744 



THE SENSES 



The Field of Vision. The field of vision of an eye is that part of the 

 external world which can be seen by it when the eye is fixed. Under such 

 circumstances objects near the axis of vision stimulate points in the retina 

 near the fovea or on it, while objects at an angle of 60 to 90 from the axis 

 of vision stimulate regions of the opposite side of the retinal cup, i.e., the 

 retinal field is inverted. 



The perimeter is an instrument for measuring the field of vision in terms 

 of angular measure. When a field is charted by means of the perimeter it 

 is revealed that objects can be seen further out in the field in some directions 



330 ' 



270 a 



FIG. 471. Perimeter Chart, Showing Extent of Field of Vision for White Light and to the 

 Colors Red, Green, Yellow, and Blue. (Krapart.) 



than in others. For example, objects in the temporal field can be seen at 

 an angle of 90 to 100, while on the nasal side they are seen only 60 to 70 

 If the head is turned to the right or the left while keeping the eye fixed, it is 

 found that objects are seen at a greater angle. This shows that the limita- 

 tions are due to the facial boundaries of the eye preventing the light from 

 entering the eye and not from lack of sensitiveness of the retina. In fact, 

 the retina is sensitive to light out to the ora serrata. 



Localization in the Retina. Careful exploration of the retina with 

 the perimeter gives a measure not only of the extent of the visual field, but of 

 its acuteness and localization in different areas toward the periphery. Con- 

 sidering the minimal distance apart which two luminous points must be to 

 be distinguished as two, it is found that when the image falls on the fovea 



