Page:Cyclopaedia, Chambers - Volume 2.djvu/971

 V I s

C 3H 1

V I s

3", A vijitle Object moving with any Velocity, appears to be at reft.if the Space defcribed in the Interval of one Se- cond, be imperceptible at the diftance of the Eye. — Hence it is, that a near Object, moving very flowly, as the Index of a Clock ; or a remote one very iwiftly, as a Star ; feem at reft.

4°, An Object moving with any degree of Velocity, will appear to reft, if the Space it runs over in a Second of Time be to its diftance from the Eye, as i to 1400 : Nay, in fact, if it be as 1 to 1500.

5°, The Eye proceeding ftrait, from one Place to ano- ther ; a lateral Object, either on the right or left, will feem to move the contrary way.

«°, If the Eye and the Object move both the fame way, only the Eye much fwifter than the Object, that laft will appear to go backwards.

7°, If two or mote Objects move with the fame Velocity, and a third remain at reft ; the Moveables will appear fix'd, and the quiefcent in motion the contrary way. — Thus, Clouds moving very fwiftly, their Parts feem to preferve their Situa- tion, and the Moon moves the contrary way.

If the Eye be moved with a great Velocity, lateral Ob- jects at reft, appear to move the contrary way. — Thus, to a Perfon fitting in a Coach, and riding brifldy thro' a Wood, the Trees feem to retire the contrary way ; and to People in a Ship, &c. the Shores feem to recede.

Visible Species. See Species.

VISION, the Aft of Seeing ; or of perceiving external Objects by the Organ of Sight. See Seeing, and Sight.

Vifion is well defin'd to be a Senfation, whereby, from a certain Motion of the Optic Nerve, made in the bottom of the Eye, by the Rays of Light emitted, or reflected from Objects, and hence convey'd to the common Senfory in the Brain 5 the Mind perceives the luminous Object, its Quan- tity, Quality, Figure, &c. See Visible.

The Phenomena of Vifion, the Caufes thereof, and the Manner wherein it is effected, make one of the greateft and moft important Articles in the whole Syftem of Natural Knowledge. — Indeed, a great Part of the Phyfical, Mathe- matical, and Anatomical Difcoveries and Improvements of the Moderns terminate here ; and only tend to fet the Bufi- nefs of Vifion in a clearer Light.

Hitherto, refer what Sir /. Newton and others have dif- cover'd of the Nature of Light, and Colours 5 the Laws of Inflexion, Reflexion, and Refraction of the Rays ; the Structure of the Eye, particularly the Retina, and Optic Nerves, ££;c.

It is not neceffary we mould here give a minute Detail of the Procefs of Vifion from its firft Principles : The greateft

Part is already deliver'd under the refpective Articles. ■

The Eye, the Organ of Vifion, we have defcrib'd under the Article Eve; and its feveral Parts, Tunics, Humours, (Sc. ' under their proper Heads, Cornea, Crystalline, &c.

The immediate and principal Organ of Vifion, viz. the Retina, according to fome, and the Cboroides, according to others, are diftinctly confider'd : as alfo, the Structure of rhe Optic Nerve, which conveys the Impreffion to the Brain 5 and the Texture and Difpoiition of the Brain it felf, which receives them, and reprefents 'em to the Soul. See Re- tina, Choroides, Optic Nerve, Brain, Sensory, £S?e.

Again, the Nature of Light, which is the Medium, or Ve- hicle whereby Objects are carried to the Eye, is laid down at large under the Article Light, and Colours 5 and the chief Properties thereof concern'd in Vifion, under Refle- xion, Refraction, £$c. And many of its Circumftances under Ray, Medium, &c. What remains for this Ar- ticle, therefore, is only to give a general Idea of the whole Procefs 5 in which all the feveral Parts are concern'd.

Opinions or Syjlems of Vision.

The 'Platonifts and Stmts held Vifion to be effected by the Emiflion of Rays out of the Eyes ; conceiving, that there was a fort of Light thus darted out, which with the Light of the external Air, raking, as ir were, hold of the Objects, render'd 'em vifible ; and thus returning back again to the Eye alter'd and new modify'd by the Contact of the Object, made an Impreffion on the Pupil, which gave the Senfation of the Object.

The Reafons whereby they maintain their Opinion, are fetch'd, i°, from the Brightnefs and Luftre of the Eye : a°, From our feeing a remote Cloud, without feeing one which we are incompafs'd withal 5 (the Rays being fuppofed too brifk and penetrating to be ftop'd by the near Cloud, but growing languid at a greater diftance, are return'd to the Eye :) 3^, From our not feeing an Object laid on the Pupil: 4°, From the Eye's being weary with feeing, i. e. by emitting great Quantities of Rays : And laftly, from Animals which fee in rhe Nighr 5 as Cats, Lions, Moles, Owls, and fome Men.

The Epicureans held Vifion to be perform'd by the Ema- nation of corporeal Species, or Images from Objects j or, a fort of Atomical Effluvia, continually flying off from the in- timate Parts of Objects to the Eye.

