Page:Cyclopaedia, Chambers - Volume 2.djvu/594

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fame Plate will, in the fame Part, repli all the Rays of one Co- lour, and tranfmit all thole of the otner ; but in different Parts, will repli the Rays of one and the fame Colour, at one tbick- nefs, and tranfmit them at another,- and thus alternately, and in infinitum— Now it can never be imagined that at one Place the Rays which, for Inftance, exhibit a blue Colour, fhould have the Fortune to ftrike on the folid Parts, and thofe which exhibit a Red to hit on the void Parts of the Body; and at another Place, where the Body is either a little thicker, or a little thinner, that on the contrary the Blue fhould hit on the Pores, andtheRed upon the folid Parts.

ija. In the Paffage of Light out of Glafs into Air, there is a Repxion as ftrong as in its Paffage out of Air into Glafs, or ra- ther a little ftronger, and by many Degrees ftronger than in its Paffage out of Glafs into Water.

Now it feems improbable that Air fhould have more repBing Parts than Water or Glafs : But if that fhould be fuppofed, yet it will avail nothing; for the Repxiox is as ftrong or ftronger when the Air is drawn from the Glafs by the Air-Pump, as when it is adjacent to it. — If any fhould here objed, on Des Carte's Hypo- thefls, that though the Air be drawn out, there is a fubtle Mat- ter remaining to fupply its Place, which being of a denfer Kind, is better fitted for the Repxion of Light than any other Body : Belidc that we have elfewhere (hewn fuch fubtle Matter to be fictitious; and that fuppoiingitsExiftence, and its replling Power, no Light could ever have been propagated, but muft have been all replied back to the lucid Body, immediately after it was firft emitted ; The following Experiment does evidently convict it of fallity.

f. It Light in its Paffage out of Glafs into Air ftrike more obliquely than at an Angle of 40 or 4.1 Degrees, it is wholly re- plied, it lefs obliquely it is in great Meafure tranfmitted.— Now it is not to be imagined, that Light at one Degree of obliquity fhould meet with Pores enough in the Air to tranfmit the grea- ter Part of it, and at another Degree fhould meet with nothing but Parts to repll it wholly ; efpccially conlidering that in its Palfage out of Air into Glafs, how oblique foever be its Inci- dence, it finds Pores enough in the Glafs to tranfmit a great Part of it. — It any fuppofe that it is not replied by the Air, but by the outmolt fuperricial Parts of the Glafs, there is dill the fame Difficulty : Belides, that fuch a Suppolition is unintelligible, and will alfo appear to be falfe by applying Water behind fbme Part of the Glals inftead of Air: For fb, in a convenient obliquity of the Rays, fuppofe of 4; or 45. Degrees, at which they are all re- plied where the Air is adjacent to the Glafs; they (hall be in great Mealure tranfmitted where the Water is adjacent to it : Which argues, that their Reflexion or Tranfmiffion depends on the Conftitution of the Air and Water behind the Glafs, and not on the ftriking of the Rays upon the Parts of the Glafs; the Rays not being replied 'ii\ they have reached the laft Part of the Sur- face, and are begun to go out. For if in going out they fall up- on the Surface of Oil or Water, they proceed; the Attradion of the Glafs being balanced by an equal Force the contrary Way; and prevented from having its erfed by the Attraction of the Liquor adhering to it : But if the Rays in palling out of this laft Surface fall into a Vacuum which has no Attraction, ' or into Air which has but little, not enough to coun'er-balance the effed of the Glafs ; in this Cafe the Attradion of the Glafs draws them back, and repels them.

This will appear ftill more evident, by laying two Glafs Prifms, or the objeit Glalles of two Teiefcopes, the one Plane and the other a little Convex, upon each other, fo as they may neither touch, nor yet be too far a-part. For that Light which falls on the hind Surface of the firft Glafs, where the GlalTes are not a- bove t ~Jt~ Part of an Inch a-part, will be tranfmitted through the Surface, andthrough the Air or Vacuum between the Glafles, and pals into the fecond Glafs : But if the fecond GlaS be taken away, then the Light palling out of the fecond Surface of the firft Glafs into the Air or Vacuum, will not proceed but return into the tint Glafs, and be replied.

Whence it follows, that the Rays are drawn backagain by fome Force in the firft Glafs; there being nothing elfe to occafion their return.— And hence too, it follows, that the RepBion is not effected by means of any fubrle Matter, contiguous to the hind Surface, according to the Principles of Dei Cartes; fince that Matter ought to reflet! them when the Glafles were nearly con- tiguous, as well as when the fecond Glafs is quite removed.

