THE SENSE OF VISION 



639 



perimeter (see Fig. 367) consists of a quadrant upon which a white 

 spot can be moved, and this quadrant can be revolved about a line 

 continuous with the optic axis. At K is the chin-rest, double, so as 

 to enable either eye to be adjusted against 0. The subject, having 

 taken his position, covers one eye, and fixes the eye that is to be 

 examined on the mark at /. The quadrant is then placed, say, in 

 the vertical meridian, and at the back of the wheel which revolves 

 with the quadrant is inserted in the frame a special chart adapted for 

 recording perimetric observations. Starting at the extreme distance, 

 the mark Ob is gradually moved along the quadrant, and at a certain 

 angle the white spot will be just visible. The angle indicates the limit 

 of vision in this meridian, and can be recorded on the chart. Similar 

 observations are Made in other meridia. In this manner the limits 



10 10 



170 IgQ "0 



FIQ. 368. PERIMETRIC CHART OF THE EIGHT EYE. 



The thick line surround? the area within which white is visible, the point C being 

 fixed by the eye -(right); the fine dotted line surrounds the area within which 

 green is visible. The area of red would be somewhat larger, that of blue larger 

 still, though not so large as that of white. B= the situation of the blind spot. 



of vision in the different meridia of the field of vision can be recorded. 

 It is essential that the subject keep his eye fixed on / the whole time 

 the spot is being moved (Fig. 368). 



The area bounded by a line drawn through the limiting points in 

 the different meridia is properly the area of the field of vision. If the 

 meridia be inverted, the figure traced corresponds to the sensitive 

 portion of the retina. Perimeters are generally so constructed that 

 the limiting marks in the different meridia are inverted on the chart, 

 so that the latter becomes a chart of the extent of the sensitiveness 

 of the retina. This is indicated in the figure above. 



The Ophthalmoscope. The ophthalmoscope (Fig. 369) is an ap- 

 paratus to enable an observer to direct his vision along the axis of 

 the pencil of light illuminating the subject's eye, thereby enabling 



