Page:Cyclopaedia, Chambers - Volume 2.djvu/972

 V I s

and thence to N, and thence to the others in O. But

the Rays between BE and BF, being fomewhat lefs re- fracted, will not meet precifely in the dime Point O.

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V I s

admitted at one little Aperture ; to that Aperture apply the nu S me Animal new| y kill '<i. having firft dextroully Thus will the radian'r Point 'B off fl".i:"V"j"r"u 1:. P ul1 d °f the Mem bfanes that cover the bottom of the vi- lli feme manne as if the FtaS hrih A I f Z^ti T™ ? Um ° Ur ' oik the hind Pa " ° f the Sclerotica, Cboroi- the tame manner as it the I up,, had had no breadth, or &j and even part of the Retina ; then will the Images of

all the Objects without Doors be feen diftindly painted on

as if the Radiant it felf had only emitted one fingle Ray; fuch as were equal in Power to all thofe between BE and BF.

In like manner, the Rays proceeding from the Point A, will be fo refrafted in paffing thro' the Humours of the Eye, as to meet near the Point X : And the Rays from any intermediate Point between A and B, will nearly meet in fome other Point in the Fund of the Eye, between X andO.

Upon the whole, it may be afferted univerfally, that every Point of an Objeft arrefls only one Point in the Fund of the Eye ; and on the contrary, that every Point in the Fund of the Eye, only receives Rays from one Point of the Object, — Tho this is not to be understood in the utmoft rigour. Sec Radiant.

Now, if the Objefl recede from the Eye, in fuch manner as that the radiant Point B does not decline from the Line B D ; the Rays which would proceed from B not enough divaricated, would be fo refracted in paffing the three Sur- faces, as that they would meet ere they reach'd the Point O : On the contrary, if the Object mould be brought near- er the Eye, the Rays paffing from the Point B ro the Pu- pil, too much divaricated, would be refracted fo, as not to meet till beyond the Point O. Nay, the Object may be fo near, that the Rays proceeding from any Point may be fo divaricated, as that they ftiall never meet at all. — In all which Cafes, there would be no Point of the Object but would move a pretty large Portion of the Fund of the Eye ; and thus the Action of each Point would be confounded with that of the contiguous one.

And this would commonly be the Cafe, but that Na- ture has provided againft.ir; either by contriving the Eye fo as its Bulb may be lengthen'd, or Ihorten'd, as Ob- jects may be more or lefs diftant ; or, as others will have it, fo as that the Cryftalline may be made more convex or more flat ; or, according to others, fo as that the diftance be- tween the Cryitalline and the Retina may be lengthen'd or fhorten'd.

The firft Expedient is the molt probable ; on the footing of which, when we direct our Eyes to an Object fo remote as that it can't be diftinclly view'd by the Eye in its ac- cuftom'd Figure, the Eye is drawn back into a flatter Fi- gure, by the Contraction of four Mufcles ; by which means, the Retina becoming nearer the cryitalline Humour, re- ceives the Rays fooner : and when we view an Object too near, rhe Eye being comprefs'd by the two oblique Mufcles, is render'd more globular ; by which means, the Retina being fet further off from the Cryftalline, does not receive the Rays ot any Point before rhey meet.

It may be here added, that this accefs and recefs of the Cryftalline, is lb neceffary to Vifion, that whereas in fome Birds the Coats of the Eye are of fuch a bony confiftence, that Mufcles would not have been able to contract and diltend'em ; Nature has taken other means, by binding the Cryftalline down to the Retina, with a kind of blackifti Threads, not found in the Eyes of other Animals. — Nor mult it be omit- ted, that of the three Refractions above mention'd, the firft is wanting in Fifties ; and that to remedy this, their Cryftal- line is not lenticular, as in other Animals, but globular. Laftly, fince the Eyes of old People are generally worn flatter than thofe of young ones ; fo that the Rays from any Point fall on the Retina ere they become collected into one 5 they mull exhibit the Object fomewhat confufedly : nor can fuch Eyes fee any but remote Objects diftinclly. See Presbytje.

In others, whofe Eyes are too globular, the Cafe is juft the reverie. See Myopes.

From what has been lfiewn, that every Point of an Ob- ject moves only one Point of the bottom of the Eye 5 and, on the contrary, that every Point in the Fund of the Eye, only receives Rays from one Point of the Object ; 'tis eafy to conceive, that the whole Object moves a cerrain Part of the Retina ; that in this part there is a diftinct and vivid Collection of all the Rays receiv'd in at the Pupil ; and that as each Ray carries, its proper Colour along with it, there are as many Points painted in the Fund of the Eye, as there were Points vifible in the Object. — Thus is there a Spe- cies or Picture on the Retina, exactly like the Object ; all the difference between 'em, is, that a Body is here repre- fented by a Surface ; a Surface frequently by a Line, and a Line by a Point: that the Image is inverted, the right hand anfwering to the left of the Object, &c. and that 'tis ex- ceedingly fmall, and ftill the more fo, as the Object is more remote. See Visible.

