Page:Cyclopaedia, Chambers - Volume 2.djvu/101

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6f Light, by impinging on any reflecting or refracting Sur face, excite Vibrations in the reflecting or refracting Me- dium, and by that means agitate the folid Parts of the Body. Thefe Vibrations, thus propagated in the Me- dium, move faiter than the Rays, fo as to overtake them j and when any Kay is in that part of the Vibration which confpires with its Motion, its Velocity is increased, fo that it eaiily breaks thro a refracling Surface : but when it is in a contrary parr of the Vibration, which impedes its Motion, it is eaiily reflected, and confequently that every Ray is fucceffivcly difpofed to be eaiily reflected or trans- mitted by every Vibration which overtakes it. The Re- turn of which DiSpofition of any Ray to be reflected, he calls Fht of eafy Reflexion ; and thofe of its Difpofition to be transmitted, he calls Fits of eafy Tranf miff on; and the Space between the Returns, the Interval of the Fit, The Reafon then why the Surface of all thick tranfparent Bodies reflect part of the Light incident on 'em, and re- fract the refl, is, that fome Rays at their Incidence are in Fits of eafy Reflexion, and others of eafy TranSmiffton. For the Properties and Laws of reflected Light, fee Re- flexion and Mir r our.

Further, a Ray of Light pairing out of one Medium into another of different Denfity, and in its paffage making an oblique Angle with the Surface that feparates the Me- diums, will be refracted or turn'd out of its right Line 5 by reafon the Rays are more Strongly attracted by a den- fer than a rarer Medium.

That thefe Rays are not refracted by Striking on the folld Parts of Bodies, but without any Contact, by that fame force wherewith they are emitted and reflected, exerting itfelf differently in different Circumflances, is proved in great meafure by the fame Arguments which demonstrate Reflexion to be perform 'd without Contact. For the Properties,- &c. of refracted Light, fee Ref-afhon, Lens, &c. In Illand Chryital is obferved a kind of double Refraction, very different from what we find in any other Body : the Rays that fall obliquely being not only dif- perfecl, with a double Refraction in one and the fame Surface, but even the perpendicular Rays themfelves are mofl of 'em divided into two Beams by means of the fame double Refraction ; which Beams are of the fame Colour with the incident Beams, and are equal in degree of X-ightj at leafl nearly, to each other: Whence the great Philofopher, So often cited, takes occasion to fufpect that there are in Light fome other original Properties beiides thofe hitherto defcribed ; and particularly, that the Rays have different Sides endued with feveral original Properties. For of thefe Refractions, the one is per- formed in the ufual manner, i. e. the Sine of Incidence is to that of Refraction as 5 is to 3 ; and the other in an un- ufual manner : and yet the fame Ray is refracted Some- times in the one manner, and fometimes in the other, according to the various Poiitions which its Several Sides have, in refpect of the Chryflal. Thefe Difpoiitions, he fhews, mufi have exiited originally in the Beams, with- out having undergone any Alterations in that refpect, by theChryltal. Every Ray of Light therefore has two oppo- site Sides, the one originally endued with a Property whereon its unufual Refraction depends, and the other not endued with that Property.

Sir J. Newton having obferved the vividly colour'd Image projected on the Wall of a darken'd Room, by the Sun- Beams tranfmitted thro a Prifm, to be five times as long as broad 5 fctting him Self to inquire into the Reafon of this Difproportion, was led from other Experiments to the Exferimentum Cruets ■> whence he difcovered the Caufe of the Phenomenon to be, that Some of the Rays of Light were more refracted than others, and therefore ex- hibited feveral Images of the Sun under the appearance of one, extended lengthwife. Thence he proceeded to conclude, that Light itfelf is an heterogeneous Mixture of Rays differently refrangible. Hence he dittinguifhes Light into two kinds, iiz,. that whofe Rays are equally refrangible, which he calls Homogeneal, Similar or Uni- form Light; and that whofe Rays are unequally refran- gible, which he calls Hcterogeneal Light.

There are but three Affections of Light, wherein he obferved its Rays to differ, was. Refrangibility, Reflexi- bility, and Colour ; and thofe Rays which agree in Re- franoibility, agree alfo in the other two : whence they may be well defined Homogeneal, tho in fome other re-

gects they may poffibly be Heterogeneal. Again, the Dlours exhibited by Homogeneal Light, he calls Homo- geneal Colours 5 and thofe produced by Heterogeneal Light, Heterogeneal Colours. Thefe Definitions laid down, he advances feveral Propofitions.

