Os Optics. 131

by the distance of the object, and divide the product by their dif-ference; the quotient will be the distance of the picture.

The picture will be as much bigger or lefs than the object, as its Fig.distance from the glass is greater or lefs than the distance of the object.for, as Be is to eB, fo is AC to ca. So that if ABC be the object, cbawill be the picture 5 or, if cba be the object, ABC will be thepicture.

Having defcribed how the rays of light, flowing from objects and The mannerpaffing through convex glastes, are coliected into points, and form of visl0n -the images of the objects; it will be eafy to understand how the raysare affected by paffing through the humours of the eye, and arethereby coliected into innumerable points on the bottom of the eye,and thereon form the images of the objects which they flow from.

For, the different humours of the eye, and particularly the chrystallinehumour, are to be considered as a convex glase; and the rays in paffingthrough them to be affected in the fame manner as in paffing througha convex glass.

The eye is nearly globular. It consists of three coats and three Fig.humours. The part JDHHG of the outer coat, is called the fcle-rotica , the rest DEFG the cornea. Next within this coat is thatcalled the choroides , which serves as it were for a lining to the other,and joins with the iris mn, mn. The iris is compofed of two fetsof muicular fibres; the one of a circular form, which contracts thehole in the middle called the pupil, when the light would otherwifebe too strong for the eye; and the other of radial fibres, tending The eye de-every where from the circumference of the iris towards the middle of scribed *the pupil j which fibres, by their contraction, dilate and enlarge thepupil when the light is weak, in order to let in the more of itsrays. The third coat is only a fine expansion of the optic nerve L,which fpreads like net-work all over the inside of the choroides, andis therefore called the retina ; upon which are painted (as it were)the images of all visible objects, by the rays of light which either flowor are reflected from them.

Under the cornea is a fine transparent fluid like water, which istherefore called the aqueous humour. It gives a protuberant figure tothe cornea, filis the two cavities mm and nn, which communicate by thepupil P, and has the ferne limpidity, fpecific gravity, and refractive poweras water. At the back of this lies the chryßalline humour II, whichis ffiaped like a double convex glass; and is a little more convexon the back than the fore part. It converges the rays, which passthrough it from every visible object to its focus at the bottom of the

S 2 eye.