f such errors is to be found in that mass of obscure
sensation which is connected with the organic processes, as digestion,
respiration, etc., together with those varying tactual and motor
feelings, which result from what is called the subjective stimulation of
the tactual nerves, and from changes in the position and condition of
the muscles. Lying commonly in what is known as the sub-conscious region
of mind, undiscriminated, vague, and ill-defined, these sensations, when
they come to be specially attended to, readily get misapprehended, and
so lead to illusion, both in waking life and in sleep. I shall have
occasion to illustrate this later on.

With these sensations, the result of stimulations coming from remote
parts of the organism, may be classed the ocular impressions which we
receive in indirect vision. When the eye is not fixed on an object, the
impression, involving the activity of some-peripheral region of the
retina, is comparatively indistinct. This will be much more the case
when the object lies at a distance for which the eye is not at the time
accommodated. And in these circumstances, when we happen to turn our
attention to the impression, we easily misapprehend it, and so fall into
illusion. Thus, it has been remarked by Sir David Brewster, in his
_Letters on Natural Magic_ (letter vii.), that when looking through a
window at some object beyond, we easily suppose a fly on the window-pane
to be a larger object, as a bird, at a greater distance.[15]

While these cases of a confusion or a wrong classification of the
sensation are pretty well made out, there are other illusions or
quasi-illusions respecting which it is doubtful whether they should be
brought under this head. For example, it was found by Weber, that when
the legs of a pair of compasses are at a certain small distance apart
they will be felt as two by some parts of the tactual surface of the
body, but only as one by other parts. How are we to regard this
discrepancy? Must we say that in the latter case there are two
sensations, only that, being so similar, they are confused one with
another? There seems some reason for so doing, in the fact that, by a
repeated exercise of attention to the experiment, they may afterwards be
recognized as two.

We here come on the puzzling question, How much in the character of the
sensation must be regarded as the necessary result of the particular
mode of nervous stimulation at the moment, together with the