HO INVERSE VISION. [MEMOIR VI. 



But the inverse of this is possible. Impressions ex- 

 isting in the brain may take, as it were, an outward 

 direction, and be projected or localized among external 

 forms ; or if the eyes be closed, as in sleep, or the ob- 

 server be in darkness, they will fill up the empty space 

 before him with scenery of their own. 



Inverse vision depends primarily on the condition 

 that former impressions, enclosed in the optic thalami, or 

 registering ganglia at the base of the brain, assume such 

 a degree of relative intensity that they can arrest the 

 attention of the mind. The moment that an equality 

 is established between the intensity of these vestiges 

 and sensations contemporaneously received from the 

 outer world, or that the latter are wholly extinguished, 

 as in sleep, inverse sight occurs, presenting, as the occa- 

 sions may vary, apparitions, visions, dreams. 



From the moral effect that arises, we are very liable 

 to connect these with the supernatural. In truth, how- 

 ever, they are the natural results of the action of the 

 nervous mechanism, which of necessity produces them 

 whenever it is placed, either by normal or morbid or 

 artificial causes, in the proper condition. It confounds 

 the subjective and the objective together. It can act 

 either directly, as in ordinary vision, or inversely, as in 

 cerebral sight, and in this respect resembles those instru- 

 ments which equally yield a musical note whether the 

 air is blown through them or drawn in. 



The hours of sleep continually present us, in a state 

 of perfect health, illusions that address themselves to 

 the eye rather than to any other organ of sense, and 

 these commonly combine into moving and acting scenes, 

 a dream being truly a drama of the night. In certain 

 states of health appearances of a like nature intrude 

 themselves before us even in the open day, but these, 

 being corrected by the realities by which they are sur- 



