Page:Cyclopaedia, Chambers - Volume 2.djvu/503

 PRI

( 880 )

PRI

of taking Proviiions for the Support of the Garrifon of his Gillie, upon paying for them within 40 Days. See Stat. 12 Car. 2. cap- 34.

PRISM, in Geometry, an oblong Solid or Body, whofe Planes are all rectilinear and regular, and the oppofite ones equal. See Solid, &c.

It is thus call 'd from the Greek vjHrpz, fomething/JjWM, or cut off.

The Prifm is generated by the Motion of a rectilinear Figure, as ACB (Tab. Geometry, fig. io\) 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 Prifm ; if fquare, a quadrangular one, iSc.

From the Gencfis of the Prifm, 'tis evident it has two equal and oppofite Bafes ; that it is terminated by as many Parallelograms as the Bafe confifts of Sides ; and that all the Sections of a Prifm parallel to its Bafe, are equal.

Every triangular Prifm may be divided into three equal Pyramids. See Pyramid.

To meafure 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 5 find the Areas of the Planes or Pa- rallelograms, that include or circumfcribe it, and add their Sum to the former Product.. The Sum is the whole Sur- face of the 'Prifm.

Multiply then, the Bafe BAG, by the Altitude CD; the Product is the Solidity of the Cube A B C D E F. See Centro-baric.

Ail Prifms are in a Ratio compounded of their Bafes and Altitudes : If then their Bafes be equal, they are to each other as their Heights ; znAviceverfi. Similar Prifms, $$c, are in a triplicate Ratio of their homologous Sides, as alfo of their Altitudes.

Prism, in Dioptricks, is a Glafs inform of a triangular Prifm, much ufed in Experiments about the Nature of Light and Colours. See Lic/iit, ££c.

The Phenomena and Ufe of the Prifm, arife from its feparating the Rays of Light in their Paffage thro' it. See Ray.

The more general of thefe Phenomena are as follow : For, to enumerate all, would be endlefs; and even thefe are fufficient to demonitrare, that Colours don't either con- fift in the Contorfion of the Globules of Light, as 2)es Car- tes imagined; nor in the Obliquity of the Pulfes of the jEtherial Matter, as Hook fancied ; nor in the Conftipation of Light, and its greater or lefs Conciration, as Dr. Barrow conjectured: but that they are original and unchangeable Properties of Light itfelf.

of the Prism.

1°. The Sun's Rays tranfmitted thro' a Prifm to an op- pofite Wail, project an Image like the Rainbow, of va- rious vivid Colours ; the chief whereof are red, yellow, green, blue, and violet. See Rainbow.

The rcafon 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 Prifm, and thrown, each Colour, by itfelf. See Refr angibility.

For the blue Rays, e. gr. reprefented by the dotted Lines, (Tab. Optjcks, fig. 50.) beginning to be fepa- rated from the reft in the Sidec*?, of the Prifm abc, by the firft Refraction in d d $ are again feparated further in the other face of the Prifm be, by a fecond Refraction, the fame way, in ee. Whereas in a plain Glafs, or even ia a Prifm, in a differenr Pofirion ; the blue Rays feparated by the firft Refraction in the firft Surface, are again mixed by the fecond Refraction at the other Sutface, which is made a contrary way. See Refraction.

2 . The Image rhus projected, is not round; but when the Angle of the Prifm is 00 or 65 Deg. about five times as long as broad.

Becaufe fome of the Rays are refracted more than 0- thers, and therefore exhibit feveral Images of the Sun, ftretch'd out in length, as if it were but one.

3 Q. Thofe Rays which exhibit the yellow Colour, fwerve more from the rectilinear Courfe, than thofe which exhibit the Red ; and the Green more than the Yellow 5 and the Violet moll of all.

4 . If the Prifm thro' which the Rays are tranfmitted, be turn'd about its Axis ; fo as the red, yellow, green, &c. Rays be receiv'd in order, on another Prifm, about twelve Foot diftant from the former, thro a little hole, and thence proje3ed further; the yellow, red, £Jfc. Rays, tho' they fall in the fame manner on the fecond Prifm, yet will not be projected on the fame place as the red, but will be de- flected further, that way towards which the Refraction is.

