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posed of three coats or skins, and three other substances called humours. This figure (Plate 111. Fig 25.) represents the section of an eye, that is, an eye cut down the middle ; and Fig. 26, the front view of an eye as it appears in the head.
Charles. Have these coats and humours all different names ?
Tutor. Yes: the external coat, which is represented by the outer circle A B C D E, is called the sclerotica; the front part of this, namely, C x D, is perfectly transparent, and is called the cornea; beyond this, towards B and E, it is white, and called the white of the
eye. The next coat, which is represented by the second circle, is called the choroides.
James. This circle does not go all round.
Tutor. No: the vacant space a b is that which we call the pupil, and through this alone the light is allowed to enter the eye.
Charles. What do you call that part, which is of a beautiful blue in some persons, as in cousin Lydia ; and in others brown, or almost black ?
Tutor. That, as a c, b e, is part of the cho roides, and is called the iris.
Charles. The iris is sometimes much lar ger than it is at another.
Tutor. It is composed of a sort of net work, which contracts or expands according to the force of the light in which it is placed. Let James stand in a dark corner for two or three minutes:--now look at his
eyes. Charles. The iris of each is very small, and the pupil large.
Tutor. Now let him look steadily, rather close to the candle.
Charles. Theirisis considerably enlarged, and the pupil of the eye is but a small point in comparison of what it was before.
Tutor. Did you never feel uneasy after sitting some time in the dark, when candles were suddenly brought into the room?
James. Yes:Iremember last Friday evening we had been sitting half an hour almost in the dark at Mr. W. 's, and when candles were introduced, every one of the company complained of the pain which the sudden light occasioned.
Tutor. By sitting so long in the dark, the iris was contracted very much, of course the pupil being large, more light was admitted than it could well bear, and therefore till time was allowed for the iris to adjust itself, the uneasiness would be felt.
Charles. What do you call the third coat, which, from the figure, appears to be still less than the choroides ?
Tutor. It is called the retina, or net-work, which serves to receive the images of objects produced by the refraction of the different humours of the eye, and painted, as it were, on the surface.
Charles. Are the humours of the eye in: tended for refracting the rays of light, in the same manner as glass lenses?
Tutor. They are; and they are called the vitreous, the crystalline, and the aqueous humours. The vitreous humour fills
space z 2, at the back of the eye ; it is nearly of the substance of melted glass. The crystalline is represented by df, in the shape of a double convex lens : and the aqueous, or watery humour, fills up all that part of the eye between the crystalline humour, and the corner C X D.
James. What does the part A at the back of the eye represent?
Tutor. It is the optic nerve, which serves to convey to the brain the sensations produced on the retina.
Charles. Does the retina extend to the brain ?
Tutor. It does : and we shall, when we meet next, endeavour to explain the office of these humours in effecting vision. In the mean time, I would request you to consider again what I have told you of the different
and examine, at the same time, both figures ; viz. 25 and 26.
James. We will: but you have said nothing about the uses of the eye-brows and eyelashes.
Tutor. I intended to have reserved this to another opportunity : but I may now say, that the eye-brows defend the eye from too strong a light; and they prevent the eyes
parts of the
from injuries by the sliding of substances down the forehead into them.
The eye-lids act like curtains to cover and protect the eyes during sleep: when we are awake, they diffuse a fluid over the eye, which keeps it clean and well adapted for transmitting the rays of light.
The eye-lashes, in a thousand instances, guard the eye from danger, and protect it from floating dust with which the atmosphere abounds.