Through Magic Glasses and Other Lectures / A Sequel to The Fairyland of Science

by Arabella B. Buckley

Fig. 11.

Section of an eye looking at a pencil. (Adapted from Kirke.)

c, c, Cornea. w, White of eye. cm, Ciliary muscle. a, a, Aqueous humour. i, i, Iris. l, l, Lens. r, r, Retina. on, Optic nerve. 1, 2, Pencil. 1´, 2´, Image of pencil on the retina.

"Now look at the second diagram (Fig. 11) and notice the chief points necessary in seeing. First you will observe that the pupil is not a mere hole; it is protected by a curved covering c. This is the cornea, a hard, perfectly transparent membrane, looking much like a curved watch-glass. Behind this is a small chamber filled with a watery fluid a, called the aqueous humour, and near the back of this chamber is the dark ring or iris i, which you saw from the front through the cornea and fluid. Close behind the iris again is the natural 'magic glass' of our eye, the crystalline lens l, which is composed of perfectly transparent fibres and has two rounded or convex surfaces like an ordinary magnifying glass. This lens rests on a cushion of a soft jelly-like substance v, called the vitreous humour, which fills the dark chamber or cavity of the eyeball and keeps it in shape, so that the retina r, which lines the chamber, is kept at a proper distance from the lens. This retina is a transparent film of very sensitive nerves; it forms a screen at the back of the chamber, and has a coating of very dark pigment or colouring matter behind it. Lastly, the nerves of the retina all meet in a bundle, called the optic nerve, and passing out of the eyeball at a point on, go to the brain. These are the chief parts we use in seeing; now how do we use them?

"Suppose that a pencil is held in front of the eye at the distance at which we see small objects comfortably. Light is reflected from all parts of the surface of the pencil, and as the rays spread, a certain number enter the pupil of the eye. We will follow only two cones of light coming from the points 1 and 2 on the diagram Fig. 11. These you see enter the eye, each widely spread over the cornea c. They are bent in a little by this curved covering, and by the liquid behind it, while the iris cuts off the rays near the edges of the lens, which would be too much bent to form a clear image. The rest of the rays fall upon the lens l. In passing through this lens they are very much bent (or refracted) towards each other, so much so that by the time they reach the end of the dark chamber v, each cone of light has come to a point or focus 1´, 2´, and as rays of this kind have come from every point all over the pencil, exactly similar points are formed on the retina, and a real picture of the pencil is formed there between 1´ and 2´."

Fig. 12.

Image of a candle-flame thrown on paper by a lens.

"We will make a very simple and pretty experiment to illustrate this. Darkening the room I light a candle, take a square of white paper in my hand, and hold a simple magnifying glass between the two (see Fig. 12) about three inches away from the candle. Then I shift the paper nearer and farther behind the lens, till we get a clear image of the candle-flame upon it. This is exactly what happens in our eye. I have drawn a dotted line c round the lens and the paper on the diagram to represent the eyeball in which the image of the candle-flame would be on the retina instead of on the piece of paper. The first point you will notice is that the candle-flame is upside down on the paper, and if you turn back to Fig. 11 you will see why, for it is plain that the cones of light cross in the lens l, 1 going to 1´ and 2 to 2´. Every picture made on our retina is upside down.