The ordinary distance of distinct vision for small objects, such as the letters of a book, is from ten to twelve inches. But possibly there do not exist two pairs of eyes in the world whose foci are the same. Even in the same individual it frequently happens that the focal length of the eyes differs considerably. In some persons the focus of the eye is so reduced that they are obliged to bring the object they are examining within six, and even four inches of their eyes, before they can see it. This defect is known ordinarily as _short sight_, and results from the too great convexity of the cornea and crystalline lens. It is corrected by wearing spectacles with concave glasses. Others again, on the contrary, place the book or object they are looking at, at a greater distance from the eye than that named. Such people are called long-sighted, and the defect results from the too great flatness of the cornea and the crystalline lens. The fault is of course corrected by the use of spectacles containing convex lenses. Long-sightedness is generally the result of old age, and it may be taken as a fact that the older we grow the flatter becomes the crystalline lens. Hence short-sighted people have been known to recover their sight perfectly as they advance in years through the natural process of the flattening of the crystalline lens. These matters, however, will be more fully treated of when we begin to speak of the properties of lenses of different forms and curvatures. ------------------------------------------------------------------------ CHAPTER III. THE ERRORS OF THE EYE.