27

Optics.

of external Objects are by that means form dand represented (CXXVI). -

Over

(CXXVI) i. Ib order to exhibit a just Idea of thetrue Theory of Vision , I shall here give a more exact andparticular Description of the Eye and of its severalParts, with an Account or Calculation of the variousRefractions of the Rays of Light through the severalHumours, for forming the Images of Objects on theRetina at the Bottom of the Eye.

2. To this End I have here represented a section

of the Human Eye in its true or natural Magnitu e,which consists of two Segments of two different opheies*viz. one larger, as BNB, and a lesser, BIB. Thp p] a telarger Segment consists of three Tunics or Coats, or XL^Awhich the outmost is of a hard, thick, white, opake 17.

Substance, called the Sclerotica, as BNB. j

this is another thin, soft, and blackish Tunic, cai ethe Choroida ; which serves as it were for a Lining tothe other, or rather as a delicate Stratum for the thirTunic called th z Retina, which is a curious hne Ex-pansion of the Optic Nerve Y Z over all the larger Seg-ment of the Eye, every Way to BB.

3. The lesser Segment consists of one Coat or Tunic,called the Cornea , as resembling a Piece of transparentHorn ; this is more convex than the other, and is de-noted hyBIB. Within this Coat, at a small Distance,

> b placed a circular Diaphragm, as B 0, B 0, called the

TJvea, or Iris , because of the different Colours it has1 in different Eyes. In this is a round Hole in the Middle

? called the Pupil as 0 s, which in some Creatures is of a

different Figure, viz. oblong, as in Cows, Cats, LA.

' 4- As the Cornea, by its'Transparency, admits the.

J Light to enter the Eye, so the Pupil is destined to re-

r gulate the Quantity of the Rays that ought to enter

3 the interior Part of the Eye for rendering V ision distinfj,

j and the Images of Objects properly illumined. Eo this

g Purpose it is composed of two Sets of muscular I ibres,

Viz, one of a circular Form, which, by corrugating,

5 £°iitract or diminish the Pupil; and the other is an An-

£ ' f ■ nuius