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or watery humour, fills up all that part of eye between the crystalline humour, and the corner cx D.

the

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 pro

duced 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 parts of the eye; 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

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.

CONVERSATION XVI.

Of the Eye, and the Manner of Vision.

CHARLES. I do not understand what you meant when you said, the optic nerve. served to convey to the brain the sensations produced on the retina.

Tutor. Nor do I pretend to tell you in what manner the image of any object painted on the retina of the eye is calculated to convey to the mind an idea of that object : but I wish to show you, that the images of the various objects which you see are painted on the retina. Here is a bullock's eye, from the back part of which I cut away the three

coats, but so as to leave the vitreous humour perfect: I will now put against the vitreous humour a piece of white paper, and hold the eye towards the window; what do you see?

James. The figure of the window is drawn upon the paper; but it is inverted.

Tutor. Open the window, and you will see the trees in the garden drawn upon it in the same inverted state, or any other bright object that is presented to it.

Charles. Does the paper, in this instance, represent the innermost coat called the retina?

Tutor. It does; and I have made use of paper because it is easily seen through, whereas the retina is opake; transparency would be of no advantage to it. The retina, by means of the optic nerve, is conveyed to the brain, or, in other words, the optic nerve is an extension of the retina.

James. And does it carry the news of every object that is painted on the retina? Tutor. So it should seem; for we have

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an idea of whatever is drawn

I

it. upon direct my eyes to you, and the image of your person is painted on the retina of my eye, and I say I see you. So of any thing

else.

Charles. You said the rays of light proceeding from external objects were refracted in passing through the different humours of the eye.

Tutor. They are, and converged to a point, or there would be no distinct picture drawn on the retina, and of course no distinct idea conveyed to the mind. I will show you what I mean by a figure, taking an arrow again as an illustration.

As every point of an object A B C (Plate IV. Fig. 27.) sends out rays in all directions, some rays from each point on the side next the eye, will fall upon the cornea between x y, and by passing through the humours of the eye they will be converged and brought to as many points on the retina, and will form on it a distinct inverted picture c b a of the object.

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