460 THE POPULAR SCIENCE MONTHLY. 



Although less directly, the optic nerve, as well as the retina, is ac- 

 cessible to our observation. Hence we ascribe certain scintillations, 

 visible to us in a rapid motion of the eye, to a twist of the nerve ; sur- 

 gical operations, dating from a period when narcotics were not em- 

 ployed, have likewise shown that contact with this nervous cord pro- 

 duces only sensations of light, not those of pain. 



Lastly, we can point out the seat of the root, or the central termina- 

 tion of the optic nerve, by anatomically tracing the fibrils of the visual 

 nerve into this tract ; and partly, too, by an analysis of the phenomena 

 observable in healthy and diseased states. When the brain has been 

 excited by a narcotic, and the irritation is transmitted to the afore- 

 said tract, there arise sensations of light, which, combining with ideas 

 of luminous objects, simultaneously excited, are transformed into what 

 we call phantasms. The same thing takes place when the blood, as in 

 fever, heats the brain ; or when that part of the organ is excited from 

 other causes. And thus it is with our visual impressions during 



dreams, or even in a half-waking condition. 

 .* . 



But all this does not constitute any relation between sensation and 



the objects of the external world; that is, proper sensory action, 

 whether it be the gay visions that surround us in the intoxication 

 caused by opium; the comic phantasmagoria that hashish conjures up; 

 the compact shapes that belladonna brings so near us ; the airy forms 

 seen in our dreams, or the scintillations produced by pressure, they all 

 proceed from irritation of the special sensory power, and it is indiffer- 

 ent to the brain whether it receives its impressions from direct vision, 

 or only from internal influences. All those operations, therefore, which 

 proceed from direct irritation of the nervous part of the visual organ, 

 without the medium of the eye and of light, under the term subjective 

 vision, are opposed to those phenomena produced by the media of 

 eye and light, and known as objective vision. 



Great as are the influences of this subjective sight for the refresh- 

 ment of our brain during sleep, and powerfully as they affect the tem- 

 perament of the blind, they cannot connect us with the outer world. 

 The yellow light which floods our field of vision, on rubbing the retina, 

 is of no use to light up external objects. Hence, when, some years ago, 

 a man pretended to recognize a delinquent who had attacked him in 

 the night, by the sparks of fire produced by a blow on his eye, and 

 founded an accusation thereon, it was, of course, unjustifiable, although 

 the authorities consulted did not declare against the impossibility of 

 the fact. Baron Munchausen went still further in the use to which he 

 put those visual sparks ; for, when attacked by bears in the night, he 

 not only struck out light enough by which to prosecute the chase, but 

 fire, too, for his guns with the same blow. 



We cannot entirely overlook the question whether sensations of 

 light can be produced with the assistance of any other mechanism in 

 the body but that of the visual nerve. As only the part where sensa 



