692 



THE SENSES 



the patient's right eye. At the same time he rests his left little finger lightly' 

 upon the patient's right temple, and holding the lens between his thumb and 

 forefinger, two or three inches in front of the patient's eye, directs the light 

 through the lens into the eye. The red reflex, and subsequently the white one, 

 having been gained, the operator slowly moves his mirror, and with it his eye, 

 toward or away from the face of the patient, until the outline of one of the 

 retinal vessels becomes visible, when very slight movements on the part of the 

 operator will suffice to bring into view the details of the fundus above described, 

 but the image will be much smaller and inverted. 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 



120 



f,r, 



150 



255 



270" 



285" 



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 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 

 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. 