Their chief Reafons are, 1°, That the Object muft ne- ceffarily be united to the vifive Faculty ; and fince it is not united by it felf, it muft be fo by fome Species that repre- fents it, and that is continually flowing from Bodies : 2°, That it frequently happens, that old Men fee remote Ob- jects better rhan near ones ; the diftance making the Species thinner, and morecommenfurate totheDebility of their Organ.

The Peripateticks hold, with Epicurus, that Vifion is perform'd by the Reception of Species ; but differ from him in rhe Circumftances : For they will have the Species, (which they call Intentionales) to be incorporeal.

'Tis true, Ariftotle's Doctrine of Vifion, deliver'd in his Chapter de AfpeSu, amounts to no more than this ; That Objects muft move fome intermediate Body, that by this they may move the Organ of Sighr : To which he adds, in another Place, That when we perceive Bodies, 'tis their Species, not their Matter, that we receive : as a Seal makes an Impreffion on Wax, without the Wax's retaining any thing of the Seal.

But this vague and obfcure Account, the 'Peripateticks have thought fit to improve. — Accordingly, whar their Maf- ter call'd Species, the Difciplcs undcrftanding of real proper Species, affert, That every vifible Object expreffes a perfect Image of it felf, in the Air contiguous to it ; and this Image another, fomewhar lets, in the next Air ; and the third ano- ther, Sfc. till the laft Image arrives at the Cryftalline, which they hold for the chief Organ of Sighr, or that which immediately moves the Soul. — Thefe Images they call'd Intentional Species^ See Species.

The modern Philosophers, as the Cartefian! and Newto- nians, give a better account of Vifion.— They all agree, that it is perform'd by Rays of Lighr reflected from the feveral Points of Objects, received in at the Pupil, refracted and collected in their paffage thro' the Coats and Humours, ro the Retina 5 and thus linking, or making an Impreffion on fo many Points rhereof: which Impreffion is convey'd by the correfpondent Capillaments of the Optic Nerve ro rhe Brain, ££?c.

As for the Peripatetick Series or Chain of Images, 'tis a mere Chimera, and Ariftotle's meaning is better underftood without than with 'em. — In effect, fetting thefe afidc, the Ari- Jlotelian, Cartefian, and Newtonian Doctrines of Vifion are very confiftent : For Sir I.Neviton imagines, that Vifion is per- form'd chiefly by the Vibrations of a fine Medium which penetrates all Bodies, excited in the bottom of the Eye by the Raysof Light ; and propagated thro' the Capillaments of the Optic Nerves, to the Senforium. And Des Cartes maintains, that the Sun preffing the MateriaSnl>tilis,vihevewj th the World is fill'd, every way ; the Vibrations or Pulfes of rhat Matter reflected from Objects, are communicated to the Eye, and thence to the Senfory : So rhat the Action or Vibrar'ion of a Medium is equally fuppofed in all. See Medium.

Modern 'Theory of Vision.

In order to Vifion, we are certain, itis required rhat the Rays of Light be fent from rhe vifible Objects to the Eye. — What befals them in the Eye, will be conceiv'd from what follows.

Suppofe, e.g. Z rhe Eye, and A B C rhe Object, (Tab. Opticks, Fig. 53.) Now, tho every Point of an Object be a radianr Point, that is, tho there be Rays reflected from every Point of the Object to every Point of the circumam- bient Space ; each carrying with ir its refpective Colour, (which we falfly imagine to be thofc of the Object ;) yet, as only thofe Rays which pafs thro' the Pupil of the Eye affect the Senfe, we (hall here confider none elfe.

And, again, tho there be a great number of Rays paffing from one radiant Point, as B, thro' the Pupil, yet we (hall only confider the Action of a few of 'em ; as BD, BE, BF.

Now then, the Ray BD, falling perpendicularly on rhe Surface E D F, will pafs out of the Air into the aqueous Humour, without any Refraction, and proceed right to H ; where falling perpendicularly on the Surface of the cryftal- line Humour, it will go on without any Refraction to M 5 where, again, falling perpendicularly on the Surface of the vitreous Humour, it will proceed ftrait to the Point O, in the Fund of bottom of the Eye.

Again, the Ray BE paffing obliquely out of Air upon the Surface of the watery Humour E D F, will be refracted, and approach towards rhe perpendicular EP : Thus, pro- ceeding ro the Point G, in the Surface of the Cryftalline, it will be there refracted ftill nearer to the Perpendicular. — So alfo E G, falling obliquely out of Air into a harder Body, will be refracted towards rhe Perpendicular GR; and fall- ing on rhe Point L of the Surface of the vitreous Humour, it will ftill be brought nearer to M.

Laftly, GL, falling obliquely out of a denfer, upon the Surface of a rarer Body L M N, will be retracted, and re- cede from the Perpendicular L T ; in receding from which, it is evidenr ir approaches towards rhe Ray B D O, and

may be fo refracted, as to meet the other in O/ In like

manner, the Ray B F being refracted in B, will turn to I,

and