574 THE POPULAR SCIENCE MONTHLY. 



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 actu- 

 ally forms the picture on his retina ; his erroneous interpretation of 

 that picture being the result of a prepossession derived from antece- 

 dent 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 ^(pon 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 work- 

 ing at the same time and from the same point of view, show how dif- 

 ferently they interpret the same visual picture, according to their ori- 

 ginal 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; ihej 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 tlie 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 j)hysiological signification that changes in' the 

 cerebrum which answer to the higher mental states act doinncard 

 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 sen- 



