First, you should realize that the image formed by a convex lens is not always larger than the object. Repeat Experiment 41, but this time move the lens from near the candle toward the paper, past the point where it makes its first clear image. Keep moving the lens slowly toward the paper until a second image is formed. Which image is larger than the flame? Which is smaller? [Illustration: FIG. 79. A section of the eye.] The important point in this experiment is for you to see that if the lens is nearer to the image on the paper than it is to the candle, the image is smaller than the candle. That is why a photograph is usually smaller than the thing photographed; it would be impossible to take a picture of a house or a mountain if the lens in the camera gave a _magnified_ image. [4]Your eye is a small camera. It has a lens in the front; it is lined with black; and at the back there is a sensitive part on which the picture is formed. This sensitive part of the eye is called the _retina_. It is in the back part of your eyeball and is made of many very sensitive nerve endings. When the light strikes these nerve endings, it sends an impulse through the nerves to the back part of the brain; then you know that the image is formed. And, of course, since your eyeball is small and many of the things you see are large, the image on the retina must be much smaller than the object itself, and this is because the lens is so much nearer to the retina than it is to the object. [Footnote 4: The following explanation may be omitted by any children who are not interested in it. Let such children skip to the foot of page 156.] [Illustration: FIG. 80. How an image is formed on the retina of the eye.] [Illustration: FIG. 81. A simpler diagram showing how an image is formed in the eye.] [Illustration: FIG. 82. A diagram showing how a reading glass causes things to look larger by making the image on the retina larger.] [Illustration: FIG. 83. Diagram showing how a reading glass enlarges the image on the retina. More lines are drawn in than in Figure 82.] You can understand magnification best by looking at Figures 80, 81, 82, and 83. In Figure 80 there are a candle flame, the lens of an eye, and the retina on which the image is being formed. Figure 81 is the same as Figure 80, with all the lines left out except the outside ones that go to the lens. It is shown in this way merely for the sake of simplicity. All the lines really belong in this diagram as in the first. In both diagrams the size of the image on the retina is the distance between the point where the top line touches it and the point where the bottom line touches it. In order to make anything look larger, we must make the image on the retina larger. A magnifying glass, or convex lens, if put in the right place, will do this. In the next diagram, Figure 82, we shall include the magnifying glass, leaving out all lines except the two outside ones shown in Figure 81.