EXERCISE 92 



The eye in backboned animals and in certain mollusks (the octopus, for example) consists of a lens 

 that projects an image upon a sensitive surface, the retina. The focusing, instead of being brought 

 about, as in a camera, by changing the distance between the lens and the retina, is brought about by 

 changing the convexity, or bulge, of the lens. Very few human eyes are perfect, although most of 

 them are quite usable for all ordinary purposes. The most frequent defects are nearsightedness 

 and farsightedness, and an unevenness of the curvature called astigmatism. 



Problem. What condition of the eye brings about farsightedness or nearsightedness ? 



What to use. A convex lens ; a vertical screen or sheet of paper held in a vertical position ; a ruler. 



What to do. Support the lens in a vertical position a measured distance from the window say 

 100 or 1 20 inches. Place the screen back of the lens and move it backward and forward until you 

 have a clear image of the window. Measure the distance between the lens and the screen ; this is the 

 focal distance. Now bring the lens five or six inches nearer the window, focus again, and measure 

 the focal distance. Move the lens five or six inches farther from the window and find the third 

 focal distance. 



Record. 



From the measurements made, can you tell what condition of the eyeball brings about farsightedness 

 and what condition brings about nearsightedness ? 





Questions. 1. If the eyeball is too short, what can be done to save eyestrain ? 



2. If the eyeball is too long, what can be done to save eyestrain ? 



3. Why is a person more likely to need spectacles at an advanced age than at an early age ? 



[99] 



