THE SENSE OF SIGHT. 191 



miual apparatus in conformity with natural laws ; mure than this we shall never 

 be able to establish until we have succeeded in explaining the nature of that ner- 

 vous activity itself. 



We liave thus pursued our subject, so far as it relates to the proximate per- 

 formances of our organ of vision, as far as this is at present practicable ; we 

 have seen the undulations of the luminoiis ether penetrate through the dioptric 

 screen of the eye and the rays of light unite in the surface of the retina into 

 images of outward objects; we liave seen, finally, sensations of definite quality 

 arise from those etherial undulations, by means of the apparatus of the retina 

 and the terminal apparatus of the optic nerve. But these simple sensations are 

 by no means visual perceptions, are in themselves no communications to our 

 minds from the outer world of its existence and its action ; they are but dead 

 letters, which must first be reduced to living speech. As Avith all the rest of 

 the senses, so here the naked sensation is but a special accident of our conscious- 

 ness, which must undergo farther elaboration, be interpreted by indirect means, 

 and connected witli appropriate ideas, before we can realize its inappreciable 

 value. Only by our learning to refer the sensation of light to the external 

 object which occasions it, thus giving it objectivity, only by our learning to 

 associate with the simple sensations ideas of the local relations of external 

 objects, is sensation transformed into vision. AVhat would it avail us that every 

 object from which a luminous undulation proceeds should cause a sensation 

 which might be of red, or green, or other color, according to the length of the 

 wave, if this sensation remained a purely subjective feeling, disclosed to us 

 nothing respecting the source from which it came ; to what pui-pose the wonder- 

 ful arrangements which unite the rays of light on the retina into a faithful 

 image of the object whence they proceed, if this image could not. act upon the 

 mind in such a way as to make it sensible of the relations of the object in out- 

 ward space, its form and magnitude, its position and movement? The laity are 

 not generally accustomed to distinguish clearly the sensation from the associated 

 idea, and therefore overlook the wide chasm between a sensation of sight and 

 seeing. We will attempt to overbridge this chasm as briefly and intelligibly as 

 possible ; and we may the better afford to be brief, since we have already treated 

 at large, in reference to the sense of touch, of the process by which objectivity 

 is conferred on our sensations and of the origin of our local preceptions. 



We transfer every impression of the sight to the outer world, even those 

 whose cause exists in the eye itself ; we refer none to our organ of vision ; the 

 gleam of fire which strikes the eye, as well as the flash occasioned by an 

 electric stream directed through it, seems to us to be outside of the eye. While, 

 as regards the sense of touch, we arrive at a perception of the sensitive sur- 

 face, and hence of the w-gan on which the impression takes effect, as a gen- 

 eral thing we never attain, in the case of the sense of sisrht, a consciousness of 

 the perceptive surface of the retina, the organ of this sense; never perceive an 

 image formed on that membrane as cause of the sensation we experience. It 

 was for science first to investigate and verify the existence of such a surface 

 and image ; and notwithstanding this, the physiologist himself is as little capa- 

 ble as the novice of transferring his own sensations to the membrane of the 

 retina. And why 1 There are circuitous means, already discussed in our doctrine 

 of the sense of touch, by which we attain in childhood to the conviction that 

 there is space external to ourselves and outward things as opposed to our own per- 

 sonality; that these outward things are the causes of those conditions of the 

 mind which we call sensations. But while, as regards the sense of touch, we 

 are enabled, through the double sensation which arises from the touch of one 

 part of the surface by another, to distinguish this surface from outward things, 

 no such resource is available as regards the eye, because the means thereto, the 

 power of testing the organ of vision with the organ of vision itself, is wanting ; 

 we learu to refer the sensations of sight directly and exclusively to the external 



