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James. I once saw a bullock's eye. dissected, and was told that it imitated a human eye in its several parts.
Tutor. The eye, when taken from the socket, is of a globular form, and it is composed of three coats or skins, and three other substances called humours. This figure (Plate . 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 choroides, and is called the iris.
Charles. The iris is sometimes much larger 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. The iris is 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: I remember 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 occasionedu
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 intended 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 up all the space Z z, at the back of the eye; it is nearly of the substance, of melted glass. The crystalline is represented by d f, 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 humoui, and the corner C, D. ;
James. What does the part A at the back of the eye represent?