Page:Cyclopaedia, Chambers - Volume 2.djvu/579

 RAY

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RAY

hhofhon; by theGm^j Aeyjw; by the LatiM Rationale and Pe#o- rale; and by the Englijb Tranfiators, Breajl-Plate. See PEC- torale.

The Rationale was a Piece of embroidet'd Stuff wore on the Bteaft, about a Span-fquare.

D11 Cange defcribes it as a double Square of Thread ;of four Colours, interwove with Gold, and fet with twelve precious Stones in four Rows, whereon were engraven the Names oi the twelve Tribes ; and faften'd to the Shoulder by two Chains and two Hooks of Gold.

The Form of the Rationale was prefcribed by God himfelf, Exod. 28.

The Rationale appears to have been alfo antiently wore by the Bifhops under the new Law. But Authors are in doubt about its Form; ibme will have it referable that of the Jews; others take it to be only the Pallium. See Pallium.

RATION A.LIS, an Officer mentioned in feveral antient In- fcriptions

Lampridius, in the Life of Alex. Severn!, ufes Rationalis as fy- ncnimous with Procurator. See Procurator..

The Rationale; were Intendants, or Surveyors under the Em- perors; and though Lampridius pretends they were firft eftablifh- ed by Severn! ; 'tis evident there were fome under Augustus.

RATIOMIS Oi, in Anatomy, the Bone of the Fore-head, 0- therwife cail'd Os Frontis. See Frontis.

RATTLE, among the Antients, was accounted a Mufical In- ftrument, of the pulfatile Kind; cail'd by the Romans, Crepitacn- hm. See Musick.

Mr. Malcolm takes the Tintirnabulunl, Crotalum, and Sijlrum, to have been only lb many different Kinds of Rattles. See Crotalum, Sistrum, <&c.

The Invention of the Rattle is afcribed to Archytas; whence Ariftotle calls it, «$xy*v n-terscy^, Arcbytas's Rattle.

Diogeniaiius adds the Occalion of the Invention; v'm. that ha- ving Children, he contriv'd this Inftrument to prevent their tum- bling other things about the Houfe. So that how much foever fome Inflruments have chang'd their ufes, the Rattle we are fure has preferv'd its.

RATLINES, or as the Seamen call them, Ratlings; thofe Lines which make the Ladder Steps, to get up the Shrouds and Puttocks : Hence cail'd the Ratling! of the Shroudi.

RAVELIN, in Fortification, was andently a flat Baftion, pla- ced in the middle of a Curtain. See Bastion and Curtain.

Tis fince made a detach'd Piece, compofed only of two Faces, which make a Salient Angle, without any Flanks ; and raifed be- fore the Curtains or Counterfcarp, and commonly cail'd an Half- Moon.

A Ravelin is a Triangular Work refetnbling the Point of a Baftion, with the Flanks cut off.

Its ufe before a Curtain, is to cover the oppolite Flanks of the two next Baftions. 'Tis ufed alfo to cover a Bridge or a Gate ; and is always placed without the Moat.

What the Engineers call a Ravelin, die Soldiers generally call a Demi-Lu?ie, or Half-Mom. See DeMi-Lune, <&c

There are alfo double Ravelins, which ferve to defend each other. They are faid to be double when they are join'd by a Curtain.

RAVISHMENT de Card, is a Writ which lies for him who took from the Guardian, the Body of his Ward. See Gard, Guardian, Ward, &e.

Ravishment, in Law, an unlawful taking either a Woman, or an Heir in Ward.

Somedmes it is alfo ufed in the fame Senfe as Rape. See Rape.

RAY, Radius, in Opdcks, a Line of Light, propagated from a radiant Point, through an unrefifting Medium. See Light, Radiant, &c.

Sir lfaac Newton defines Rays to be the leaft Parts of Light, whether fuccellive in the fame Line, or Cotemporary, in feve- ral Lines.

For, that Light confifts of Parts of both Kinds, appears hence, that one may Hop what comes this Moment in any Point, and let pais that which comes the next; and again, may ftop what come3 in this Point, and let pafs that in the next. Now, the leaft Light, or Part of Light which may be thus ftopp'd alone, he calls a Ray of Light.

If the Parts of a Ray of Light do all lie ftraight between the Radiant and the Eye, the Raj is faid to be direel: The Laws and Properties whereof make the Subjcd of Opticks. See Op-

TICKS.

If any of them be turned out of the Direction, or bent in their Paffage, the Ray is faid to be refraSed. See Refrac- tion.

If it (hike on the Surface of any Body, and be driven back, it is faid to be reflected. See Reflection.

In each Cafe, the Ray, as it falls either directly on the Eye, or on the Point of Reflection, or of Refraflion, is faid to beln- cident. See Incidence.

Again, if fevetal Rays be propagated from the Radiant Equi- dittantiy from one another, they are cail'd Parallel Rays. See Pa- rallel.

If they come inclining towards each other, they are cail'd Converging Rays. See CONVERGING.

