SENSE OF SIGHT. 431 



fall upon the retina (Fig. 116, 1) : such eyes are called hyper- 

 metropic, and this want of convergence (or shortness of 

 the ocular cone) constitutes hypermetropia. On the other 

 hand, the ocular medium of some persons has so much con- 

 verging power that the ocular cone is always too short, and, 

 in order to see objects distinctly, they are obliged to place 

 them very near the eye ; so that, as the cone lengthens, its 

 summit may fall upon the sensitive membrane: this is the 

 case with short-sighted persons (Fig. 116, 2), and this short- 

 ness of the ocular cone constitutes what is called myopia? 



We see that hypermetropia and myopia are two opposite 

 conditions ; in the former, the eye, when in the state of repose, 

 and making no effort to adapt itself to objects, sees only those 

 which are at the greatest possible distance from it ; while in 

 the latter, under similar circumstances, it sees those only 

 which are close at hand. Another condition of the eye which 

 is frequently mistaken for hypermetropia is that called pres- 

 byopia / this derangement of the functions of the ocular 

 medium consists in a diminution of the power of adaptation 

 which can be no longer exercised on objects near the eye ; 

 this is generally found to be the case as persons advance in 

 age. In hypermetropia the ocular cone is always too long / 

 in myopia, it is always too short ; in both cases, however, the 

 power exists of modifying the cone, by adaptation, especially 

 of shortening .it, as we shall presently see. A far-sighted 

 person, on the contrary, or one suffering from presbyopia, has 

 scarcely any power of modifying the cone in order to see 

 objects near the eye ; thus we see that if a normal eye may 

 become presbyopic, a hypermetropic or a short-sighted eye 

 may do so likewise, and that short-sightedness and presbyobia 

 may exist together. 



Means of remedying these defects of sight have, however, 

 been found in optics : for the purpose of modifying an ocular 

 cone which is too long or short, either a concave or a convex 

 glass is placed before the eye. If we have the slightest 

 knowledge of physics, we shall be able to understand that a 

 concave or diverging glass, lengthens the ocular cone by 

 diminishing the converging power of the eye : short-sighted 

 persons, for this reason, use concave glasses. On the other 

 hand, a convex or converging glass shortens the ocular cone 



* See, in the " Nouv. Diet, de Me'd. et de Chirur. Prat.," 

 articles by Liebreich and Javal: " Accommodation, Emmetropie, 

 Diplopie, Asthenopie," etc. 



