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Chambers, Ephraim, 1680 (ca.)-1740 / Cyclopædia, or, An universal dictionary of arts and sciences : containing the definitions of the terms, and accounts of the things signify'd thereby, in the several arts, both liberal and mechanical, and the several sciences, human and divine : the figures, kinds, properties, productions, preparations, and uses, of things natural and artificial : the rise, progress, and state of things ecclesiastical, civil, military, and commercial : with the several systems, sects, opinions, &c : among philosophers, divines, mathematicians, physicians, antiquaries, criticks, &c : the whole intended as a course of antient and modern learning

Vision - urine,   pp. 314-333 PDF (20.3 MB)

Page 314

;, A vifible Objefl: moving with any Velocity, appears
to be at reff, if the Space delcribed in the Interval of one Se-
cond, be imperceptible at the aiflance of the Eye.-Hehce it is,
that a near Objeil moving very flowly, as the Index of a
Clock; or a tetnote one very fiwiftly, as a Star; feeth at
40, An Obje& moving With any degree of Velocity, will
appear to reff, if the Space it runs over in a Second of Time
be to its diftance from the Eye, as I to i4oo: Nay, in fadt,
if it be as i to1300.
5, The Eye peoceedi   fitrait, from one Place to ano-
ther; a lateral Objed, either on the right or left, wiill feem
to move the contrary way.
6f, If the Eye and the Obje& mo#e both the fame way,
only the Eye much fwifter than the Objedt, that laft will
appear to go backwards.
70, If two or more Objeds move with the fame Velocity,
and a third remain at refl; the Moveables will appear fix'd,
and the quiefcent in motion the contrary way.-Thus, Clouds
moving very fwlftly, 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-
jecds at tel, appear to move the contrary way.-Thus, to a
erfon fitting in a Coach, and riding brifkly thro' a Wood,
the Trees feem to retire the contrary way ; and to People in
a Ship, F~c. the Shores feem to recede.
VISION, the Ad 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 refleaed from
Objecs, and hence convey'd to the common Senfory in the
Brain; the Mind perceives the luminous Objed, its Quan-
tity, Quality, Figure, Oc. See VISIBLE.
The Phxnomena of Vifon, the Caufes thereof, and the
Manner wherein it is efFe&6ed, make one of the greareff and
mofi important Articles in the whole Syflem 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 I. Newton and others have dif-
cover'd of the Nature of Light, and Colours ; the Laws
of Inflexion, Reflexion, and Refraffion of the Rays; the
Strudlure of the Eye, particularly the Retina, and Optic
Nerves, Ec.
It is not neceffary we Ihould here give a minute Detail of
the Procefs of Vif on from its firfi Principles: The greatefi
Part is already deliver'd under the refpeffive Articles.-
The Eye, the Organ of Vifion, we have defcrib'd under the
Article EYE; and its feveral Parts, Tunics, Humours, Wic.
under their proper Heads, CORNEA, CRYSTALLINE, SC.
The immediate and principal Organ of Vifon, viz. the
Retina, according to fome, and the Choroides, according to
others, are diftindly confider'd : as alfo, the Struaure of the
Optic Nerve, which conveys the Impreffion to the Brain;
and the Texture and Difpofition of the B rain it felf, which
receives them, and represents 'em to the Soul. See RE-
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; and the
chief Properties thereof concern'd in [i/ion, under REFLE-
XION, REFRACTION, ec. And many of its Circumrfances
under RAY, MEDIUM, Sc.-What remains for this Ar-
ticle, therefore, is only to give a general Idea of the whole
Procefs i in which all the feveral Parts are concern'd.
Opiinions or Syftems of VISION.
The Platonifts and Stoicks held Vifon to be effered by
the Emiffion 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, taking, as it were, hold of the
Objecfs, 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 Impreflion on the Pupil, which gave the
Senfation of the Objef.
The Reafons whereby they maintain their Opinion, are
fetch'd, I0, from the~ Brightnefs and Lufire of the Eye.
20, From our feeing a remote Cloud, without feeing one which
we are incompafs'd withal; (the Rays being fuppofed too brink
and penetrating to be fiop'd by the near Cloud, but growing
languid at a greater diflance, are return'd to the Eye :) 3f,
From our not feeing an Objedt laid on the PLpil: 4', From
the Eye's being weary with feeing, i. e. by emitting great
Quantities of Rays: And lafily, from Animals which fee in
the Night ; as Cats, Lions, Moles, Owls, and fome Men.
The Epicureans held V/if on to be perform'd by the Ema-
