THE EYE AS AN OPTICAL INSTRUMENT. 



201 



erly adjusted concave glasses to carry the focus farther back. 

 When the eye is too short, the focus is behind the retina, and the 

 person is far-sighted and must wear convex glasses. The first 

 condition is called myopia, and the second, hypermetropia ; but 

 in most persons who are obliged to wear convex glasses in ad- 



CHOROID 



OPTIC 



CHOROID 



Fio. 2. DIAGRAMMATIC SECTION OF THE HUMAN EYE. 



vanced life, the crystalline lens has become flattened and inelastic, 

 the diameter of the eye being unaltered. This condition is called 

 presbyopia, which means a defect in vision due to old age. 



One of the wonderful things about the eye is the mechanism 

 by which a perfect image is formed. What is called the area of 

 distinct vision is a depression in the yellow spot of the retina, 

 which is probably not more than a thirty-sixth of an inch in di- 

 ameter. It is with this little spot that we examine minute details 

 of objects. If we receive the rays of light from an object upon a 

 double-convex lens and throw them upon a screen in a darkened 

 room, the image of the object appears upon the screen ; but in 

 order to render this image even moderately distinct it is necessary 

 to carefully adjust the lens, or the combination of lenses, to a cer- 

 tain distance, which is different for lenses of different curvatures. 

 In the human eye the adjustment is most accurately made, almost 

 instantaneously, for any desired distance, not by changing the 

 distance between the crystalline lens and the retina, but by chang- 

 ing the curvature of the crystalline lens itself. The way in 



