VISION. 



(i) Hold any small object directly in front of the subject and have 

 him fix his eye upon it constantly. Move the object quickly 

 toward the subject's left and notice the immediate fixation of the 

 object in its new position. What muscles are brought into action, 

 in this movement? (2) Move the object quickly to the right, 

 upward, downward, and diagonally, noticing the immediate fixa- 

 tion in all the fields. What muscles are brought into action in 

 each position, and are all movements equally rapid ? (3) Bring 

 the object exactly in front about one metre distance and note the 

 range of lateral movement without causing any appreciable change 

 in the visual axis. (4) Bring the object to the central position and 

 move it very slowly outward in various directions and observe 

 whether the changes of d, 



direction of the visual 

 axes are equally slow and 

 regular. 



(b) BINOCULAR FIXA- 

 TION. Convergence. It 

 was probably noticed dur- 

 ing the above exercises 

 that, though one eye was 

 screened, it shared in all 

 movements with its fel- 

 low. With both eyes 

 open, let the subject fix 

 a small object, held about 

 one metre distant. Let 

 the observer move the 

 object slowly in all 

 fields, downward, upward, laterally, and around, observing the 

 perfect continuous fixation with both eyes. 



What muscles or pairs of muscles are involved in the movements 

 in the different directions ? If any variations are noticed in the 

 subjects examined, describe them. 



STEREOSCOPIC VISION. Binocular Single Vision. By this is 



FlG. 44.- a, 6, Two objects, the images of which 

 (a,, b l and a 3 , *,) fall on corresponding parts of 

 both retinae, K and /?,. 



