13 A horizontal section of the right globe.
14 The sclerotica.
15 The cornea.
16 The choroid.
17 The dark pigment.
18 The retina.
19 The iris, having the pupil widely open.
20 and 21 The anterior and posterior chambers containing the aqueous humour.
22 The lenticular system.
23 The vitreous body.
24 The central artery.
25 The optic nerve.
26 The chiasma of the optic nerves (longitudinal section of which is made), the numerous fibrils composing these nerves, their course, and decussation.
27 The part of the optic nerves coming from the brain.
28 An object which reflects the light in the direction of the lines drawn from the points a to the eyes directed towards the object, just as every other point in the object would do. The lines b are pencils of light, as they find entrance through the pupil into the interior of the eye-ball, where each of them is by refraction made to converge to a point, called the focus, which, falling exactly on the retina, forms a distinct image thereon of the point a of the external object from which the pencil of light emanated. This is what takes place at least in the eye in its normal state; but when there is any deviation from this state, and the condition of the eye is such as to bring the rays of each pencil of light to a focus at the line c or d, no distinct image can in this case be formed on the retina; since, when the focus occupies the place of the line c, the rays, in meeting the retina, are again divergent; and, when the focus is situated at the line d, the rays, in
