THE FIELD OF VISION 



657 



view the details of the fundus above described, but the image will be much smaller and in- 

 verted. The lens should be kept at a fixed distance of two or three inches, the mirror 

 being alone moved until the disc becomes visible: should the image of the mirror obscure 

 the disc, the lens may be slightly tilted. 



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 



105 



60* 



180 



10, 



S10 



225 



21*0 



255 



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

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



of vision stimulate regions of the opposite side o"f 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 

 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 seefi 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 

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