BINOCULAR VISION. 117 



these same secondary axes, which all cross at the same point, 

 it is evident that an exact or erect sensation, as well as the 

 object which produces it, should necessarily correspond to an 

 inverted or reversed image. But it is neither habit, educa- 

 tion, nor information derived from the sense of touch, that 

 enables us, as it is said, to see objects erect by means of re- 

 versed images. The retina sees or localizes objects where 

 they are ; that is what we call ' erect.' If the picture be re- 

 versed, it is a mere matter of geometry." : 



In discussing the same question, Helmholtz says that " our 

 natural consciousness is completely ignorant even of the ex- 

 istence of the retina and of the formation of images : how 

 should it know any thing of the position of images formed 



upon it?" 1 



Binocular Vision. 



"We have thus far considered the mechanism of the eye 

 and its action as an optical instrument, in simple, or monocu- 

 lar vision. It is evident, however, that we habitually use 

 both eyes, and that their axes are practically parallel in look- 

 ing at distant objects and are converged when objects are 

 approached to the nearest point at which we have distinct 

 vision. In fact, an image is formed simultaneously upon the 

 retina of each eye, but is nevertheless appreciated as a unit. 

 If the axis of one eye be slightly deviated by pressure upon 

 the globe, so that the images are not formed upon correspond- 

 ing points upon the retina of each eye, our vision is more 

 or less indistinct, and is double. In strabismus, when this 

 condition is recent, temporary, or periodical, as in recent 

 cases of paralysis of the external rectus muscle, when both 

 eyes are normal, there is double vision. When the strabismus 

 is permanent and has existed for a long time, double vision 

 may not be observed, unless the subject direct the attention 

 strongly to this point. As it is usual, in such cases, for one 



1 GIRAUD-TEULON, La vision Unoculaire. Revue des cours scientifiques, Paris, 

 1867-1868, tome v., p. 223. 



9 HELMHOLTZ, Optique physiologique, Paris, 1867, p. 771. 