As i/lr, That the Sun's Light confifts of Rays differing by indefinite Degreesof Refrangibility. zdty, That Rays which differ in Refrangibility, when parted from one ano- ther, do proportionably, differ in the Colours which they exhibit. $dly t That there are as many fimple and ho-

mogeneal Colours, as Degrees of Refrangibility, for to every Degree of Refrangibility belongs a different Colour. 4.thly, Whitenefs in all refpects like that of the Sun's im- mediate Lights and of the ufual Objects of our Senfes, cannot tie compounded of fimple Colours, without an in- definite Variety of them ; for 10 fuch a Compofiticn there are required Rays endued with all the indefinite Degrees of Refrangibility, which infer as many fimple Colours. 5 i/j/y, The Rays of Light do not act on one another in parting thro the fame Medium. 6tbly> The Rays of Light do not fuffer any Alteration of their Qualities from Re- fraction, nor from the adjacent quiefcent Medium. Ithly, There can no Homogeneal Colours be produced out of Light by Refraction, which are not commix'd in it be- fore; fince Refraction, as was before obferv'd, changes not the Qualities of the Rays, but only Separates thofe which have divers Qualities by means of their different Refrangibility. Sthly, The Sun's Light is an Aggregate of Homogeneal Colours ; whence Homogeneal Colour* may be call'd Primitive or Original.

We have already obferved that the Rays of Light are compofed of diffimilar or heterogeneous Parts j fome of them being in all probability greater, others lefs. Now the Smaller the Parts are, by fo much the more refrangi- ble they are, i.e. they are fo much the in ore eafily di- verted out of their rectilinear Courfe ; and thofe Part3 which differ in Refrangibility, {i.e. in Bulk) we have alfo obferved differ in Colour. Hence ariSes the whole Theory of Colours. Thofe Parts, o. g. which are the mofl refrangible, constitute Violet Colour ; that is, the molt minute Particles of Light when Separately impelled on the Organ, do there excite the Shorter! Vibrations in the Retina, which are thence communicated by the Solid Parts of the Optic Nerve into the Brain, and excite in us the Senfation of Violet Colour, the dimmeft and mofl: languid of all other Colours : And thofe Particles, on the contrary, which are the leafl refrangible, constitute a Ray of a Red Colour 5 *'. e. the greater! Particles of Light excite the longelt Vibrations in the Retina, and fo convey the Senfation of a Red Colour, as being the raoft bright and vivid of all others. The other Particles being diftinguiihed into little Rays, according to their refpective Magnitudes and Degrees of Refrangibility, excite inter- mediate Vibrations, and fo occafion the SenSatrons of the intermediate Colours ; in like manner as the Vibrations of the Air, according to their different Magnitudes, ex- cite the Senfations of different Sounds. The Colours then of thefe little Rays not being any adventitious Mo- difications of them, but connate, primitive, andneceffary Properties, refulting, in all probability, from their diffe- rent Magnitudes, raufl be perpetual and immutable, not to be altered by any Reflexion, Refraction, or other fubfequent Modification. See the DoBrine of Colours^ laid down under Colour. For the Manner in which Light affects our Senfes, and how it contributes to Vifion^ fee Vifion.

LIGHT is alfo ufed to fignify the Difpofition of Ob- jects with regard to the receiving of Light 5 thus we fay, a Painting is feen in its proper Light, when its Situation, with regard to the Light, is the fame with that for which it was painted.

LIGHTS ABOARD SHIPS. See Signals.

LIGHTS, in Architecture, are understood of the Openings of Gates and Windows, and other Places thro* which the Air and Light have a Paffage. In the Pantheon all the Light comes from on high 5 it has no Lights but in the Dome.

LIGHTS, in Painting, are thofe Parts of a Piece that are illumined, or that lie open to the Luminary, by which the Piece is SuppoSed to be enlightened ; and that for this reafon are painted in bright vivid Colours : in which SenSe Light is oppofed to Shadow. Light is alSo uSed Sor the luminous Body that emits it. There are various kinds of Lights; general Lights, as the Air; particular Lights, as a Fire, a Candle, and even the Sun. Different Lights have very different Effects on the Piece, and occafion a Difference in the Management of every Part. A great deal therefore depends on the Painters chufing a proper Light for his Piece to be illumined by, and a great deal more in the Conduct of the Lights and Shadows when the Luminary is pitched upon. The Strength and Relievo of a Figure, as well as its Gracefulnefs, depends entirely on the Management of the Lights, and the joining of thofe to the Shadows. The Light a Figure receives, is either di- rect or reflected, to each of which fpecial Regard rauft be had? The Doctrine of Lights and Shadows makes that Part of Painting, called Clair-obfcure, which fee.

LIGHT-HORSE, an antient Term in our E*%l\fl» Cufloms, Signifying an ordinary Cavalier or Horfeman lightly armed, and fo as to enter a Body or Regiment ; in opposition to the others, who were heavily accoutred, and armed at all Points,

LIGHTER,