175 



THE RECENT PROGRESS OF THE 

 THEORY OF VISION, 



A Course of Lectures delivered in Frankfort and Heidelberg, and Republished 

 in the Prevssische Jahrbiicher, 1868. 



I. THE EYE AS AN OPTICAL INSTRUMENT. 



THE physiology of the senses is a border land in which the two 

 great divisions of human knowledge, natural and mental science, 

 encroach on one another's domain; in which problems arise 

 which are important for both, and which only the combined 

 labour of both can solve. 



No doubt the first concern of physiology is only with 

 material changes in material organs, and that of the special 

 physiology of the senses is with the nerves and their sensations, so 

 far as these are excitations of the nerves. But, in the course 

 of investigation into the functions of the organs of the senses, 

 science cannot avoid also considering the apprehension of external 

 objects, which is the result of these excitations of the nerves, and 

 for the simple reason that the fact of a particular state of mental 

 apprehension often reveals to TIS a nervous excitation which 

 would otherwise have escaped our notice. On the other hand, 

 apprehension of external objects must always be an act of our 

 power of realization, and must therefore be accompanied by con- 

 sciousness, for it is a mental function. Indeed the further exact 

 investigation of this process has been pushed, the more it has 

 revealed to us an ever-widening field of such mental functions, 



