Page:Language of the Eye.djvu/40

22 13 &emsp;A horizontal section of the right globe.

14 &emsp;The sclerotica.

15 &emsp;The cornea.

16 &emsp;The choroid.

17 &emsp;The dark pigment.

18 &emsp;The retina.

19 &emsp;The iris, having the pupil widely open.

20 and 21 &emsp;The anterior and posterior chambers containing the aqueous humour.

22 &emsp;The lenticular system.

23 &emsp;The vitreous body.

24 &emsp;The central artery.

25 &emsp;The optic nerve.

26 &emsp;The chiasma of the optic nerves (longitudinal section of which is made), the numerous fibrils composing these nerves, their course, and decussation.

27 &emsp;The part of the optic nerves coming from the brain.

28 &emsp;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