To get a sharp picture, or image of the flame on the cardboard screen you will have to move the lens toward and away from the cardboard, and this process is called focusing. If you will fix the lens in the front of a light-tight box and place a sheet of ground glass in the back of the box you will have a simple, though crude, camera.
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.
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.
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.