Page:Works of Plato his first fifty-five dialogues (Taylor 1804) (Vol 2 of 5) (IA Vol2worksofplato00plat).pdf/463

 INTRODUCTION TO THE TIM.EUS.

453

it muſt be obferved that Democritus, Heraclitus, the Stoics, many of the Peripatetics and antient geometricians, together with the Platonifts, were of opinion that vifion fubfids through a lucid fpirit emitted from the eyes : and this fpirit, according to Plato and his followers, is an unburning viviflc fire fimilar to celedial fire, from which it originally proceeds.

But this

fire, the illuminations of which, as we have already obferved, give life to our mortal part, is abundantly colleded in the eye as in a fat diaphanous fubftance, whole moidure is moll fhining and whofe membranes are tender and tranfparent, but yet fufficiently firm for the purpofe of preferving the inhe¬ rent light.

But a mod ferene ray fhines through the more folid pupil; and

this ray originates internally from one nerve, but is afterwards derived through two fmall nerves to the two eves.

And thefe nerves, through the

fat humours of the eyes, winding under the tunics, arrive at length at the pupils.

But a light of this kind, thus preferved in the fmall nerves, and

burding through the narrow pupils as foon as it fhines forth into difperfed rays, as it commenced from one ray, l'o it immediately returns into one, from the rays naturally uniting in one common rav : for the eyes alfo, on account of their lubricity, roundnefs, and fmooth lubdance, are eafily moved hither and thither, with an equal and fimilar revolution. This vifual ray, however, cannot proceed externally and perceive objects at a didance, unlefs it is conjoined with external light proceeding conically to the eyes ; and hence our ray infinuating itfelf into this light, and becoming drengthened by the alfociation, continues its progreffion till it meets with fome, oppofing objed.

But when this is the cafe, it either diffufes itfelf through

the fuperficies of the object, or runs through it with wonderful celeritv, and becomes immediately affeded with the quality of the objed.

And a relid-

ance, motion, and affection of this kind produces vifion, viz. from the vibra¬ tion of the ray thus affeded gradually arriving at the indrument of fio-ht, and by this means exciting that image of the objed which is naturally inhe¬ rent in the indrument, and through which when excited perception enfues. For there are three particulars which belong in general to all the lenfes ; fird, an image or mark of the fenlible thing impreded in the fenfitive indrument; and this condituted both in paffion and energy in a certain limilitude to the fenlible object : but afterwards we mud confider an impreffion of this kind as now perfed, and ending in fpecies, viz. in the common compofite life : and