426 ILLUSTRATIVE EXPERIMENTS 



and the middle point of the circumference with an X. This forms a rough 

 ' Perimeter." 



The subject of the experiment closes one eye and places the other at 

 the centre of the semi-circle, directing his gaze at the middle point of the 

 circumference. The observer slowly draws a small object, say a piece of 

 chalk, along the circumference from below. The subject states when it 

 becomes visible and the other marks this point. The process is repeated, 

 starting from above, and the point when the object comes into view is 

 again marked. The angle formed by joining these points to the centre of 

 the circle subtends the vertical field of vision. 



By turning the blackboard into the horizontal plane, or by using the top 

 of a table, the horizontal field of vision may be mapped out. 



Movements of the Eyeball. The movements of the eyeball may be 

 studied by means of a ball of wool. Make some mark on it to represent 

 the pupil, and thrust three knitting needles through the ball to represent 

 the axes round which the eyeball rotates (Fig. 51 , p. 223). The movements 

 can then be reproduced with a fair degree of accuracy. 



