tion of a scene in which the objects, as the
human figures and horses, have a distinctness that belongs to near
objects, but an apparent magnitude that belongs to distant objects. So
again, it is found that the degree of luminosity or brightness of a
pictorial representation differs in general enormously from that of the
actual objects. Thus, according to the calculations of Helmholtz,[44] a
picture representing a Bedouin's white raiment in blinding sunshine,
will, when seen in a fairly lit gallery, have a degree of luminosity
reaching only to about one-thirtieth of that of the actual object. On
the other hand, a painting representing marble ruins illuminated by
moonlight, will, under the same conditions of illumination, have a
luminosity amounting to as much as from ten to twenty thousand times
that of the object. Yet the spectator does not notice these stupendous
discrepancies. The representation, in spite of its vast difference, at
once carries the mind on to the actuality, and the spectator may even
appear to himself, in moments of complete absorption, to be looking at
the actual scene.

The truly startling part of these illusions is, that the direct result
of sensory stimulation appears to be actually displaced by a mental
image. Thus, in the case of Meyer's experiment, of looking at the
distant viaduct, and of recognizing an artistic representation,
imagination seems in a measure to take the place of sensation, or to
blind the mind to what is actually before it.

The mystery of the process, however, greatly disappears when it is
remembered that what we call a conscious "sensation" is really
compounded of a result of sensory stimulation and a result of central
reaction, of a purely passive impression and the mental activity
involved in attending to this and classing it.[45] This being so, a
sensation may be modified by anything exceptional in the mode of central
reaction of the moment. Now, in all the cases just considered, we have
one common feature, a powerful suggestion of the presence of a
particular object or local arrangement. This suggestion, taking the form
of a vivid mental image, dominates and overpowers the passive
impression. Thus, in Meyer's experiment, the mind is possessed by the
supposition that we are looking at the grey spot through a greenish
medium. So in the case of the distant viaduct, we are under the mastery
of the idea that what we see in the distance is a red brick structure.
Once more,