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.