230 RECENT PROGRESS OF THE THEORY OF VISION. 



Action of the Senses. They are no longer so novel that 

 they can be reckoned among the latest advances of the 

 theory of vision, which form the subject of the present 

 essay. Moreover, they have been frequently expounded 

 in a popular form by others as well as by myself. 1 But 

 that part of the theory of 'vision with which we are now 

 occupied is little more than a further development of the 

 theory of the specific action of the senses. I must, there- 

 fore, beg my reader to forgive me if, in order to give him 

 a comprehensive view of the whole subject in its proper 

 connection, I bring before him much which he already 

 knows, while I also introduce the more recent additions 

 to our knowledge in their appropriate places. 



All that we apprehend of the external world is brought 

 to our consciousness by means of certain changes which 

 are produced in our organs of sense by external impres- 

 sions, and transmitted to the brain by the nerves. It is 

 in the brain that these impressions first become conscious 

 sensations, and are combined so as to produce our concep- 

 tions of surrounding objects. If the nerves which convey 

 these impressions to the brain are cut through, the sensa- 

 tion, and the perception of the impression, immediately 

 cease. In the case of the eye, the proof that visual per- 

 ception is not produced directly in each retina, but only 

 in the brain itself by means of the impressions transmitted 

 to it from both eyes, lies in the fact (which I shall after- 

 wards more fully explain) that the visual impression of 

 any solid object of three dimensions is only produced by 

 the combination of the impressions derived from both 

 eyes. 



What, therefore, we directly apprehend is not the imme- 

 diate action of the external exciting cause upon the ends 



1 ' On the Nature of Special Sensations in Man,' Kbnigsberger naturwis- 

 senschaftliche Unterhaltungen, vol. iii. 1852. ' Human Vision,' a popular 

 Scientific Lecture by H. Helmholtz, Leipzig, 1855. 



