THE SENSE ORGANS 



191 



way. The sheet of paper, however, must then be adjusted in front of 

 this eye and bear a cross mark near its right margin. 



If a perimeter is available, the subject should be comfortably seated 

 at a table with his chin resting upon the support. Either the right or 

 the left eye should then be focalized steadily upon the white dot in the 

 center of the semicircle. The metal arch bears a sliding path in which 

 is moved the object, generally a small white square, from without 

 inward, its movement being indicated in degrees upon a scale inscribed 

 upon the arch. The point at which the object becomes first visible 



FIG. 111. THE PERIMETER. 



is then indicated upon a small piece of paper upon which different 

 meridians have been drawn out to correspond to those of the metal arch. 



Study the general outline of the visual field of each eye. Why is 

 it not round, but oval, with its more pointed area toward the nasal side? 

 Do you note any irregularities in its outline which cannot be ascribed 

 to a faulty technic? Enumerate the causes which might be held re- 

 sponsible for such restrictions of the visual field. 



6. The Fields for Colored Objects. Proceed as described in para- 

 graph 5, but insert red, blue, and green squares in the holder upon the 



