CHAP, ii.] SIGHT. 545 



Elevation. Rectus superior and obliquus inferior 





g. f Elevation. Rectus superior and obliquus inferior 



-g Depression. Rectus inferior and obliquus superior. 



At0 



extemu, 



f Elevation with Rectus superior and internus with obliquus 



^ I adduction. inferior. 



o> -g | Depression Rectus inferior and internus with obliquus 



.T | I with adduction. superior. 



Elevation with Rectus superior and externus with obliquus 



abduction. inferior. 



Depression Rectus inferior and externus with obliquus 



with abduction. superior. 



Coordination of Visual Movements. Thus even in the move- 

 ments of a single eye, a considerable amount of coordination takes 

 place. When the eye is moved in any other than the vertical and 

 horizontal meridians, impulses must descend to at least three 

 muscles, and in such relative energy to each of the three as to pro- 

 duce the required inclination of the visual axis. But the co- 

 ordination observed in binocular vision is more striking still. If 

 the movements of any person's eyes be watched it will be seen that 

 the two eyes move alike. If the right eye moves to the right, so 

 does also the left; and, if the object looked at be a distant one, 

 exactly to the same extent; if the right eye looks up, the left eye 

 looks up also, and so in every other direction. Very few persons 

 are able by a direct effort of the will to move one eye inde- 

 pendently of the other; though some, and among them one 

 distinguished both as a physiologist and an oculist, have acquired 

 this power. In fact, the movements of the two eyes are so arranged 

 that in the various movements the images of any object should fall 

 on the corresponding points of the two retinas, and that thus single 

 vision should result. We cannot by any direct effort of our will 

 place our eyes in such a position that the rays of light proceeding 

 from any object shall be brought to a focus on parts of the two 

 retinas which do not correspond, and thus give rise to two distinct 

 visual images. We can bring the visual axes of the two eyes from 

 a condition of parallelism to one of great convergence, but we 

 cannot, without special assistance, bring them from a condition 

 of parallelism to one of divergence. The stereoscope will enable us 

 to create a divergence. If in a stereoscopic picture the distance 

 between the pictures be increased very gradually so as carefully to 

 maintain the impression of a single object, the visual axes may be 

 brought to diverge. Similarly if a distant object be looked at 

 with a prism before one eye, and the image of the object be kept 

 carefully single, while the prism is turned very slowly up or down, 

 then on suddenly removing the prism a double image is for a 

 F. 35 



