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 laws of
sensibility, and how much must be put down to the reaction of the mind
in the shape of attention and discrimination? For our present purpose we
may say that, whenever a deliberate effort of attention does not suffice