PROPERTIES OF THE RETINA. 



321 



The extent and portion of the retina affected may be used to aid in 

 locating the seat of the lesion. For physiological and for clinical 

 purposes it is necessary to distinguish between the central (or direct) 

 and the peripheral (or indirect) fields of vision. The former term 

 is meant to refer to that portion of the field which falls upon the 

 fovea centralis; in other words, it is the projection, in any fixed 



Fig. 

 n obj 



136. Perimeter. The semicircular bar may be placed in any meridian. A 

 gven oject is then moved along the bar from without in until it is just perceived. The 

 angular distance at which this occurs is marked off on the corresponding meridian on the 

 chart seen at the left of the figure. The eye examined gazes over the top of the vertical 

 rod at the right at a fixed point in the middle of the semicircular bar. 



position of the eye, of the fovea into the external world. The 

 peripheral field refers to the rest of the visual field involving the 

 retina outside the fovea. As a matter of fact, all of our distinct and 

 most useful vision in the daytime at least is effected through the 

 fovea. When the eye is kept fixed the small portion of the external 

 world that falls upon the fovea is seen distinctly. All the rest is 

 seen more or less indistinctly in proportion to the distance of its 



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