The eye has all of the things which the highest priced camera has and a good deal more, for all of its adjustments are automatically made, and you don’t even have to think about them. [Illustration: FIG. 139.—FORMING AN IMAGE WITH A LENS.] The eye is almost as round as a ball and it can be turned a little in its bony socket in any direction. The outer part of an eye which takes the place of the box of a camera, is stretched round the whole eye like the cover of a baseball, as shown in Fig. 140. The front part of this cover forms the white of the eye, and fitting into the cover and over the lens, like a watch crystal in its rim, is the _cornea_, which is a tough, but transparent film and protects the _iris_ and the _lens_. [Illustration: FIG. 140.—THE HUMAN EYE.] Between the lens and the cornea is a thin disk, or diaphragm with a hole in its center and this is the iris; the purpose of the iris is to let in only a certain amount of light, just like the shutter of a good camera. The hole in the iris forms the _pupil_ of the eye, and you can see the hole, or pupil, grow larger or smaller, just as the eye needs more or less light. The lining of the eye is called the _retina_ and this forms a screen at the back of the eye on which the light waves in the ether project the image of the object at which the eye is looking. Instead of being white like our cardboard screen the retina is very black. The retina upon which the image is formed is connected with the optic nerve; in fact, the retina is a part of the optic nerve and is covered with a lot of little nerve ends or filaments called _rods_ and _cones_. Now when the waves in the ether sent out by the flame of a candle, or by the Sun, reach the eye, they pass through the cornea, then through the pupil, or hole in the iris, and finally through the lens which focuses the waves on the retina and forms the image there. The different colors of light are caused by waves in the ether of different lengths; when very short waves strike the retina we say the color is violet; waves a little longer we call green and the longest waves which the eye can see form in our brains the sensation of red. Waves in the ether which are longer than the wave-lengths the eye can see produce heat and when these waves fall on any part of the body the nerves detect them and we call the sensation heat. On the other hand waves in the ether which are too short to affect the nerves of the eye will impress a photographic plate. The iris of the eye acts as a self-regulating shutter, which makes the hole, or pupil in front of the lens larger or smaller according to the amount of light which is needed to see an object well. If the light is strong the iris contracts, which means that the hole gets smaller and so cuts off some of the light. If the light is weak the hole gets larger and we say the pupil expands and this lets more light through the lens.