THE SENSE OF SIGHT. 193 



every nerve-fibre terminates in a cone, and can, as is really the case, only be 

 excited by the light through this organization ; the above explanation may then 

 be interpreted as follows : Each cone elaborates the lumiuons wave which strikes 

 it into an irritant of the nerve, which sends a cm-rent to the brain, and there 

 evokes a pei ception of light. The current is essentially the same in all the nerve 

 fibres from the similar constitution of the waves, but it is characterized in each 

 fibre by some modification, some nuance, however slight it may be, which defi- 

 nitely distinguishes it from the current in all the other fibres ; this modification 

 is imparted to it by the appendage in some manner wholly unknown to us. It 

 is this modification of the nervous current, and the sen.sation thereby generated, 

 which we learn to interpret as a token of the place in the field of vision, to which 

 we must assign the perception ; how this is effected has been shown in our remarks 

 on the sense of feeling. The shading of the sensation, which an impression 

 on the central cones situated opposite to the pupil produces, we have learned to 

 connect with the idea that the object to which we refer the sensation lies in the 

 middle of the field of vision, and it is from the same specific shading or modi- 

 fication of the sensation that we determine the relative position of the other 

 points in that field. This brings us to a further interesting fjuestion pertaining 

 to this head of our subject. When the novice is informed that all outward objects 

 are portrayed on the retina in an inverted position, it seems to him incompre- 

 hensible why we should see the objects upright, as they really are, and not 

 inverted; indeed, up to the latest times there have been physiologists who, in 

 order to clear up this seeming contradiction, have taken great pains to find some 

 anatomical or physical mechanism for once more reversing the image of the 

 retina. This is wholly unnecessary ; such a mechanism were only to be thought 

 of, if we perceived the image as such and its inverted position were capable of 

 directly acting upon the sensorium. But that, as we have seen, is not the case. 

 Notwithstanding the inversion of the image, the fact that the object is seen up- 

 right admits of the most simple and natural explanation, when we accept the 

 fact that we connect with the excitation of a sensitive point lying to the left in 

 the retina the idea of a cause of the sensation lying to the right in outer space, 

 and have learned in like manner to refer the excitation of a point below to an 

 object situated above. If Ave consider how ideas of place come to be connected 

 with the sensations of sight, we must see that it cannot be otherwise. An ob- 

 ject which we see we can at the same time touch, and from the position of our 

 organ of touch can determine, in the manner formerly pointed out, the position 

 of the object in space. When we know that, in order to reach a point which 

 has evoked a sensation, we must move the touching finger in the direction which 

 we call to the right, we refer the visual sensation also to the right in outer space, 

 unembarrassed by the fact, of which we are wholly unconscious, that the im- 

 pression of the light has been received on the left and not on the right of the 

 retina. As we formerly saw, it is the appropriate sensations connected Avith 

 the movements of our limbs, Avhose surface is sensitive to the touch, from which 

 we form a judgment of the direction of the movement and the position of the 

 objects touched. Now the eyes also are movable ; their movciiients are con- 

 nected Avith the feeling of movement like those of our arm, and this feeling Ave 

 learn to interpret and to use for the formation of ideas. We know, if our sense 

 of vision has once been educated, that an object lies to the right of us, Avhen, in 

 order to perceive it clearly, Ave must turn our eyes to the right — that is to say, 

 when the movement of the eye creates a muscular feeling, which Ave have 

 learned to recognize as a movement to the right. These muscular feelings 

 which accompany the movements of our eye are, in yet another relation, highly 

 important auxiliaries to the perceptions of sight, and in a manner wholly 

 analogous to the sense of touch. They instruct us respecting the size, form, 

 and distance of observed objects and the direction of their movements. Not to 

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