THE EYE AND THE PHYSIOLOGY OF VISION. 555 



is an element of optical hygiene to insist that persons with 

 normal eyes to begin with shall hold objects, such as a 

 printed page, at a reasonable distance from the eye. 



2. Long Sight or Hypermetropia. Hypermetropia 

 occurs when the image in the eye would naturally fall be- 

 hind the retina, or since this is actually impossible, the 

 real state of things is the absence of a focus altogether, the 

 rays of light falling on the retina in a circle before they 

 have met in a point. The causes of this defect are two. 

 (a) It may consist of a shortening of the eyeball from be- 

 fore backwards, so that the retina now is in front of its 

 natural position. Or, () since the rays of light are not 

 converged enough the lens may be too nearly flat. The 

 remedy in this instance is equally obvious. The converg- 

 ing power of the eye must be increased, and so convex 

 glasses are needed. 



As the image of an object is pushed further and further 

 back in the eye the closer the object itself is placed to the 

 eye, and as in the far-sighted individual this shifting of the 

 image, backwards exaggerates the difficulty, such persons 

 have a clearer vision when the object is removed some dis- 

 tance, whence, of course, the name "far-sightedness." 



Fig. 173. THE HYPERMETROPIC EYE, AND ITS CORRECTION. 



It is well to point out, though, that in many cases far- 

 sighted children will develop a habit of holding the book 

 too close to the eye and so leave the impression on the ob- 

 server that they are near-sighted. Such a far-sighted child 

 in looking at the page before him does not see it distinctly, 

 and prompted by the desire to look more closely and mis- 

 led by his usually correct experience that approach to an 

 object increases the distinctness of its vision, he moves the 



