402 INVEESE VISION. 



But the inverse of this is possible. Impressions already existing 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, or the ob- 

 server is 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 impres- 

 sions, which are inclosed 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 equal- 

 ity 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 vision occurs, presenting itself, 

 as the conditions may vary, under different forms, apparitions, visions, 

 dreams. 



From the moral effect to which these give rise, we are very liable to 

 regard them as connected with the supernatural. In truth, however, 

 they are the natural result 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 can act either di- 

 rectly, as in ordinary vision, or inversely, as in cerebral sight, and in this 

 respect resembles those instruments which equally yield a musical note 

 whether the air is blown through them or drawn in. 



The hours of sleep constantly present us, in a state of perfect health, 

 Difference be- illusions which appear to address themselves to the eye rath- 

 aii^wakin^^f er ^ nan ^ an 7 other sense, and these commonly combine into 

 lusions. moving and acting sceneries, a dream being truly a drama 



of the night. In certain states, appearances of a like nature intrude 

 themselves before us even in the open day, but these, being corrected by 

 the realities with which they are surrounded, impress us very differently 

 to the phantoms of our sleep. The want of unison between such im- 

 ages and the things among which they have intruded themselves, the 

 anachronism of their advent, or other obvious incongruities, restrain the 

 mind from delivering itself up to that absolute belief in their reality 

 which so completely possesses us in our dreams. Yet, nevertheless, 

 such is the constitution of man, the bravest and the wisest encounter 

 these fictions of their own organization with awe. 



If we measure the importance of events occurring to us by their fre- 

 Frequencyof ( l uenc 7 the depth of the impression they make, the influ- 

 mentaihaiiuci- ence they exert on our own individual career, or have ex- 

 erted on the progress of the whole human race, there are 

 very few more deserving the discussions of physiology than visual hal- 

 lucinations. With respect to frequency, it may be reasonably said that, 

 if images arise in the mind by night as numerously as sensible forms 



