772 



PERIMETRIC CHARTS. 



I Scotoma is the term applied to dimness or blindness in certain parts of the field of vision, 



which may be central, marginal, or in patches.] 



The capacity for distinguishing colours di- 

 minishes more rapidly at the periphery of the 

 retina, than that for distinguishing differences in 

 the brightness or intensity of light In fact, the 

 periphery of the retina is slightly red blind. 

 The diminution is greater in the vertical meridian 

 of the eye than in the horizontal, and it dimin- 

 ishes with the distance from the fixation point 

 (Aubert and Forster). These observers also 

 state that, during accommodation for a distant 

 object, the diminution of the capacity to distin- 

 guish brightness and colour towards the peri- 

 phery of the lens, occurs more rapidly than with 

 near vision. The excitability of the retina for 

 colours and brightness is greater at a point 

 equally distant from the fovea centralis on the 

 temporal than on the nasal side of the eye 

 (Schon). 



Perimetric Chart. If the arc of the perimeter 

 (fig. 559) be divided into 90 degrees, beginning at 

 the fixation point (central point), and proceeding 

 to L and M (fig. 560) ; and if a series of con- 

 centric circles be inscribed on this, with the 

 point of fixation as their centre, we can con- 

 struct a topographical chart of the visual capa- 

 city of the normal or healthy eye from the data 



Fig. 559. -Priestley Smith's perimeter. ol *? ine * ty the examin ? tion of the retina. \ 



t lg. 560 is an example ; the thick lines indi- 

 cate a diseased eye the corresponding thin lines a healthy eye. The continuous line indicates 



VI 



Fig. 560. 



Perimetric chart of a healthy and a diseased eye. 



the limits for the perception of white ; the interrupted line that for blue ; the punctuated and 