And if in lieu of the fecond Prifm, they be receiv'd on a Lens a little gibbous; the yellow, green, g?c, Rays,

will be collected, each in its order, into a nearer Focu- than the red ones : The real'on of which two laft Phenomena is, that the yellow Rays are refracted more than the red ones, the green ones more than the yellow ones, and the violet ones moil of all.

neither be deftroy'd, nor in any manner alter'd by repeated Refraaions thro' a number of Prifms ; nor by paffing thro' an illumined Space, nor by their mutual Decuflatiom nor by the Neighbourhood of the Shade, nor by being re- flected from any natural Bodies.
 * •>. The Colours of colour'd Rays well feparated, can

Becaufe their Colours are not Modifications arifing from Refraction, but original and immutable Properties there- ot. See Colour.

6". All colour'd Rays collected together in any manner either by feveral Prifms, or a convex Lens, or concave Speculum, form Whitenefs ; but being again feparated af- ter Decuflation, each exhibits its proper Colour. See Whiteness.

Becaufe, as the Ray was white e'er its parts were fepa- rated by Refraction ; fo thofe parts being re-mix'd, it re- covers its Whitenefs; and the colour'd Rays when they meet, don't deftroy one another, but only intermix.

Hence Dufts.or Pouders,red,yellow, green, blue,violct,££?c. mix'd in a certain proportion, become grey; 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 fmear'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.

^o. If the Sun's Raysftrike very obliquely on the inner Superficies of a Prifm; the Rays reflected will be violet; thofe tranfmitted, red.

8°. If there be two Prifms, 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 tranfmitting none but blue, the other none but red Rays; the two together will tranfmit none at all. See Blue.

9°. All natural Bodies, efpecially white ones, view'd thro a Prifm held to the Eye ; feem fringed or hemmed, on one fide with red and yellow, on the other with blue and vio- let.

io°. KtwoPrifms be fo placed, that the red of the one, and the purple of the other, meet in a proper Paper encom- pafs'd with Darknefs; the Image will be pale : but view'd thro' a third Prifm held to the Eye at a due diftance, will appear double, red and purple.

And if two kinds of Dufts, the one perfectly red, the other blue, be mixed ; a little Body being cover'd thick with the Mixture, will exhibit a double Image, the one red, the other blue, thro' a Prifm apply'd to the Eye.

ii°. If the Rays tranfmitted thro' a convex Lens, be receiv'd on a Paper before they meet in the Focus,' the Confine of Light and Shadow will feem tinged with a red Colour; if beyond the Focus, with a blue.

12 . If the Rays about to be tranfmitted thro' one part of the Pupil, be intercepted by the Interpofition of fome opake Body near the Eye ; the Extremes of Bodies lying beyond it, will feem ting'd with Colours, as if feen thro' a Prifm ; tho' lefs vivid.

Becaufe the Rays tranfmitted thro' the reft of the Pupil, are feparated by RefraCtion into Colours; and the intercep- ted Rays, which would be refracted a contrary way, are Prevented from mixing and diluting them : Whence' alfo it is, that a Body viewed with both Eyes thro' two little Holes made in a Paper, does not only appear double, but tinged with Colours too.

PRISMOID, Prismoides, in Geometry, a folid Figure, bounded by feveral Planes, whofe Bafes are right-angled Parallelograms, parallel and alike fituated. See Prism.

PRISON. See Goal.

PRISONER, in Law, one that is reftrain'd of his Liberty upon any AClion, Civil or Criminal ; or upon Commandment.

A Man, again, may bePrifoner either upon Matter of FaCt, or of Record.

Prifoner upon Matter of Record, is he who being pre- fent in Courr, is by the Court committed to Prifon, only upon an Arreft, be it by the Sheriff, Conftable, or other.

PRIVATION,, theAbfence, Want, or Defect, of fome- thing needed. /

In the Canon Law, Privation is ufed for an InierdiSio'i or Sufpenfion. See Deprivatton.

The Myftic Divines call Privation of God, the DryneflVs a Soul experiences, to whom God docs not make hirnfelf felt.

The Church of Rome teaches, that Children dying without Baptifm, go into* Ijmbus, where they undergo & Privation of the Sight of God.

PRI"