La ftfy> If it be ask'd how fome of the Rays come to be re- plied, and others tranfmitted : And why they are nor all alike re- Plied; fuppofmg the RepBion owing to the Action of the whole Surface? The fame great Author Chews, that there are both in the Rays of Light, and in the Bodies themfelves, certain Vibrations, (or tome fuch Property) imprefs'd on the Rays, by the Action either of the Luminary that emits them, or of the Bodies that re- PO them; by means whereof it happens that thofe Rays in that

art of their Vibration which confpires with the Motion of the

a, tB ° f 'he Body, enter the Body, are refraded and tranfmit-

> hut thole in a contrary Part of their Vibration reflected.

ee \ior ation ant j Medium. See alfo Refraction and


 * '-.'NSMlssioN.

nwS d ' f thacev «yRayof Light.in itsPaffage through any refrad- g urtace,is put into a certain tranfient Conftitution or State, which

in the Progrefs of the Ray returns at equal Intervals, and difpofcj the Ray at each return to be eaffly tranfmitted through the next refraBmg Surface; and between each Return to be ealily rePBed by it. ' J

Thefe alternate Dirpofitions, which Sir tfiac Newton calls fits of eafy Reflexion, and of eafy Tranfmijfto,,, he accounts for by fup- pofmg that the Rays of Light, in imping ng on Bodies, excite Vibrations therein, which happening to move falter than the Kays, when a Ray is in that Part of the Vibration which con- fpires with its Motion, it paffes through; but when in the con- trary Part of the Vibration, is beat back again : Whence every Ray is fucceftively difpofed to be ealily repaid, or eaffly tranf- mitted, by every Vibration which overtaKes it. See Trans- mission.

Reelection, in Catoptricks, is the return of a Ray of Light from the poliflr'd Surface of aSpeculum or Mirrour, diiven thence by fome Power refiding therein. See Mirrour and Catop- tricks.

The Ray thus returned is called a Repx, or replied Raj, or Ray of Repxion ; and the Point of the Speculum whence the re- turn commences, the Point of Repxion.

Thus the Ray AB (Tab. Optickt, Fig. 26.) proceeding from the Radiant A, and ftriking on the Point of theSpeculum B ; being returned thence to C ; BC rcprefents the replied Rap and B the Point of Repxion : In refped whereor AB reprcfents the Incident Ray, or Ray of Incidence, and B the Point of Incidence. See Point and Ray.

Again, a Line as CG drawn from any Point as C of the replied Ray BC, perpendicular to the Speculum, is call'd the Catbetus of Repxion, or Catbetus of tbe Eye: As a Line AF, drawn from the Radiant Perpendicular to the Speculum, is call'd the Catbetus of Incidence. See CathetUS.

Of the two Augles which the refleliedRay BC makes with the Mirrour, the fmaiieft, CBE, is called the single of Repxiox : As, of the two Angles the incident Ray makes with the Speculumy the fmaiieft ABD is called the Angle of Incidence. See Angle.

If the Mirrour be either Concave, or Convex, the fmaiieft Angles the Ray makes with a Tangent to the Point of RepBi- on and Incidence, are the Angles of Reft xion and Incidence.

The Angle CBH which the replied Ray makes with a Perpen- dicular to the Point of R ft liion, is call'd the Inclination of tbe re- fietted Rays ; as the Angle ABH is called the Inclination ot theferi- dent Ray.

lbs General Laws of Reflection.

1°. If a Ray of Light be replied from a Speculmn of any form; tbe Angle of Incidence is ever equal to the Angle of F.rfl liion.— This Law obtains in Percullions of all Kinds of Bodies, and con- fequently muft do fo in thofe of Light. See Laivs of Per- cussion.

It might therefore be here aflumed as an Axiom : But 'tis of that Importance, and its Demonftration fo beautiful, that we cannot omit it — Suppofe, then, DC (Fig. 54.) an incident Ray, propagated from the Radiant A : Here, though the Motion of the Ray be fimplc, yet its Determination in the Line DC being ob- lique with refpect to the Obftacle, is really compounded of two Determinations; the one along DE, the other along DG. See Compound Motion.

The Force along DC, therefore is equal to the two Forces a- long DG and DH. But the Obftacle GF only oppofes one of the Determinations; viz. that along DG, (for it cannot oppofe a Determination Parallel to it felf, as DE ;) therefore only the Force along DG will be loft by the ftroke ; that along DH or GC remaining entire.— But a Body perfectly Elaftic, (fuch as we fup- pofe the Ray of Light) will recover by its Elaiticity, the force ic loft by the Shock. See Elasticity.

The Ray, therefore, will recover the Force DG or CH. Thus, retaining both its Forces, and both its former Determina- tions HC and CF; after Percuflion; it will be impell'd along CF and CH, by the fame Forces as before along DH and DG. By its compound Motion, therefore, ic will defcribe the right Line CE, and that in the fame time as DC; and HE and DH will be equal, as being defcribed by the fame Force. Now, the two Triangles DCH and CHE are equal, and confequently their fimilar Angles equal. Since then, HCA=HCF; DCA the An- gle of Incidence, is equal to ECF the Angle of RepBion. Q_

This Law is confirmed in Light b'y an eafy Experiment. For a Ray of the Sun falling on a Mirror, in a dark Room, through a little Hole; you'll have the Pleafure to fee it rebound, fo as to make the Angle of- Repxion equal to that of Incidence. See Camera Qbfcura.

The fame may be lliewn various other Ways : Thus, e. gr. Placing a Semicircle FIG (Tab. Oplids, Fig. 26.) on a Mirror DE, its Centre on B, and its Limb Perpendicular to the Specu- lum; and afltiming equal Arches, FaandGr; place an Object in A, and the Eye in C : Then will the Object be feen by a Ray replied from the Point B. And if B be cover'd, the Ob- jed will ceafe to be feen.

Hence, i°. If a Ray of Light, as HB, fall Perpendicularly on the Surface of a Speculum DE, it will be replied back upon it felf. 2 . From,