546 THE NERVOUS SYSTEM. 



shorter ranges than a normal one, but gives an imperfect image for 

 remote objects. 



The remedy employed for the myopic eye is a concave eye-glass, 

 which increases the divergence of the incident rays. This serves to 

 carry the focus of parallel or nearly parallel rays farther backward, so 

 that it falls upon the retina, producing distinct vision. As the accom- 

 modating power is normal in amount, this contrivance restores the per- 

 fection of sight, if the eye be otherwise well-formed ; and the patient 

 can then accommodate 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 coming from it enter the eye. The perception of light neces- 

 sarily marks the direction in which it has arrived, and therefore the 

 apparent position of its source. It is difficult to understand fully the 

 physiological cause for this appreciation of the path followed by a 

 luminous beam ; though it seems probable that it may be connected 

 with the position of the rods and cones, which are everywhere perpen- 

 dicular to the curved surface of the retina, and thus receive the impres- 

 sion of a ray, if at all, in the direction of their longitudinal axes. But 

 whatever may be the optical mechanism of the process, its result is 

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

 the field of vision ; and one coming from above is referred to the upper 

 part cf the same field. Thus if two luminous points appear simulta- 

 neously in the field of vision, they present themselves in a certain posi- 

 tion 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. 



It is evident accordingly that the lower half of the retina receives the 

 rays coming from above, and its upper half those coming from below ; 

 while tho right half of the visual field is perceived by the left half of 

 the retina, and vice versa. The image formed upon the retina is con- 

 sequently an inverted and reversed image of the object. But as it is 

 tbo direction of the visual ray at its impact on the retina which deter- 

 mines the apparent position of its source, objects will appear erect, 

 though their images on the retina are inverted ; and the eye perceives 

 every object in the field of vision above or below, to the right or left, 

 according to the position which it really occupies in regard to the centre 

 of the field and the line of direct vision. 



Point of Fixation, in Vision with Two Eyes. For either eye, distinct 

 perception is possible, as shown above (p. 538), only for objects in a 

 single range, known as the " line of direct vision." Since the eyes are 

 placed in their orbits at a lateral distance from each other of about six 

 centimetres, when they are both directed at the same object, within a 

 moderate distance, their lines of direct vision have a sensible conver- 

 gence, and meet at a certain point. At this intersection of the two lines 

 of direct vision, an object may be seen distinctly by both eyes. But 

 at every other point it must appear indistinct to one of them ; because 



