THE SENSATION OF SIGHT. 203 



an epoch in science, was able to lay down the most important 

 principles of the theory of the impressions derived from the 

 senses. These principles have not only been confirmed in all 

 important points by subsequent investigation, but have proved 

 of even more extensive application than this eminent physio- 

 logist could have suspected. 



The conclusions which he arriysd at are generally compre- 

 hended under the name of the theory of the Specific 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 develop- 

 ment of the theory of the specific action of the senses. I must, 

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

 a comprehensive view of the whole subject in its proper connec- 

 tion, 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 pro- 

 duced in our organs of sense by external impressions, 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 conceptions of surrounding ob- 

 jects. If the nerves which convey these impressions to the 

 brain are cut through, thje sensation, and the perception of the 

 impression, immediately cease. In the case of the eye, the 

 proof that visual perception 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 

 afterwards more fully explain) that the visual impression of any 

 solid object of three dimensions is only produced by the combi- 

 nation of the impressions derived from both eyes. 



1 ' On the Nature of Special Sensations in Man,' KSnigsberger naturu>iss",n- 

 schaftliche Unterhaltungen, vol. iii. 1852. ' Human Vision,' a popular Scien- 

 tific Lecture by H. Helmholtz, Leipzig, 1855. 



