When we look at an object with both eyes we have a separate image thrown upon each retina, and therefore two sets of impulses are sent to the sensorium, one from the right and one from the left eye. Yet we are only conscious of the occurrence of one stimulation. The reason of this is, that experience has taught us that similar images thrown upon certain parts of the two retinae correspond to the same object, and in our minds we fuse the sensations caused by the two images so that they produce but one idea.
These points of the retina which are thus habitually stimulated by the same objects are called "corresponding points".
Besides being of great use in making up for such deficiencies as the blind spots (which are not corresponding points), binocular vision is useful for the following purposes: -
To judge of distance. When using one eye only, some knowledge of distance may be gathered by the force employed to accommodate, but a much more accurate judgment can be formed when both eyes are used and the muscular sense of the ocular muscles, employed in converging the eyeballs for near objects, gives further evidence of their distance.
In judging of size, in the same way, with one eye, we can only have an idea of the apparent size of an object, which will vary with its distance. With a knowledge of apparent size and distance such as is gained by binocular vision, we can come to a fairly accurate conclusion as to the size of an object.
To judge of the relative distances of objects so as to see depth in the picture before our eyes, binocular vision is necessary. If one eye alone is used we see a flat picture, without having an accurate idea of the relative distances of the different objects. With each eye, however, we get a slightly different view of each object, and thus we are helped to a conclusion as to their exact, distances and shapes, and arrive at fairly correct judgments as to their form, etc.