Thus you can see how it is with your eye, and why you cannot see things distinctly held close to it. The lens in the eye can change its shape a little, so that it will focus objects a mile off, or ten inches off, but it cannot be pushed in and out like the tube in your camera. You can do this, however, if you take another lens and hold it outside your eye and let the light go through that first before it comes to the lens in your eye, and in this way you can get a focus in your retina, and the outside lens thus forms a part of that optical instrument called your eye. Does your grandma know that her spectacles are a part of the cameras that she calls her eyes? How is it that a lens bends (refracts is the big word for it) the rays of light? You will learn by and by. You can see that it does so by a few experiments with your sun glass or any such lens. Hold it between the sun and a piece of white paper until the white spot in the centre is as small as you can make it. You will see that the rest of the lens casts a shadow although it is all glass; this is because the rays of sunlight that fall on the lens are all bent towards the centre, and so you have a small white spot on which is concentrated the light and the heat, and before you have found out how it is all done, your paper takes fire and the experiment ends in smoke. Take another piece of paper, and when the white spot is at its smallest, measure the distance between the lens and the paper, and you will have the focal distance of the lens. You have now found out how to get your eye close to an object and see something that is very small; this is usually called magnifying it, because it seems to make it look large. Suppose you have a lens that will let you see a flea through it held just one inch from it, this lens is now an addition to your eye, as we measure from the lens. If you had another flea held ten inches off, so big that it would just be hidden by the little flea, the one farthest off would be ten times as large as the near one. (Fig. 3.) In this case it is said that the lens having a focal length of one inch magnifies ten times, or has a power of ten. [Illustration: FIG. 3.] The shortest usual distance of objects seen distinctly being taken as ten inches, microscopists have agreed to consider that as the standard of measurement, and objects seen through a lens are considered magnified to the size they would have if projected ten inches off, like our little flea. II.--THE OUTFIT. Now that we have got hold of the idea that the eye is an optical instrument, and that to increase its capacity for seeing small things we add to it other optical contrivances, making with it one instrument composed of several parts, let us look at such additions more particularly. [Illustration: FIG. 1.] [Illustration: FIG. 2.] [Illustration: FIG. 3.--OPEN AND CLOSED.]