THE SPECIAL SENSES 301 



ina. The probable source of this knowledge is the habitual 

 comparison of those objects with the position of our own 

 bodies ; thus, to see an elevated object, we know we must raise 

 the head and eyes ; and to see one at our right hand, we must 

 turn the head and eyes to the right. 



79. Long-sight or Hyperopia, and Short-sight or Myopia. 

 The eye is not in all cases perfectly formed. I^or example, 

 persons may from birth have the cornea too prominent or too 

 flat, or the lens may be too thick or too thin. In either of 

 these conditions sight will be more or less defective from the 

 first, and the defect will not tend to disappear as life advances. 

 The most common imperfection, however, is in the shape of 

 the globe ; which may be short (Fig. 68, H), as compared with 

 the natural eye, N, or it may be too long, M. 



80. When the globe-is short,- only objects that are at a 

 distance can be clearly seen, and the condition of the vision is 

 known as " lonfi-sight." or hyperopia. It will be observed, by 



FIG. 68. THE DIFFERENT SHAPES OF THE GLOBE OF THE EY* 



N, The Natural Eye M, The Short-sighted Eye 



H, The Long-sighted Eye 8, Parallel Rays from the Sun 



reference to Fig. 68, that the focus of the rays of light would 

 fall behind the retina of this eye. When the globe is too long, 

 only objects that are very near to the eye can be clearly seen, 

 and the condition resulting from this defect is termed " short- 

 sight," or myopia. The focus of the rays of light is, in this 

 case, formed in the interior of the eye in front of the retina. 



79. The uniform perfection of the eye ? Examples ? The most common imperfection ? 



80. How is " long-sight " explained ? " Short-sight " f 



' 



If Am 



