212 THE SPECIAL SENSES. 



the retina observes no difference, so to speak, between the 

 right and left or the upper and lower positions of objects. 

 In fact, the mind is never conscious of the formation of a 

 retinal image, and until instructed, has no knowledge that 

 it exists. Consequently, our knowledge of .the relative 

 location of external objects must be obtained from some 

 other source than the retina. 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. For 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 



Fro. 51. THE DIFFERENT SHAPES OF THE GLOBE OF THE EYE. 



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



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



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. 51, H), as compared with the natural 

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



80. When the globe is short, objects can only be clearly 



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

 perfection ? 



80. How is " long-sight" 1 explained? "Short-sight?" 



