feem fringed or hemme(
fide with red and yellow, on the other with blue
xOf. If two Prifis be fo placed, that the red of
and the purple of the other, meet in a proper Pape
pafs'd with Darknefs; the Image will be pale : bu
thro' a third Prifm held to the Eye at a due diflai
appear double, red and purple.
And if two kinds of Dulls, the one perfealy
other blue, be mixed; a little Body being covei
with the Mixture, will exhibit a double Image,
red, the other blue, thro' a MPrirfm apply'd to the ]
11. If the Rays tranfmitted thro' a convex I
receiv'd on a Paper before they meet in the F
Confine of Light and Shadow will feem tinged wii
Colour; if beyond the Focus, with a blue.
120. If the Rays about to be transmitted thro'
of the Pupil, be intercepted by the Interpofition
opake Body near the Eye; the Extremes of Bodi
beyond it, will feem ting'd with Colours, as if fi
a Prinfm; tho' lefs vivid.
Becaufe the Rays tranfmitted thro' the refi of tl
are feparated by Refrafion into Colours; and the i
ted Rays, which would be refraled a contrary %
prevented from mixing and diluting them: Whe
it is, that a Body viewed with both Eyes thro' v
Holes made in a Paper, does not only appear doul
tinged with Colours too.
PRISMOID, PRIsMoIDES, in Geometry, a folk
bounded by feveral Planes, whofe Bafes are righ
Parallelograms, parallel and alike fituated. See Pi
PRISONER, in Law, one that is reflrain'd of his
upon any Affion,Civil or Criminal; or upon Comma
A Man, again, may be Prifoter either upon A
Faa, or of Record.
Prifoner upon Matter of Record, is he who be
fent in Court, is by the Court committed to Prif
upon an Arrefi, be it by the Sheriff, Conflable, or
PRIVATION, the Abfence, Want, or Defec?,
thing needed.
In the Canon Law, Privation is ufed for an Inn
or Sufpenfion. See DEPRIVATroN.
The Myflic Divines call Privation of God, the I
a Soul experiences, to whom God does not makt
The Church of Rome teaches, that Childre
without Baptifm, go into a Limbus, where they a
Privation of the Sight of God.

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