PROPERTIES OF THE RETINA. 337 



the fovea centralis. As a matter of fact, the configuration of the 

 face is such as to cut off a considerable part of this field, in any 

 fixed position of the eyes, and to give to the field as it actually 

 exists an irregular outline. The bridge of the nose, the projecting 

 eyebrows, and cheek bones serve to thus hmit the field; and, in 

 addition, the sensitivity of the peripheral portion of the retina 



Fip;. 143. — Perimeter. The semicircular bar may be placed in any meridian. A 

 given object 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 a>, 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. 



may not extend equally far toward the ora serrata in different 

 eyes or in clift'erent meridians of the same eye. To obtain the 

 exact outline and extent of the visual field in any given case it 

 is only necessary to keep the eye fixed and then to move a small 

 object in the different meridians and at the same distance from 

 the eye. The limits of vision may be obtained in this way along 

 each meridian and the results combined upon an appropriate 

 chart. An instrument, the perimeter, has been devised to facilitate 

 the process of charting the visual field. It has been given a nunibei* 

 of different forms, one of which is illustrated in Fig 143. The shape 



22 



