It is made exactly like the first one except that the larger tube is 14 inches long and the smaller tube is 5 inches long. In this telescope both lenses are double convex, the large one, or object glass, having a diameter of 1½ inches and a focal length of 12 inches, while the smaller lens, or eyepiece, has a diameter of 1 inch and a focal length of 3 inches. It is shown in cross section in Fig. 156. A spyglass usually has four or five _plano-convex lenses_ in it and these not only magnify the image but they also erect it so that you see the object as it really is. While the homemade telescopes which I have described will not magnify as highly as a cheap telescope which you can buy, yet you ought to make one, for it will let you into the secret of combining lenses, and this is as interesting as seeing the stars. By all means make your first telescope and then if you want a better one buy it and get one as large as you can afford. [Illustration: FIG. 157.—MAGNIFYING POWER OF TELESCOPE.] =To Find the Power of a Telescope.=—In the last chapter, I explained what the _focal length_ of a convex lens is, see Fig. 146, and how to measure it. To repeat, it is the distance in inches between the center of a lens and the point where the rays come together. You can find exactly what the _magnifying power_ of your telescope is, when both lenses are convex, by dividing the focal length of the object glass by the focal length of the eyepiece, or lens. For instance, suppose the focal length of the convex object glass of your telescope is 12 inches and the focal length of the convex eye lens is 3 inches, then 12 ÷ 3 = 4 and the quotient 4 is the magnifying power of your telescope. To find the focal length of a concave lens is a little harder, but Garrett P. Serviss tells us in his good little book on _Astronomy with an Opera Glass_ of an easy way to judge the magnifying power of an opera glass and it is just the same for a telescope. Look at a brick wall through one of the tubes with one eye while the other naked eye sees the wall direct. Now notice how many bricks which the naked eye can see, are needed to equal the thickness of one brick as seen through the glass. The number of bricks seen with the naked eye represents the magnifying power of the glass. Fig. 157 shows how the bricks are compared. =The Stars Seen Through a Spyglass.—The Moon.=—When Galileo lived the people believed that the Moon was a ball as smooth and bright as a glass marble and they also thought that the dark spots on its surface were the continents of our Earth reflected by it. So the first thing Galileo did when he got his telescope was to turn it on the Moon, for he wanted to know about these dark spots, and you can imagine his surprise and delight to find that they were really great mountains and extinct volcanoes.