054 COORDINATION OF OCULAR MOVEMENTS. [BOOK in. 



divergence. If in a stereoscope the distance between the pic- 

 tures be increased very gradually so as carefully to maintain 

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

 to diverge , and the subject of the experiment may himself be 

 made aware of the divergence, by the sudden removal of the 

 instrument from his eyes ; his vision of external objects is for 

 a moment double, but for a moment only. This experiment 

 shews the reason of the limitation of which we are speaking. 

 So long as the visual axes are parallel or appropriately con- 

 vergent the images of external objects fall on corresponding 

 parts of the two retinas, and single vision results; when the 

 visual axes are carried beyond parallelism, the images on the 

 two retinas are not on corresponding parts and vision is double. 

 Thus, as regards convergence or divergence of the visual axes, 

 the movements of the two eyes are governed by the principle 

 that the will can of itself only carry out those movements which 

 are consistent with images of external objects falling on corre- 

 sponding parts of the two retinas. There is an exception to this 

 in the case of extreme convergence ; we can as in squinting 

 make the visual axes converge too much, and in consequence by 

 a simple effort of the will can obtain double vision ; but this is 

 probably in order to leave a margin which shall secure our being 

 able to use to the utmost our accommodation mechanism for 

 near objects ; otherwise the rule holds good. Not only so, but 

 as the above experiment also shews, when by artificial assist- 

 ance, which is in itself directed towards securing single vision 

 with the two eyes, we obtain divergence of the visual axes, 

 immediately that the assistance is done away with the axes 

 return, by an involuntary movement, to parallelism ; the double 

 vision occurring at the moment of removal of the instrument 

 rapidly gives way to normal single vision. Other illustrations 

 of the same principle may be met with. For instance, if a dis- 

 tant object be looked at with both eyes, but with a prism held 

 horizontally before one eye, and if the image of the object be 

 kept carefully single while the prism is turned very slowly 

 from the horizontal to the vertical position, then on suddenly 

 removing the prism a double image is for a moment seen ; this 

 shews that the eye before which the prism was placed had 

 moved in disaccordance with the other. The double image, 

 however, immediately after the removal of the prism, becomes 

 single on account of the eyes coming into accordance. 



When we examine all the various movements of the eyes 

 which we are capable of making by a direct effort of the will, 

 we find that they are all of such a kind that through them the 

 two images of an external object are brought upon correspond- 

 ing parts of the two retinas ; conversely the movements which 

 could be effected by the contractions of this or that ocular mus- 

 cle, but the effect of which would be to bring the two images 



