Page:The wonders of optics (1869).djvu/36

 been already familiarized with the appearance of our satellite, he will not know whether the image he sees is reversed or not. It is the brain, therefore, and the brain only, that has the power of determining the position of objects around us, without taking into consideration the reversed picture of them that is depicted on our retina. The student who takes an interest in the structure of this important organ, would do well to procure a sheep's or bullock's eye from the butchers, and dissect it carefully with a sharp penknife and pair of scissors. The image formed on the retina may be easily seen by cutting away the sclerotic and choroid coatings at the back of the eye.

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