It is generally supposed that there are what are called "corresponding points" on the retinas of the two eyes. The muscles of the eyes adjust the eyes in such a manner that, when one eye is looking at anything, the second image of the same thing received by the other eye falls on the corresponding point. The impression imparted to the brain is that of a single object seen from two slightly different points of view, which gives a solid object the appearance of solidity.
If the adjusting muscles should be put out of order by illness or drunkenness, things are seen double. One can at any time produce double vision by looking at, say, a lamp flame and forcing one eye slightly out of position with a finger-tip.