Page:Cyclopaedia, Chambers - Volume 2.djvu/970

 VIS [313]

For fince ah ObjecT: is.feen by thofe Rays which carry its Image to the Retina, and in that Place to which the vifive Power is directed by the Rays it receives ; as it perceives the Impulfe of the Rays to come from any Place, £S?e. it is abundantly admonifk'd of the Object's being in that Place See Place.

vis

and Situation.

5 u i If the Eye be hVd in A, (Fig.52.) and the right Lfii© BC be moved in fuch manner, as that the Extremes thereof always fall on the Periphery ; it will always' appear of the fame Magnitude.— Hence, the Eye being placed in any An- gle of a regular Polygon, the Sides will appear equal.


 * 1) °» If the Magnitude of an ObjecT: direftly oppofite td

From this Principle, feveral remarkable Phenomena of the Eye, be equal to its diftance from the Eye ; the whple

Vilion are accountable for ; as, Object will be taken in by the Eye, but nothing more.

i°, That if the Diftance between two vijihle ObjecTs, be "Whence, the nearer you approach an Object, the leS pare

feen under an Angle that is infenfible 5 the diftant Bodies will you fee of it. See Flame.

appear as if contiguous : Whence, a continuous Body being

the Refult of feveral contiguous ones ; the Diftances of fe- IV. The Figure vf Vifible OfjeSs is efiinmted "'chiefly veral Vifibtes appearing under an infenfible Angle, they from OUT Opinion of the Situation of the fever alT arts thereof will appear one continuous Body : which gives a pretty II- This Opinion of the Situation, &c. enables the Mind to luftration of the Notion of a Continuum. See Continuity, apprehend an external ObjecT: under this or that Figure^ 2, If the Eye be placed above a horizontal Plane, the more juftly than any Similitude of the Image in ihe Retina remoter Parts will appear as if higher and higher, till the with the Object can; the Irriages being frequently elliptU laft be feen in a level with the Eye. Whence it is, that the cal, oblong, &c. when the Objects they exhibit to the Mind Sea, to Perfons ftanding a-fhore, feems to rife higher and are Circles, Squares, &c,

1 -_1 ..L„ r..„^1 ..1 I I- ° nril -r * f -r-r.A

higher the further they look

3 Q, If any number of Objects be placed below the Eye ; the moft remote will appear the higheft : If they be above the Eye, the moft remote will appear the loweft.

4, The upper Parts of high Objects, appear to ftoop, or incline forwards ; as the Fronts of Churches, Towers, &c. And Statues a-top of Buildings, to appear upright, muft re- cline, or bend backwards. See further under the Article Horopter, ££?<;.

The Laws of Vifion, with regard to the Figures df the Vifible Objects, are,

i°, That if the Centre of the Pupil be exactly againft\ or in the Direction of a right Line j the Line will appear as one Point.

2, If the Eye be placed in the Direction of a Surface, fo that only one Line of the Perimeter can radiate upon it$ it will appear as a Line.

3, If a Body be oppofed diredTly towards the Eye, fo

as only one Plane of the Surface can radiate tin it 5 it will

II. The Mind perceives the Diftance of Vifible Objects, appear as a Surface.

from the different Configuration of the Eye, and the man- ner wherein the Rays ftrike the Eye and the Image is im- prefCd thereon.

For the Eye difpofes it felf differently, according to the different Diftances it is to fee, viz. for remote ObjecTs, the Pupil is dilated, and the Cryttalline brought nearer the Re- tina, and the whole Eye made more globous : on the con- trary, for near Objects, the Pupil is contracted, the Cryftal- line thruft forwards, and the Eye lengthen'd. See Pupil, Crystalline, &c.

The Diftance, again, is judg'd of by the Angle the ObiecT: makes, from the diftincT or confufed Reprefentation of the ObjecT: 5 and the Briiknefs or Feeblenefs, the Rarity or Spif- fitude of the Rays.

To this it is owing, i°, that Objects which appear ob- fcure or confufed, are judg'd to be the more remote : a Prin- ciple which the Painters ufe, to make fome of their Figures appear farther diftant than others on the fame Plane. See Perspective, ££?c.

To this it is likewife owing, that Rooms whofe Walls are whiten'd, appear the fmaller : That Fields cover'd with Snow, or white Flowers, fhow lefs than when clothed with Grafs : That Mountains cover'd with Snow, in the Night-time, ap- pear the nearer : That opake Bodies, appear the more re- mote in the Twilight, SJfc. See Distance.

III. 'ihe Magnitude or Quantity of Vifible Objects, is known chiefly by the Angle comprehended between two Rays drawn from the two Extremes of the Object, to the Centre of the Eye. See Optic Angle.

