But of course the light from the base of the flame is focused at the same time as the light from the point; so what really happens is that which is illustrated in Figure 74. In this diagram, we have drawn unbroken lines to show the light from the point of the candle flame and dotted lines to show the light from the base of the flame. This is so that you can follow the light from each part and see where it goes. Compare this diagram with the one where the light is shown all crisscrossed (Fig. 70), and you will see why the lens makes an image, while you have no image without it. By looking at the last diagram (Fig. 74) you can also see how the image happens to be upside down. EXPERIMENT 48. Set up the candle and piece of paper as you did for the last experiment, but move the magnifying glass back and forth between the paper and the candle. Notice that there is one place where the image of the candle is very clear. Does the image become clearer or less clear if you move the lens closer to the candle? if you move it farther from the candle? The explanation is this: After the light comes together into a point, it spreads out again beyond the point, as shown in Figure 75. So if you hold the lens in such a way that the light comes to a focus before it reaches the paper, the paper will catch the spreading light and you will get a blur instead of a sharp image. It is as shown in Figure 76. [Illustration: FIG. 75. The light spreads out again beyond the focus.] [Illustration: FIG. 76. So if the light comes to a focus before it reaches the paper, the image will be blurred.] On the other hand, if you hold your lens in such a way that the light has not yet come to a focus when it reaches the paper, naturally you again have a blur of light instead of a point, and the image is not sharp and definite (Fig. 77). [Illustration: FIG. 77. Or if the light reaches the paper before it comes to a focus, the image will be blurred.] And that is why good cameras have the front part, in which the lens is set, adjustable; you can move the lens back and forth until a sharp image is formed on the plate. Motion-picture machines and stereopticons likewise have lenses that can be moved forward and back until they form a sharp focus on the screen. Even the lens in your eye has muscles that make it flatter and rounder, so that it can make a clear image on the sensitive retina in the back of your eye. The lens in the eyes of elderly people often becomes too hard to be regulated in this way, and so they have to wear one kind of glasses to see things near them clearly and another kind to see things far away.