634 POPULAR SCIENCE MONTHLY, 



derangement in the meclianism of perception, and has nothing to 

 do with what the patient is thinking of. In the second, one may- 

 have recourse to the theory of subconscious states and assume 

 that every hallucination had a true mental existence, if not con- 

 scious, then subconscious, before being brought to the upper con- 

 sciousness. The first is the old orthodox theory of illusion, and, 

 to make it intelligible, I must say a little of the normal processes 

 of perception. 



Our sense-impressions are primarily initiated, as I have said, 

 by currents from the periphery of the body, but their final com- 

 plexion is only in part determined by the peripheral currents; 

 it owes much to the condition in which the cortex happens to be 

 and to the manner in which it is constituted. We may roughly 

 compare it to a penny-in-the-slot machine. Without the penny 

 there would be no response, but the precise character of the 

 response is determined by the constitution of the machine. The 

 case of the brain is similar, but infinitely more complex. Most 

 of our sense organs send in very complex currents : from the eye, 

 for instance, we get currents which, taken alone, would cause sen- 

 sations of color, touch, and movement — the slightest change in 

 the number and relative adjustment of these currents, even 

 though it be so slight that we can not possibly be aware of it as 

 a change in the simpler sensations, will totally change the char- 

 acter of the sense-impression. These sensory currents are like 

 the keys and stops of an organ, and any one who knows just what 

 stops and keys to rnanipulate can get any response he pleases. 

 Thus the technical part of painting consists in so imitating the 

 ordinary sensory determinants of vision by means of colors on a 

 flat surface as to produce that cortical process which is usually 

 produced by a real thing. 



It is not often possible to trace the operation of these factors 

 in our sense-impressions. Each seems an indivisible mental 

 whole. But sometimes we can distinguish them. In the first and 

 third of the three types of hallucination which I have already 

 analyzed, the character of the hallucination is clearly determined 

 by the first or central factor, and its development also seems to be 

 due to some central factor or factors. In the second of the three 

 the character is determined as before by the central factor, while 

 its development to sensory intensity is due to its accidental coin- 

 cidence with the arrival of a sensory current, which is a periph- 

 eral factor. These are all termed hallucinations centrally ini- 

 tiated, or true hallucinations. In the types which I shall now 

 take up the character is always chiefly determined by peripheral 

 currents, and, presumably, they are also responsible for the sen- 

 sory intensity of the image. These hallucinations are termed 

 peripherally initiated, or simply illusions. 



