42 



and then for the two eyes together, taking care not to move 

 the head. Make a diagram of the result, and cojnpare the 

 optical angle in vision with one and with two eyes. 



2. Estimation of Contour. 



(a) Lay a prism edge on to you on the table, look at it 

 first with one then with two eyes, and consider how the idea 

 of relief is arrived at. 



(6) With the Stereoscope study how the projection of 

 slightly different pictures on the two retinae gives the idea of 

 relief. 



3. Estimation of Distance. 



Set up a stick vertically at one end of the laboratory, 

 and with one eye closed walk up to it quickly and, without 

 hesitating, try to touch it with the outstretched finger. 

 Repeat this experiment with both eyes open, and note any 

 difference of result. What conclusion do you draw from 

 this? 



II. Why is there normally Single Vision with Two 



Eyes? 



I. Is Single Vision possible if the Eyes do not move together? 



METHODS. 1. With the tip of the finger fix one eye in its 

 socket and move the head about, looking at external objects, 

 and notice whether they remain single. 



2. Looking straight forward, press with a finger upon 

 one eye to alter its direction, and note the effect upon vision. 

 CONCLUSION ? 



Study the anatomy of the eye in the socket and the action 

 of various muscles which move it. Note that they act round 

 three axes of rotation. Now get an orange and take the pip 

 to represent the pupil. Thrust a knitting needle through each 

 of the three axes of rotation and study the influence of each 



