SENSE OF SIGHT. 637 



(Fig. 204.) This produces an indistinct image for all remote objects. 

 Within, however, a certain distance from the eye, the rays enter the 

 pupil under such a degree of divergence, that their focus behind the lens 

 falls at the situation of the retina, and the object is distinctly seen. 

 Such an eye is said to be myopic, or, in ordinary language, "near 

 sighted," because its range of distinct vision is confined to objects 

 situated comparatively near the eye. The flexibility of the lens, and its 

 capacity for increasing its convexity, may be, in the myopic eye, fully up 

 to the normal standard, and consequently its power of accommodation 

 may be as great in reality, though not in distance, as that of the normal 

 eye. In the ernmetropic. condition, a certain degree of variation in the 

 curvature of the lens produces the necessary change of accommodation 

 for any distance between 15 centimetres and infintty. In the myopic 

 eye the same amount of accommodating power may be present, though 

 perfectly distinct vision be confined between the distances of 8 and 20 

 centimetres. The myopic eye consequently has distinct vision at shorter 

 distances than a natural one, but gives an imperfect image for remote 

 objects. 



The remedy adopted for the myopic eye is to employ a concave eye- 

 glass, which increases the divergence of the incident rays. This enables 

 the eye to bring parallel or nearly parallel rays to a focus situated 

 farther back than it would otherwise fall, and at the actual position of 

 the retina ; thus giving distinct vision for remote objects. As the 

 accommodative power is normal in amount, this contrivance restores 

 completely the perfection of sight, in a myopic eye which is otherwise 

 well-formed ; and the patient can then accommodate accurately for all 

 distances within the natural limits of distinct vision. 



Apparent Position of Objects, and Binocular Vision. The apparent 

 position of an object is determined by the direction in which the lumi- 

 nous rays pass from it to the interior of the eye. The perception of 

 the light itself necessarily marks the direction from which it has arrived, 

 and therefore the apparent position of its source. It is difficult to under- 

 stand fully the precise physiological conditions which cause this appreci- 

 ation of the path followed by a luminous beam ; although there seems 

 reason for the belief that it is in some way connected with the posi- 

 tion of the rods and cones which stand perpendicularly to the curved 

 surface of the retina, and thus receive the impression of a ray, if at all, 

 in the direction of their longitudinal axes. But whatever may be the 

 optical or physiological mechanism of the process, its plain result is that 

 a ray coming from below attracts attention to the inferior part of the 

 field of vision ; and one coining from above is referred to its point of 

 origin in the upper part of the same field. Thus if two luminous points 

 appear simultaneously in the field of vision, they present themselves in a 

 certain position with regard to each other, above or below, to the right 

 or the left, according to the direction in which their light has reached 

 the eye. 



