II. TEE SENSATION OF SIGHT. 



IN the first section of our subject we have followed the 

 course of the rays of light as far as the retina, and seen 

 what is the result produced by the peculiar arrangement 

 of the optical apparatus. The light which is reflected 

 from the separate illuminated points of external objects 

 is again united in the sensitive terminal structures of 

 separate nerve fibres, and thus throws them into action 

 without affecting their neighbours. At this point the 

 older physiologists thought they had solved the problem, 

 so far as it appeared to them to be capable of solution. 

 External light fell directly upon a sensitive nervous 

 structure in the retina, and was, as it seemed, directly 

 felt there. 



But during the last century, and still more during the 

 first quarter of this, our knowledge of the processes which 

 take place in the nervous system was so far developed, 

 that Johannes Miiller, as early as the year 1826, 1 when 

 writing that great work on the ' Comparative Physiology 

 of Vision,' which marks an epoch in science, was able to 

 lay down the most important principles of the theory of 

 the impressions derived from the senses. These prin- 

 ciples 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 arrived at are generally com- 

 prehended under the name of the theory of the Specific 



1 The year in which he was appointed Extraordinary Professor of Phy- 

 siology in the University of Bonn. 



