ELEMENTARY EXPERIMENTAL PHYSIOLOGY 23 



Now, starting from above, the experimenter again draws the 

 chalk along the circumference until it becomes visible, and marks 

 the point where it comes into view. The angle thus formed with 

 the centre of the circle and these points subtends the vertical field 

 of vision. Measure this angle and record it. 



FIG. 17. A perimeter. 



Now turn the blackboard into the horizontal plane, or use the 

 top of the table, and map out the horizontal field of vision. 



Why are the angles subtended not equal in the four segments of 

 the field of vision ? Will the position of the eye in relationship to 

 the eyebrow and nose explain this ? 



