considerations help us to explain the coalescence of the retinal
impressions and its limits, the fusion of partial tones, and so on.[21]

It is plain that this fusion of sensations, whatever its exact
conditions may be, gives rise to error or wrong interpretation of the
sense-impression. Thus, to take the points of two legs of a pair of
compasses for one 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