SEAT OF VISION. 



and consequently, all being seen in the same position, that position 

 is called the erect position. 



72. Physiologists are not agreed as to the manner in which the 

 perception of a visible object is obtained from the image formed in 

 the interior of the eye. It is certain, however, that this image is 

 the cause of vision, or that the means whereby it is produced are 

 also instrumental in producing the perception of sight. It may 

 also be considered as established that the perception of a visible 

 object is more or less distinct, according to the greater or less dis- 

 tinctness of the image. But it would be a great error to assume 

 that this image on the retina is itself seen, for that would involve 

 the supposition of a second eye, beyond the first, or within it, by 

 which such image on the retina would be viewed. Now, no means 

 of communicating between the image on the retina and the sen- 

 sorium exist except the usual conduits of all sensation, the nerves. 



It has been already explained that the optic nerve, after 

 entering the eye at a point near the nose, spreads itself over the 

 interior of the globe of the eye behind the vitreous humour, and 

 that this retina or network is perfectly transparent, the coloured 

 image being formed not properly upon it, but upon the black sur- 

 face of the choroid coat behind it. Now, it has been maintained, 

 that the functions of vision are performed by this nervous mem- 

 brane in a manner analogous to that by which the sense of touch 

 is affected by external objects. The membrane of the retina, it is 

 supposed, touching the coloured image, and being in the highest 

 degree sensitive to it, just as the hand is sensitive to an object 

 which it touches, receives from the coloured image an action 

 which, being continued to the brain, produces perception there in 

 accordance with the form and colour of the image upon the choroid. 

 According to *"his view of the functions of vision, the retina feels, 

 as it were, the image on the choroid, and transmits to the sen- 

 sorium the impression of its colour and figure in the same manner 

 as the hand of a blind person would transmit to the sensorium the 

 form of an object which it touches. 



73. If this hypothesis be admitted, it would follow that the 

 Tetina itself would be incapable of exciting the sense of sight by 

 the mere action of light and colours upon it. This is verified by 

 the fact that when the image produced within the eye is formed 

 upon a point of the optic nerve which has not the choroid behind 

 it, no perception is produced. 



In order to prove this, let three wafers be applied in a hori- 

 zontal line upon a vertical screen, each separated from the other 

 by a distance of two feet. Let the screen be placed before the 

 observer at a distance of about 15 feet, the wafers being on a level 

 with the eye ; and let the centre wafer be so placed that a line 



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