444 SPECIAL PHYSIOLOGY. 



adjustment, and of distant vision, are said to be from about five inches 

 to indefinite distances, according to the intensity of the light. The 

 ordinary focal distance for easy, clear vision, as in reading, is about 

 eight inches. But the proximity of the near point is greater in early 

 life, and afterwards progressively diminishes. Thus, at the 10th year, 

 it is only 2f of an inch in front of the cornea ; at each succeeding de- 

 cennial period, its distance is 3|, 4|, 6f , 12 and 24, till at the 70th 

 year, its distance from the front of the cornea is 144 inches (Fellen- 

 berg). The far point may be said to have no limits. Under the ac- 

 tion of atropine, the near point recedes, and gradually reaches the far 

 point. The Calabar bean lessens the distance of the far point, and 

 frequently also that of the near point. 



In certain persons, the natural range of adaptation of the eye to dis- 

 tance is defective, and exceedingly limited, so that they are unable to 

 see objects except at certain distances. Such persons are either short 

 or long-sighted. In long sight, objects are only seen distinctly when 

 at a distance from the eye ; near objects, if small, are either invisible, 

 or else only confused images of them are perceptible. In short sight, 

 on the other hand, objects at a moderate distance are invisible or in- 

 distinct, the power of distinct vision being limited to objects brought 

 very close to the eye ; at the same time, a short-sighted person sees 

 small and near objects very distinctly, better illuminated, and under 

 larger visual angles, and therefore larger and brighter, than other 

 persons. 



These abnormal conditions of vision arise from a certain funda- 

 mental excess or defect in the refractive power of the eye. In long 

 sight, for example, the cornea is flatter than usual, and the antero-pos- 

 terior diameter, or optic axis, of the eye is said to be lengthened ; the 

 rays of light do not undergo sufficient refraction, but, instead, con- 

 verge to a focus behind the retina; by the use of convex glasses, the 

 convergence of the rays is increased, and they are brought to a focus 

 upon the retina. In short sight, the convexity of the cornea is too 



freat, so that the rays proceeding from an object, instead of being 

 rought to a focus on the retina, intersect each other at a point in 

 front of that membrane; the antero posterior diameter of the eye is 

 also probably shortened'; this defect is corrected by wearing concave 

 .spectacles, which cause divergence of the rays, so that the overcon- 

 Tergent effect is counteracted, and the rays are brought to a focus 

 upon the retina. It is probable that not only the curvature of the 

 cornea, but that of the lens also, is peculiar, both in long and short 

 sight. In both conditions, not only are the images thrown on the 

 retina indistinct, but luminous circles of dissipation are formed upon 

 it. Short-sighted eyes often improve by age, the cornea being said 

 then to become flatter, owing to a diminution in the quantity of the 

 fluids of the eyes. Normal eyes, from the same cause, may become 

 long-sighted in old age. 



To determine with accuracy the focal distance of the eyes, various 

 instruments, named Qptometers, have been invented. A simple plan, 

 devised by Scheiner, is to make two holes with a needle in a card, the 

 -distance between which must be less than that between the two pupils. 