An ObjecT: appears fo big as is the Angle it fubtends 5 or Bodies feen under a greater Angle appear greater j and thofe under a lefs, lefs, &e. — Hence, the fame thing appears now bigger, and now lefs, as it is lefs or more diftant from the Eye.

This we call Apparent Magnitude. See Magnituee.

Now, to judge of the real Magnitude of an ObjecT, we confider the diftance : For fince a near and a remote ObjecT may appear under equal Angles, the Diftance mult, neceffa- rily be eftimated ; that if it be great, and the Optic Angle fmall, the remote ObjecT may be judg'd great, and vice verfa.

The Magnitude of Vifible ObjecTs, is brought under cer- tain Laws, demonftrated by the Mathematicians : as,

i°, That the apparent Magnitudes of a remote ObjecT, are as the Diftances, reciprocally.

2, That the Co-tangents of half the apparent Magnitudes of the fame Objects, are as. the Diftances : Hence, the ap- parent Magnitude and Diftance being given, we have a Me- thod of determining the true Magnitude : The Canon is this, As the whole Sine is to the Tangent of half the ap- parent Magnitude ; fo is the given Diftance to half the real Magnitude.— The fame Canon, inverted, will from the Dif- tance and true Magnitude given, determine the apparent one.

3", ObjecTs feen under the fame Angle, have their Mag- nitudes proportional to their Diftances.

4, The Subtenfe AB, (Tab. Opticks, Fig. 51.) of any Arch of a Circle, apppears of equal Magnitude in all the Points D C EG 5 tho one Point be vaftly nearer than ano- ther : and the Diameter DG, appears of the fame Magni- tude in all the Points of the Periphery of the Circle. — 'Hence we take a pretty Hint for the molt commodious Form of Theatres. See Theatre.

5

4, A remote Arch, view'd by an Eye in the fame Planes,-' will appear as a right Line.

5, A Sphere, view'd at a diftance, appears a Circle.

6°, Angular Figures at a diftance appear round.

7", If the Eye look obliquely on the Centre of a regular Figure, or a Circle, the true Figure will not be feen 5 but the Circle will appear Oval, &c. See Figure.

V. The Number of Vifible Objects is perceived, not only by one or more Images formd in the Fund of the Eye j but alfo by fuch a 'Pofition of thofe 'Parts of the "Brain, whence the Optic Nerves fpring, as the Mind has been ufed to, in attending to a certain TlaCe, and that either Jingle, or ma~ nifold.

Accordingly, when either of the Eyes, with the contiguous Part of the Brain, are forced out of their juft Parallelifm with the other, v.g. by prefling it with the Finger, ^c. all Things appear double : But when they are in their requisite Parallelifm, tho there be two Images in the Fund of the: two Eyes 5 yet the ObjecT will appear fingle* — Again, one thing may appear double, or even manifold, not only with both Eyes, but even with only one of them open : Byreafon, the common Concourfe of the Cones of Rays reflected from the ObjecT: to the Eye, either falls fhort of the Retina, or goes much beyond it. See Number*

VI. Motion and Reft are feen, when the Images of the Objects reprefented in the Eye, and propagated to the Brain, are either moved, or at reft .- And the Mind perceives theje Images either moving or at reft, by comparing the moved Image to another, with refpett to which it changes place ; or, by the Situation of the Eye to the Object being conti- nually changed

So that Motion is only perceivM, in perceiving the Images to be in different Places and Situations : Nor are thefe Si- tuations perceiv'd, unlefs effected in time. So that to per- ceive Motion, a fenfible Time is requir'd. But Reft is

perceiv'd by the vifive Faculty, from the Perception of the Image in the fame Place of the Retinet t and the fame Si- tuation for fome fenfible Time.

Hence, the Reafon why Bodies moving exceedingly faft, appear at reft : Thus, a live Coal, fwung briilcly round, ap- pears a continued Circle of Fire 5 the Motion not being commenfurate with vifible Time, but much fwiftcr than the fame : So that in the Time the Soul requires to judge of any Change of Situation of the Image on the Retina, or that it is moved from this Place to that, the Thing it felf per- forms its whole Circuit, and is in its former Place again.

The Laws of Vifion t with regard to the Motion of the Vifibles, are,

i°, That if two ObjecTs unequally diftant from the Eye$ move from it with equal Velocity ; the more remote one will appear the flower : Or, if their Celerities be proportio- nable to their Diftances, they will appear to move equally fwift.

2, If two ObjecTs, unequally diftant from the Eye, fn'ove with unequal Velocities iri the fame DirecTion ; their appa- rent Velocities are in a Ratio compounded of the diredT Ra* tios of their true Velocities, and the reciprocal one of their Diftances from the Eye*

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