in the instance of looking at the picture, the spectator's
imagination is enchained by the vivid representation of the object for
which the picture stands, as the marble ruins in the moonlight or the
Bedouin in the desert.

It may be well to add that this mental uncertainty as to the exact
nature of a present impression is necessitated by the very conditions of
accurate perception. If, as I have said, all recognition takes place by
overlooking points of diversity, the mind must, in course of time,
acquire a habit of not attending to the exact quality of
sense-impressions in all cases where the interpretation seems plain and
obvious. Or, to use Helmholtz's words, our sensations are, in a general
way, of interest to us only as signs of things, and if we are sure of
the thing, we readily overlook the precise nature of the impression. In
short, we get into the way of attending only to what is essential,
constant, and characteristic in objects, and disregarding what is
variable and accidental.[46] Thus, we attend, in the first place, to the
form of objects, the most constant and characteristic element of all,
being comparatively inattentive to colour, which varies with distance,
atmospheric changes, and mode of illumination. So we attend to the
relative magnitude of objects rather than to the absolute, and to the
relative intensities of light and shade rather than to the absolute; for
in so doing we are noting what is constant for all distances and modes
of illumination, and overlooking what is variable. And the success of
pictorial art depends on the observance of this law of perception.

These remarks at once point out the limits of these illusions. In normal
circumstances, an act of imagination, however vivid, cannot create the
semblance of a sensation which is altogether absent; it can only
slightly modify the actual impression by interfering with that process
of comparison and classification which enters into all definite
determination of sensational quality.

Another great fact that has come to light in the investigation of these
illusions is that oft-recurring and familiar types of experience leave
permanent dispositions in the mind. As I said when describing the
process of perception, what has been frequently perceived is perceived
more and more readily. It follows from this that the mind will be
habitually disposed to form the corresponding mental images, and to
interpret impressions by help of these. The range of