Page:Popular Science Monthly Volume 8.djvu/592

574 I might pile up instances of visual illusion, for example, in which the subject would be ready to affirm without the slightest hesitation that he sees something which greatly differs from the object that actually forms the picture on his retina; his erroneous interpretation of that picture being the result of a prepossession derived from antecedent experience. I could show, too, that the same picture may be interpreted in two different modes: a skeleton-diagram, for example, suggesting two dissimilar solid forms, according as the eyes are fixed on one or another of its angles; and a photograph of a coin or fossil being seen as a cameo or as an intaglio, according as the position of the light affects the interpretation of its lights and shadows. Again, I have before me two pieces of card, A and B, of similar form: when A is placed above B, the latter is unhesitatingly pronounced the larger; if their relative positions be reversed, A is pronounced, with equal conviction, to be the larger; yet, when one is laid upon the other, they are found to be precisely equal in size.

So, again, in those more complex combinations of natural objects which the pictorial artist aims to represent, the different modes in which the very same scene shall be treated, by two individuals working at the same time and from the same point of view, show how differently they interpret the same visual picture, according to their original constitution and subsequent training. As Carlyle says, "The eye sees what it brings the power to see."

But mental prepossessions do much more than this; they produce sensations having no objective reality. I do not here allude to those "subjective sensations" of physiologists which depend upon physical affections of nerves in their course, the circulation of poisoned blood in the brain (as in the delirium of fever), and the like; but I refer to the sensations produced by mental expectancy, a most fertile source of self-deception. The medical practitioner is familiar with these in the case of "hysterical" subjects; whose pains are as real experiences to them as if they originated in the parts to which they are referred. And I have no reason to doubt that the "sensitives" of Reichenbach really saw the flames they described as issuing from magnets in the dark—as a very honest and highly-educated gentleman assured me that he did, not only when the magnet was there, but when he believed it to be still there (in the dark), after it had been actually withdrawn. So there are "sensitives" in whom the drawing of a magnet along the arm will produce a sensible aura or a pricking pain; and this will be equally excited by the belief that the magnet is being so used, when nothing whatever is done.

Now, the phenomena of which these are simple examples appear to me to have this physiological signification—that changes in the cerebrum which answer to the higher mental states act downward upon the sensorium at its base, in the same manner as changes in the organs of sense act upward upon it; the very same state of the