nation of corporeal Species, or Images from Objeds ; or, a
fort of Atomical Effluvia, continually flying off from the in-
timate Parts of Objeds to the Eye.
Their chief Reafons are, I0 That the Obje&t mufl ne-
ceffarily be united to the vifive Faculty; and fince it is not
united by it felf, it mudt 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-
jelts better than near ones 7 the diftance making the Species
thinner, and more commenfurate to theDebility of their Organ.
The Peripateticks hold, with Epicurus, that  fijon is
perfbrm'd by the Reception of Species; but differ from him
in the Circumilances: For they will have the Species,
(which they call Intentionales) to be incorporeal.
'Tis true, Ariflotke's Doarine of Vir/on, deliver'd in his
Chapter de l/e/petu, amounts to no more than this; That
Objecs muff move fome intermediate Body, that by this
they may move the Organ of Sight: To which he adds,
in another Place, That when we perceive Bodies, 'tis their
Species, iot 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 fPeripateticks
have thought fit to improve.-Accordingly, what their Maf-
ter call'd Species, the Difciples underilanding of real proper
Species, affert, That every vifible Objeat exprelfes a perfe&
Image of it felf, in the Air contiguous to it; and this Image
another, fomewhat lefsi in the next Air i and the third ano-
ther, s)c. till the laft Image arrives at the Cryflalline,
which they hold for the chief Organ of Sight, or that which
immediately moves the Soul.--Thefe Fmages they call'd
Intentional Species. See SPECIES.
The modern Philofophersi as the Carteflans and Newzvto-
nians, give a better account of Vi~fon.-They all agree, that
it is perform'd by Rays of Light refledted from the feveral
Points of Objeas, received in at the Pupil, refraaed and
colle&ed in their pacfage thro' the Coats and Humours, to
the Retina; and thus firiking, or making an I-mpreffion on
fo many Points thereof: which Imprefton is convey'd by
the correspondent Capillaments of the Optic Nerve to the
Brain, Cc.
As for the Peripatetick Series or Chain of Images, 'tis a
mere Chimera, and Arizfotle's meaning is better underflood
without than with 'em.-In effed, fetting thefe afide, the Ari-
flotelian, Cartefian, and Newtonian Docarines of Viflon are
very confiflent: For Sir I.Newton imagines, that Viflon 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 MateriaSubtilis,wherewith theWorld
is fill'd, every way; the Vibrations or Pulfes of that Matter
refledfed from Objefs, are communicated to the Eye, and
thence to the Senfory : So that the Affion or Vibration of a
Medium is equally fuppofed in all. See MEDIUM.
Modern fTheory of VISION.
In order to /irzon, we are certain, iris required that the
Rays of Light be fent from the vifible Objeas to the Eye.-
What befals them in the Eye, will be conceiv'd from what
Suppofe, e.g. Z the Eye, and ABC the Objea, (Tab.
Opticks, Fig. 53.) Now, tho every Point of an Objea be
a radiant Point, that is, tho there be Rays refleaed from
every Point of the Objea to every Point of the circumam-
bient Space; each carrying with it its refpedive Colour,
(which we falfly imagine to be thofe of the Objed ,) yet,
as only thofe Rays which pafs thro' the Pupil of the Eye
afcth the Senfe, we lhall 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 fhall
only confider the Affion of a few of 'em; as BD, BE, BE.
Now then, the Ray BD, falling perpendicularly on the
Surface EDEF, will pafs out of the Air into the aqueous
Humour, without any Refraffion, and proceed right to H i
where falling perpendicularly on the Surface of the cryflal-
line Humour, it will go on without any Refraffion to M;
where, again, falling perpendicularly on the Surface of the
vitreous Humour, it will proceed firait to the Point 0, in
the Fund or 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 refraaed,
and approach towards the perpendicular E P : Thus, pro-
ceeding to the Point G, in the Surface of the Cryflalline, it
will be there refraaed fill nearer to the Perpendicular.-So
alfo E G, falling obliquely out of Air into a harder Body,
will be refraaed towards the Perpendicular G R ; and fall-
ing on the Point L of the Surface of the vitreous Humour,
it will fill be brought nearer to M.
Lafily, GL, falling obliquely out of a denfer, upon the
Surface of a rarer Body L M N, will be refraaed, and re-
cede from the Perpendicular L T3 in receding from which,
it is evident it approaches towards the Ray B DO, and
may be fo refrafteT, as to meet the other in O.-In like
manner, the Ray B F being refracted in B, will turn to 1,

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