forces acting on the two retinas are not perfectly similar.
Nevertheless, such a coalescence plainly answers to the fact that these
external agencies usually arise in one and the same object, and this
unity of the object is, of course, the all-important thing to be sure
of.

This habit may, however, beget palpable illusion in another way. In
certain exceptional cases the coalescence does not take place, as when I
look at a distant object and hold a pencil just before my eyes.[23] And
in this case the organized tendency to take one visual impression for
one object asserts its force, and I tend to fall into the illusion of
seeing two separate pencils. If I do not wholly lapse into the error, it
is because my experience has made me vaguely aware that double images
under these circumstances answer to one object, and that if there were
really two pencils present I should have four visual impressions.

Once more, it is a law of sensory stimulation that an impression
persists for an appreciable time after the cessation of the action of
the stimulus. This "after sensation" will clearly lead to illusion, in
so far as we tend to think of the stimulus as still at work. It forms,
indeed, as will be seen by-and-by, the simplest and lowest stage of
hallucination. Sometimes this becomes the first stage of a palpable
error. After listening to a child crying for some time the ear easily
deceives itself into supposing that the noise is continued when it has
actually ceased. Again, after taking a bandage from a finger, the
tingling and other sensations due to the pressure sometimes persist for
a good time, in which case they easily give rise to an illusion that the
finger is still bound.

It follows from this fact of the reverberation of the nervous structures