26 



PRACTICAL PHYSIOLOGY 



slit to the centre on one side for fitting to the axle of the motor. 3. A 

 small motor with resistance to vary speeds to carry the discs, or a hand- 

 driven disc. 4. Holmgren wools. 5. Edridge Green's card tests for colour 

 blindness. 





A 



LESSON V 

 . II. Why is there Normally Single Vision with Two Eyes ? 



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

 METHODS.!. 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 and draw 

 your conclusions. 



Study the anatomy 

 of the eye in the 

 orbit, the direction 

 of the axis of the eye 

 and that of the orbit, 

 and the action of vari- 

 ous muscles which 

 move the eyeball. 

 Note that they act 

 round three axes of 

 rotation. Now get 

 an orange and take 

 the pip to represent 

 the pupil, or use a 

 ball of wool, with 

 FIG. 20. To show (A) how on looking at a near SO me mark to repre- 

 object the image of a far object may be, +i. ^ n-r.il 

 doubled, and (B) how on looking at a distant ^ n 5 i 



object the image of a near object may be Inrust a needle 

 doubled. through each of the 



(From Noel Paton's Essentials.) three axes of rota- 



tion and study the 



action of each of the three pairs of muscles upon the direction of 

 the pip or pupil. 



2. Can Double Vision be produced when the Eyes move freely together ? 

 METHOD. Set up a stick vertically about 3 feet from the eyes, 



and another at about 10 feet. Look at the near one and see what 

 happens to the image of the far one. Close one eye and observe 

 what happens. Now look at the far one and notice the image of the 

 near, and again close one eye. Make a diagram of the experiment 

 and explain the result. (Fig. 20.) 



(Read Binocular Vision in Text Book.) 



B 