And if they go continually receding from each other, they arc cail'd diverging Rays. See Diverging.

'Tis from the Circumftances of Rays, that the fereral Kinds of Bodies are diftinguilhed in Opticks. A Body, e. gr. that dir- fufes its own Light, or emits Rays of its own, is cail'd a Lucid, or Luminous Body. See Luminary.

If it only reflect Rays it receives from another, it is cail'd an illuminated Body.

IF it only tranfmit Rays, it is cail'd a travfparent Body. See Transparency.

If it intercept the Rays, or refufe them Paffage, it is cail'd an Opake Body. See Opacity.

Hence, no Body Radiates, i. e. emits Rays, unlefs it be either luminous, or illumined. See Radiation.

'Tis by means of Rays reflected from the feveral Points of il- lumined Objects to the Eye, that they become vifible, and that Vifion is perform'd ; whence fuch Rays are cail'd vifial Rays. See Visual.

In effect we find that any Point of an Object is feen in all Places to which a right Line may be drawn from that Point ■ But it is allow'd, nothing can be feen without Light, therefore every Point of an Object diffufes innumerable Rays every way.

Again, from other Experiments it appears that the Images of all Objects, whence right Lines may be drawn to the Eye, ara painted in the Eye, behind the Cryftallin, very Imall, but very diftinct.

And Iaftly, from other Experiments, that each Ray carries with it the Species, or Image of the radiating Point : And that the fe- veral Rays emitted from the fame Point are again united in one Point, by the Cryftallin, and thus thrown on the Retina. See Vision.

'Tis the Spiffitude, or clofenefs of the Rays emitted from a lu- minous Body, that conftitutes the Intenfinefs of the Light. Yet the Direction wherein the Rays ftrike the Eye, have a good Sway. In effect, a Perpendicular Ray, ftriking with more force than an Oblique one, in the Ratio' of the whole Sine to the Sine of the Angle of Obliquiry, (as follows from the Laws of Percuffion) a Perpendicular Ray will affect the Eye more vividly than an Oblique one in that Ratio.

If then the Spiffitude of the Rays be equal, the Intenlity will be as the Direction ; if the Direction be the fame, the Intenfe- nefs will be as the Spiffitude. If both differ, the Intenfencfs will be in a Ratio compounded of the Direction and the Spiffi- tude.

Hence, Firjl, If Light be propagated in Parallel Rays thro' an unrefifting Medium, its Inteuiity will not be varied by Di- ftance.

Secondly, If Light be propagated in diverging Rays, thro' an un- refifting Medium, its tntenfity will decreafe in a duplicate Ratio of theDiftances from the radiant Point, reciprocally. See Qua- lity.

Thirdly, If Light be propagated in converging Rays through an unrefifting Medium ; its Intenlity will increale in a duplicate Ratio of the Diftances from, ihe Point of Concourfe, recipro- cally.

Fourthly, If the Breadth of an illuminated Plane be to the Di- ftance of the radiant Point, as I to 2000000, 'tis the fame thing as if the Rayi ftruck upon it Parallel : And hence, lince the Dia- meter of the Pupil of the Eye, when largeft, fcarce exceeds 4 of an Inch ; the Ray! will fall upon it Parallel, as to Senfe, at the Diftance of 3860 Englijb Feet, which is nearly 6 Furlongs. See Light.

The effect of Concave Leni'es, and Convex Mirrors, is to make Parallel Rays diverge; converging Rays* become Parallel; and diverging Rays to become more divergent. See Mirrour.

The effed of Convex Lenfes, and Concave Mirrours, is to make Diverging Rays become Parallel; Parallel Rays become Convergent, and Converging Rays to Converge the more. See Lens.

The Rays of Light are not Homogeneous, or Similar, but dif- fer in all the Properties we know of, vise. Refrangibility, Reflexi- bility, and Colour. See Refrangibility, <£rc.

'Tis probable from the different Refrangibility, that the other Differences have their Rife; at leaft it appears, that thofe Rays which agree or differ in this, do fo in all the reft.

Thus from the different Senfations the differently ditpofed Rays excite in us, we call them, red Rays, yellow Rays, &c. See Colour.

The effect of the Prifm is to feparate and fort the different Kinds of Rays, which come blended promifcuoufly from theSun ; and to throw each Kind by it felf, according to its Degree of Refran- gibility and Colour, Red to Red, Blue to Blue, &c. See .Prism.

Befides, Refrangibility, and the other Properties of theRyjof Light already afcertain'd by Obfervation and Experiment, Sir lfaac Newton fufpects they may have many more; particularly, a Power of being infkfled, or bent by the Adliotl of diftant "Bo- dies ; and thofe Rays which differ in Refrangibility, he conceives likewise to differ in this Flexibility.

In palling by the Edges and Sides of Bodies, he conceives that the Rays may be bent feveral times backwards and forwards, with a Motion like that of an Eel; and that thofe Rays which falling