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 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.