of our Sensations* 261 



accordance with which certain cell-groups are specially set 

 apart for luminous transformations, other groups for sonorous 

 transformations, &c. 



If we assume for a moment that some disturbance or other 

 (for example, a more considerable afflux of blood into the cor- 

 responding region of the brain) excites one of these cell-groups, 

 there will result from it a conscious sensation, or the revival 

 of an' anterior sensation. And it is thus, under certain cir- 

 cumstances, by defect of judgment, or by defect of control, 

 that we may be led to interpret as real luminous or sonorous 

 sensations internal agitations, independent of any light or 

 sound, of the optic or auditory perceptive centres. 



In an analogous manner, in certain conditions of alteration 

 of the nervous conductors, real objective impressions may be 

 modified during their transmission to the brain. Hence come 

 false perceptions of images, or of sounds, arising from a patho- 

 logical condition of the optic or auditory nerves. 



To sum up: — A sensation excited by an external object may 

 be altered during its transmission to the brain ; and a sensa- 

 tion not corresponding to any external object may originate 

 in consequence of an internal disturbance of the sensitive 

 nerves or of the perceptive centres themselves. In the former 

 case the result has received the name of illusion; in the second 

 we have to do with hallucination. Hallucination, says Prof. 

 Ball, is a perception without an object. 



The individual who thinks he sees a brigand armed to the 

 teeth when he has before him only the trunk of a tree, or who 

 sees a body in motion when the body which meets his eyes is 

 really in repose, is under the influence of an illusion. Those 

 who see phantoms pass in the dark, who hear celestial voices 

 in the most absolute silence, or who believe that they touch 

 angels or demons when no object strikes their tactile extre- 

 mities, are under hallucination. 



These, of course, are extreme examples; but within more 

 restricted limits we are all exposed to similar errors which 

 become the source of false interpretations in the observation 

 of natural phenomena. We all know trustworthy persons 

 who assert that they have experienced some sensation or other, 

 upon which they furnish all the details one can ask of them, 

 and who in reality have been the subjects of a hallucination. 

 Facts of this kind are exceedingly frequent among the " good 

 subjects" of the mesmerists — that is to say, among persons 

 suffering from a hysterical, hypnotic, somnambulic, or other 

 neurosis, and who, by the mere fact of their peculiar nervous 

 condition, are much more exposed to the production of illusions 

 or hallucinations. 

 Phil. Mag. S. 5. Vol. 15. No. 94. April 1883. U 