What we have ftiewn under other Articles, of the Nature of Light, and Colours, readily accounts for this painting of the Object on the Retina. — The matter of Fact is proved by an eafy Experiment, firft try'd by Des Cartes ; thus : The Windows of a Chamber being /hut, and Light only

any white Body, as on an Egg-fhell, that the'E^e is laid upon.— The fame rhing is better lfiewn by an Artificial Eye, or a Camera Obfcura. See Eye, and Camera Obfcura.

The Images of Objefts, then, are reprefented on the Retina ; which is only an Expanfion of the fine Capilla- nientsof the Optic Nerve, and from which the Optie Nerve is continu'd into the Brain. Now, any Motion or Vibra- tion imprefs'd on one extreme of the Netve, will be propa- gated to the other : Hence, the Impulfe of the feveral Rays lent from the feveral Points of the Objeft, will be propagated as they are on the Retina, (i. e. in their proper Colours, gjc. or in particular Vibrations or manners of Preffure, correlpond- ing rhereto) ro the Place where thofe Capillaments are in- terwoven into the Subftance of the Brain. — And thus is Vi- jion brought to the common Cafe of Senfation.

For fuch, we know, is the Law of the Union between the Soul and Body ; that certain Perceptions of the firft, do ne- ceffarily follow certain Motions of the laft : But the different Parts of the Object do feparately move differenr Parts of the Fund of the Eye ; and thofe Morions are propagated to the Senfory : it follows, therefore, that there muft arife fo many diftinct Senfations at the fame time. See Sensation.

Hence, 1 ", we eafily conceive, that the Perception, or Imagein the Mind, muft be the clearer, and more vivid, the more Rays the Eye receives from the Object : Confe- quently, the largencfs of the Pupil will have fome Ihare in the clearnefs of Vijion. See Pop il.

=', Confidering only one radiant Point of an Object, we may fay, that that Point would move the Senfe more weak- ly, or be feen more obfeurely, as it is more remote ; by rea- fon the Rays coming from any Point, like all Qualities pro- pagated in Orbem, are always diverging ; and therefore the more remote, the fewer of 'em will be receiv'd in at the Pupil— But, as 'tis not a finale Point of an Object, but all of them together, that affeft the Organ of Senic; and as the Image of the Objefl ftill poffeffes a lefs part of the Re- tina as 'tis more remote ; therefore, tho the Rays that flow from any Poinr of an Objefl rwo Miles oft", into the Pupil, be rarer or fewer by half than thofe flowing from the fame Point at a Mile's diftance ; yet the fame Capillament of the Optic Nerve, which in the lattet Cafe would only be mov'd by that one Point, in the former, will be affected with the joint Action of the neighbouring Points ; and therefore the Image be as clear as in the other Cafe. — Add, that the Pupil dilating it felf more, as the Objefl is more remote, takes in more Rays than it would other-wife do.

5% The Diftinctnefs of Vifion is fomewhat concern'd in

the Size of the Image exhibited in the Fund of the Eye.

For there fhould be, at leaft, as many Extremes of" Capilla- ments, or Fibres of the Optic Nerve in the Space that Image poffeffes, as there are Particles in the Ob eft that fends Rays into the Pupil : otherwife, every Particle will not move its feparate Capillamenr : And if rhe Rays from two Points fall on the fame Capillament, 'twill be the fame as if only one Point had fell there ; fince the fame Capillament cannot be diffetently moved at the fame time.' — And hence it is that the Images of very remote Objects being very fmall,' they appear confus'd, feveral Points of the Image affeaing each Capillament: And hence alfo, if the Object be of different Colours, feveral Particles afftaing the fame Capillament at the fame time, only the brighteft and molt lucid will be perceiv'd : Thus, a Field furnifti'd with a good number of white Flowers among a much greater quantity of green Grafs, gfc. ar a diftance appears all white.

Our feeing of ObjeBs fingle, tho with two Eyes, in each of which is a feveral Image, or PiBure ; and our feeing of 'em ereB, whereas the Piaure is inverted ; are two great Phenomena in Vifion : which fee confider'd under the Ar- ticle Sight.

For the Manner of feeing and judging of i*e Diftance and Magnitude of Objects. See Distance, and Magnitude.

Vision, in Opticks.— The Laws of Vifion, brought under Mathematical Demonfirarions, make the Subjea of Opticks taken in the greateft Latitude of that Word : For among' the Writers of Mathematicks, Opticks is generally taken, in a more reftrain'd Signification, for the Doarine of TlireB Vifion ; Catop tricks, for the Doarine of RefeBed Vifion ■ and Dioftricks, for that of RefraBed Vifion. See Opticus^ Catoptricks, and Dioptrlcks.

2)ireB, or Simple Vision, is that perfbrm'd by means of direa Rays ; rhat is, of Rays paffing direflly, or in riolu Lines, from the radiant Point to the Eye. See Direct."

Such is that explain'd in the preceding Article Vision • fee alfo Ray.

RejleBed'VisioK, is that petform'd by Rays refleaed from Sfecilla, or Mirrors. See Reflexion.

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