Page:How and Why Library 479.jpg

 some cannot even bear the motions of swings and hammocks. But people differ very widely. Some are never made sick at sea, and others never get over the tendency, no matter how much they travel. Children rarely suffer from this malady. That is probably because they tumble about more than do grown people. It doesn't bother an active boy much to have his stomach rocked, or even turned upside down.

Of all machines a camera is the Chinaman. It does everything upside down, wrong side before, and right side out. Then the eyes in our heads are Chinamen, too, for they are Nature's cameras. Camera means a room, or chamber. Men who are very exact say camera obscura. That means dark room. The camera and the eye each have a dark room. In front of this dark room is a small, clear, thick window, or lens, for letting the light rays through. At the back of the room is a screen on which the picture is thrown, just as a screen is used in the moving-picture show. Look at the lenses of grandmother's spectacles. They are oval bits of glass, thickest in the middle. Through them grandmother can see the widest landscape and the highest sky. The light rays, falling at widely separated angles, almost meet in the tiny lenses. As they pass through the lenses they do meet in points just behind them. The rays cross each other where they meet, and spread again. But as each ray travels straight, the upper rays fall below, after crossing, and the lower rays rise to the top. That turns the picture upside down on the screen at the back of the dark room. Look into a camera and you will see the picture upside down. A photographer just turns the plate around. In your eye the optic nerve, or the brain, turns the picture right side up.

But how is this topsy-turvy picture fixed on the screen at all? The screen in the camera is a glass plate, or a gelatine film. This is coated with a sensitive chemical that is acted upon by light rays. Every gradation of light, from pure white to dead black, acts upon it differently. Rays from a white collar destroy this chemical; black cloth, giving off no rays, do not act upon it at all. The picture you get is not only upside down, it is a negative. Negative means "no." That is, it is the "no" or the very opposite of the thing pictured. In the negative, the white collar appears black. But the negative prints a positive or "yes" on a dark paper sensitive to light rays.