ne point is clearly an illusion of perception. Here is
another and less familiar example. Very cold and smooth surfaces, as
those of metal, often appear to be wet. I never feel sure, after wiping
the blades of my skates, that they are perfectly dry, since they always
seem more or less damp to my hand. What is the reason of this? Helmholtz
explains the phenomenon by saying that the feeling we call by the name
of wetness is a compound sensation consisting of one of temperature and
one of touch proper. These sensations occurring together so frequently,
blend into one, and so we infer, according to the general instinctive
tendency already noticed, that there is one specific quality answering
to the feeling. And since the feeling is nearly always produced by
surfaces moistened by cold liquid, we refer it to this circumstance, and
speak of it as a feeling of wetness. Hence, when the particular
conjunction of sensations arises apart from this external circumstance,
we erroneously infer its presence.[22]

The most interesting case of illusion connected with the fusion of
simultaneous sensations, is that of single vision, or the deeply
organized habit of combining the sensations of what are called the
corresponding points of the two retinas. This coalescence of two
sensations is so far erroneous since it makes us overlook the existence
of two distinct external agencies acting on different parts of the
sensitive surface of the body. And this is the more striking in the case
of looking at solid objects, since here it is demonstrable that the
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 impressio