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 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 eye-lashes.

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 an idea of whatever is drawn upon it. I 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.