BINOCULAR VISION. The two eyes ordinarily act together as a single instrument, and sight thus obtained is known as binocular vision. Each eye has its own field of vision: a considerable part of the inner portion of each field overlaps the other, thus forming a binocular field of vision. Although we have two eyes, we are conscious of only one visual impression; it should be remem bered that the fibres from the right half of each eye go to the visual area of the right hemisphere, from the left half to that of the left hemisphere.
There being two retinal images for each object looked at, one for each eye, these two images must fall on corresponding parts of the retina in order that single binocular vision exist; when this is the ease, the two images are projected into space to the same spot and eonsequently coincide or are superimposed. To obtain single binocular vision, the optical axes or visual lines of the two eyes oust meet at the object iocu directly; this is accomplished by the action of certain nmseles of the eye. At the same time certain objects seen in indirect vision will also appear single—namely. those which lie within a par ticular series of points known as the horopt or.
The external muscles of the eyeball serve to turn or rotate this organ in any direction; one turns the eyeball inward and outward upon the a second turns the eyeball upward and downward upon the horizontal axis; and a third set rotates the eyeball upon the antero posterior axis. By the associated action of two
or more of the muscles of both eyes the gaze can be turned toward any object; this is done rapidly, unconsciously, and automatically. Thus, in read ing, the internal muscle of the right eye acts with the internal muscle of the left eye and sufficient convergence results; when we turn the eyes to the left, the external muscle of the left eye acts with the internal muscle of the right eye, etc. As a consequence of paresis of any of the eye muscles, double vision (diplopia) results when ever the object looked at is situated in a por tion of the field corresponding to the sphere of action of the weakened muscle. If one eyeball be pressed upon with the finger so as to displace its visual line there will be double vision. In both of these instances the images of an object fall upon parts of the two retime which do not correspond. In cases of cross-eyes (squint), the eye which is habitually turned in or out, and which is responsible for the second image, grad ually learns to exclude the latter, so that the annoying double images are not perceived.