both filled with a liquid which keeps the eyeball full and rounded out. The liquid in front of the lens is called the aqueous humor, which

Choroid Coat

In the figure you can trace the choroid (kōr'oid) coat, which lines the back room of the eye. In front, this coat appears as the iris. Inside the choroid, in the back of the eyeball, we find the retina. The retina contains the sensitive endings of the nerve of sight, or optic nerve. The blind spot is caused by the coming together of nerves from nerve endings all over the retina. There they all unite, and leave the eye as the optic nerve, which carries sight messages to the brain.

FIG. 177. - The eye consists of a darkened room open to light from the front, through the pupil. The size of the pupil is regulated by the iris. The light is brought to a focus upon the retina, which in its turn sends the sight message to the brain.

Muscles attached to the eyeball hold it in its place in the head. Small muscles, inside, control the iris, making the pupil, or opening, large when the object is hard to see, and small when you are in the dazzling sunlight. Why is this adjustment needed? Why do you set your camera at 25 seconds on one day, 50 on another, and sometimes take a time exposure"?

66

Strong ligaments, or bonds, are found just back of the choroid coat; they control the shape of the crystalline

lens. When objects are near, these ligaments become loose, and the lens is rounded. This makes the light rays bend at a greater angle, and thus gets them to a focus on the retina. When you are looking at a distant object, the rays enter the eye in more of a parallel position. For that reason the ligaments are made to pull at each end of the lens, thus flattening it. As the lens becomes more flat, the rays are focused on the retina without being bent so much.

If the crystalline lens bends the light rays too much, and so brings them to a focus too soon, we are nearsighted; if it does not bend them enough, we are farsighted. If our eyes are not perfect, we should consult a skillful oculist, and have him tell us what kind of eyeglasses we need, so that the eyes may be helped to do their work properly.

As your eyes are so very delicate, you should give them the best possible care. Never rub the eyeball with your fingers, unless you are sure they are clean; the eye may become infected. Never try to read on a moving train, as the eyes are strained by trying to follow the shaking book. Reading while lying down is harmful.

253. Exercises. - 1. Why is the foaming top of a wave white ("whitecap "), while the remainder of the wave is blue or green? 2. If you try to pick up a stone from the bottom of a stream, do you find that it is where it seems to be? Why?

3. What is the rate at which light travels? How long would it take light to go once around the earth at the equator?

4. If we see a rainbow at 7 a.m., in what part of the sky will it be? At 5 p.m.?

5. Look at a mirror in the dark, and suddenly turn on the light. What happens to the pupil of the eye? Bring a cat from a dark room into a very light one, and see what happens to its eyes; why?

6. What is the back of a mirror made of? Is a water surface ever a mirror? A varnished floor? Name some other mirrors.

7. What position has the earth, with respect to the moon and sun, at the time of full moon? At the time of new moon? At which of these times can there be an eclipse of the moon? Why?

8. If you sit 4 feet from a lamp, and your brother one foot from the lamp, how much more light does he receive than you?

9. Is a finger nail transparent, translucent, or opaque? A sugar solution? Milk? Ice? Lead? Greased paper?

10. Do you think that glass bottles left in the woods might cause forest fires? Tell how.

Summary. We see objects because they send light rays to our eyes. Self-luminous bodies send us their own light; illuminated bodies send us reflected light.

Bodies may be transparent, translucent, or opaque.

Light travels in the form of waves, at the rate of about 186,000 miles a second.

Light waves from the sun bring us heat and chemical energy as well as light.

Light may be reflected by mirrors. The angle at which the light leaves the mirror is the same as that at which it strikes the mirror. Periscopes are tubes containing mirrors which reflect light "around. corners," so that men can see without being seen.

A shadow is the region from which light is cut off by an opaque object. If the source of light has a considerable area, its shadow will have two parts: an umbra and a penumbra. In an eclipse, one heavenly body passes through the shadow cast by another.

The unit of light brightness is the candle power.

When light passes at a slant from glass or water or other transparent materials into air, it is bent out of its course, or refracted.

Burning glasses are lenses for bringing many light rays together at a point called the focus.

A camera is a device in which rays of light coming from an object are brought to a focus upon a sensitive film, so that a picture of the object may be produced.

The white color of sunlight is made up of the colors of the solar spectrum. When all these colors are recombined, they produce white light.

A rainbow is a solar spectrum produced by the bending and reflecting of sunlight in many raindrops.

We can trap sunlight in a greenhouse or a cold frame. The earth's atmosphere acts as a great trap.

The eye is a wonderful camera, in which light rays from each part of an object are brought to a focus upon a sensitive "film" the retina. The optic nerve then carries the sight message to the brain.

CHAPTER XXVII

LIGHT IN THE HOUSE

254. How Do We Get Sunlight into the House? The light of the sun is our symbol of health and joy, and the eye is the organ by which we get most of our knowledge and our delight in our surroundings. Why, then, did primitive peoples, and our own ancestors, only a few generations ago, have so little sunlight in their houses? Think a minute; what was the first and most important use of the house? It was shelter, was it not? Shelter from weather and from powerful animal and human enemies. The castle itself, with its thick walls, often had only slits for windows. Why?

Even when civilization came, and man did not greatly fear enemies, except those of the weather, the problem of the window was still a hard one; for what material could he use to fill the hole in the wall? Oiled paper, isinglass, and thin sheets of horn were used at first; they are still used in some less civilized communities. Are these materials as good as glass? What, then, do you think of the debt of gratitude we owe to the man, or the men, who made it possible for us to get cheap window glass? We shall feel that the debt is much greater when we realize that in shutting out his larger enemies early man shut in many powerful but tiny enemies he did