THE SENSE OF FEELING. fe7 



occasions tte contraction of the muscles, is the same with that in the optic n*ve 



thi'ough which tlic latter produces a sensation of light; the wholly unlike effects 

 of the currents of both nerves may be completely explained by the difference of 

 the apparatus on which they operate. But it is impossible that in one and the 

 same fibre of the optic nerve exactly the same current should evoke now the 

 sensation of red and now of blue light, according as the extremity of the fibre 

 is struck by one or the other of the corresponding undulations ; yet we know 

 with absolute certainty that the veiy same fibre really produces differently qual- 

 ified sensations of color, and that there are not fibres, some for the transmission 

 of red and others of blue light. Since, then, one and the same sort of nervous 

 current in the same fibre, with the same terminal apparatus, can by no possi- 

 bility produce difiercnt effects, we must necessarily assume that there exist just 

 as many modifications of the nervous current for the fibre of the optic nerve as 

 we experience different qualities in sensations pertaining to color. Wherein 

 these modifications consist we are not at present in a condition to explain, but 

 we may with confidence assert that they can by no means pertain to processes 

 essentially differing from one another, but are rather slight modifications of a 

 process which is substantially one and the same. To this conclusion we are 

 not led by observation of the residts, that is to say, of the diff'erent sensations; 

 for the sensations of blue and of red color are so little comparable with one an- 

 other, that without other proofs of the relation we could scarcely infer from 

 themselves that they are only diff'erent modifications of the same principle. It 

 is by a comparison of the external and qualifying causes that we are guided to 

 the above conclusion. Since we know that the sensations of red and of blue 

 color are occasioned by the vibrations of the same luminous ether, and that it is 

 only a small difl'erence in the length of the -waves of the vibrating medium 

 which causes the qualitative difference of the sensations, we must also conclude 

 that the qualities of the nervous currents differ as inconsiderably from one an- 

 other as the external catises. It might be conjectured that the difference in 

 the sensations proceeds only from a dilFerence of velocity in the nervous current; 

 but we guardedly use the word "conjectured" to avert the possibility of what is 

 merely a suggestion being regarded as an assertion. 



The most important function of the eye is certainly that by which it aff'ords a 

 perception of the relative position of the objects from which the rays of light 

 are directed upon it. For this purpose there is placed before the stiperficial ex- 

 pansion of the outer extremities of the fibres of the optic nerve the retina, a 

 complete optical apparatus, a camera ohscura, which delineates on this surface 

 formed by the ends of the nerves a small and distinct image of the objects be- 

 fore the eye, just as the camera obscura of the photographer projects such an 

 image on the blank plate or sensitive collodion in the rear. On the structure of 

 this natural camera ohscura we need not here dilate. In the mere presence, in- 

 deed, of an image of external objects on the retina there lies no effective reason 

 that the image shottld be perceived as such ; on the contrary, it is clear that there 

 can be here no question of direct perceptions of space or proportion ; the ques- 

 tion with which we have to deal is this : how do the sensations occasioned by 

 an image connect themselves with such localizing representations as correspond 

 with the actual local relations of the object? This takes place after the follow- 

 ing manner : like the small stones in a Homan mosaic, the delicate points con- 

 stituting the extremities of the optic nerve-fibres are disposed near one another 

 in the above-mentioned surface of the eye in regular order. If oue such point 

 receive an impression of light there arises in the nerve- fibre proceeding from 

 that point a current of excitation which is propagated to' the brain, but which, 

 (and this is a most important law, J being confined to this one fibre, remains "ie- 

 olated," and is not taken up by the other fibres of the trunk of the optic'nerve 

 lying in immediate contact with the former. Now, if two different points re- 

 ceive simultaneously two different impressions from the light, so that two mi- 

 17 S 



