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.